Basic and Clinical Endocrinology 7th International student edition Edition
Metabolic Bone Disease
Dolores Shoback MD
Robert Marcus MD
Daniel Bikle MD, PhD
CELLULAR & EXTRACELLULAR CALCIUM METABOLISM
The calcium ion plays a critical role in intracellular and extracellular events in human physiology. Extracellular calcium levels in humans are tightly regulated within a narrow physiologic range to provide for proper functioning of many tissues: excitation-contraction coupling in the heart and other muscles, synaptic transmission and other functions of the nervous system, platelet aggregation, coagulation, and secretion of hormones and other regulators by exocytosis. The level of intracellular calcium is also tightly controlled, at levels about 10,000-fold lower than extracellular calcium, in order for calcium to serve as an intracellular second messenger in the regulation of cell division, muscle contractility, cell motility, membrane trafficking, and secretion.
It is the concentration of ionized calcium ([Ca2+]) that is regulated in the extracellular fluid. The ionized calcium concentration averages 1.25 ą 0.07 mmol/L (Table 8-1). However, only about 50% of the total calcium in serum and other extracellular fluids is present in the ionized form. The remainder is bound to albumin (about 40%) or complexed with anions such as phosphate and citrate (about 10%). The protein-bound and complexed fractions of serum calcium are metabolically inert and are not regulated by hormones; only the [Ca2+] serves a regulatory role, and only this fraction is itself regulated by the calciotropic hormones parathyroid hormone (PTH) and vitamin D. However, large increases in the serum concentrations of phosphate or citrate can, by mass action, markedly increase the complexed fraction of calcium. For example, massive transfusions of blood, in which citrate is used as an anticoagulant, can reduce the [Ca2+] enough to produce tetany. In addition, because calcium and phosphate circulate at concentrations close to saturation, a substantial rise in the serum concentration of either calcium or phosphate can lead to the precipitation of calcium phosphate salts in tissues, and this is a source of major clinical problems in patients with severe hypercalcemia (eg, malignant tumors) and in those with severe hyperphosphatemia (eg, in renal failure or rhabdomyolysis).
Table 8-1. Calcium concentrations in body fluids.
What is remarkable about calcium metabolism is that the extracellular fluid [Ca2+], which represents a tiny fraction of the total body calcium, can be so tightly regulated in the face of the rapid fluxes of calcium through it that take place during the course of calcium metabolism (Figure 8-1). The total calcium in extracellular fluid amounts to about 1% of total body calcium, with most of the remainder sequestered in bone. Yet from the extracellular fluid compartment, which contains about 900 mg of calcium, 10,000 mg/d is filtered at the glomerulus and 500 mg/d is added to a labile pool in bone; and to the extracellular fluid compartment are added about 200 mg absorbed from the diet, 9800 mg reabsorbed by the renal tubule, and 500 mg from bone.
The challenge of the calcium homeostatic system, then, is to maintain a constant level of [Ca2+] in the extracellular fluid, simultaneously providing adequate amounts of calcium to cells, to bone, and for renal excretion—and all the while compensating, on an hourly basis, for changes in daily intake of calcium, bone metabolism, and renal function. It is scarcely surprising that this homeostatic task requires two hormones, PTH and vitamin D, or that the secretion of each hormone is exquisitely sensitive to small changes in the serum calcium, or that each hormone is able to regulate calcium exchange across all three interfaces of the extracellular fluid: the gut, the bone, and the renal tubule. We will reexamine the integrated roles of PTH and vitamin D in calcium homeostasis after their actions and secretory control have been described.
Figure 8-1. Calcium fluxes in a normal individual in a state of zero external mineral balance. The open arrows denote unidirectional calcium fluxes; the solid arrows denote net fluxes. (Reproduced, with permission, from Felig P et al [editors]: Endocrinology and Metabolism, 2nd ed. McGraw-Hill, 1987.)
The challenge of the cellular calcium economy is to maintain a cytosolic [Ca2+], or [Ca2+]i, of about 0.1 ľmol/L, about 10,000-fold less than what is present outside cells, providing for rapid fluxes through the intracellular compartment as required for regulation while maintaining a large gradient across the cell membrane. The calcium gradient across the cell membrane is maintained by ATP-dependent calcium pumps and by a Na+-Ca2+ exchanger. Calcium can enter cells through several types of calcium channels, some of which are voltage-operated or receptor-operated, to provide for rapid influx in response to depolarization or receptor stimulation. The cell also maintains large stores of calcium in microsomal and mitochondrial pools. Calcium can be released from microsomal stores rapidly by cellular signals such as 1,4,5-inositol trisphosphate (IP3). Reuptake mechanisms are also present, so that cytosolic calcium transients can be rapidly terminated by returning calcium to storage pools or pumping it across the plasma membrane.
Anatomy & Embryology of the Parathyroid Glands
Parathyroid hormone is secreted from four glands located adjacent to the thyroid gland in the neck. The glands weigh an average of 40 mg each. The two superior glands are usually found near the posterior aspect of the thyroid capsule; the inferior glands are most often located near the inferior thyroid margin. However, the exact location of the glands is variable, and 12–15% of normal persons have a fifth parathyroid gland. The parathyroid glands arise from the third and fourth branchial pouches. The inferior glands are actually those derived from the third branchial pouches. Beginning cephalad to the other pair, they migrate further caudad, and one of them sometimes follows the thymus gland into the superior mediastinum. The small size of the parathyroids and the vagaries of their location and number make parathyroid surgery a challenging enterprise for all but the expert surgeon.
The parathyroid glands are composed of epithelial cells and stromal fat. The predominant epithelial cell is the chief cell. The chief cell is distinguished by its clear cytoplasm from the oxyphil cell, which is slightly larger
and has eosinophilic granular cytoplasm. Both cell types contain PTH, and it is not known whether their secretory regulation differs.
Secretion of Parathyroid Hormone
In order to carry out its function to regulate the extracellular calcium concentration, PTH must be under exquisite control by the serum calcium concentration. Thus, the negative feedback relationship of PTH with serum [Ca2+] is steeply sigmoidal, with the steep portion of the curve corresponding exactly to the normal range of serum calcium—precisely the relationship to create a high “gain” controller and assure maintenance of the normal serum calcium concentration by PTH (Figure 8-2).
To sense the concentration of extracellular [Ca2+] and thereby regulate the secretion of PTH, the parathyroid cell relies on a sensor of extracellular calcium. This calcium sensor is a 120-kDa G protein-coupled receptor, with the canonical structure of the seven-transmembrane-domain receptors of this class (Figure 3-2.) The calcium receptor has sequence homologies to the metabotropic glutamate receptors of the central nervous system, the γ-aminobutyric acid receptor-B, and a large family of pheromone receptors. The large extracellular domain of the calcium receptor is thought to be involved in ion recognition, and it is likely that calcium binds directly to sites in this domain. Like other G protein-coupled receptors, the calcium receptor has seven serpentine membrane-spanning domains. The intracellular loops that connect these domains are directly involved in coupling the receptor to G proteins, probably those with alpha q and alpha i subunits.
Figure 8-2. The relations between the serum ionized calcium level and the simultaneous serum concentration of intact PTH in normal humans. The serum calcium concentration was altered by the infusion of calcium (closed circles) or citrate (closed triangles). Parathyroid sensitivity to changes in serum calcium is maximal within the normal range (the shaded area). Low concentrations of PTH persist in the face of hypercalcemia. (Modified from Conlin PR et al: Hysteresis in the relationship between serum ionized calcium and intact parathyroid hormone during recovery from induced hyper- and hypocalcemia in normal humans. J Clin Endocrinol Metab 1989;69:593. By permission of the Journal of Clinical Endocrinology and Metabolism.)
Shortly after identification of the calcium receptor, it was shown that mutations in this receptor were responsible for familial benign hypocalciuric hypercalcemia, a disorder of calcium sensing by the parathyroid and kidney. The calcium receptor is not unique to the parathyroid. Calcium receptors are widely distributed in the brain, skin, growth plate, intestine, stomach, C cells, and other tissues. This receptor regulates the responses to calcium in thyroid C cells, which secrete calcitonin in response to high extracellular calcium, and in the distal nephron of the kidney, where the receptor regulates calcium excretion. The function of calcium receptors in many other sites is beginning to be addressed.
The primary cellular signal by which increased extracellular calcium inhibits the secretion of PTH is an increase in [Ca2+]i. The calcium receptor is directly coupled by Gq to the enzyme phospholipase C, which hydrolyzes the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) to liberate the intracellular messengers IP3 and diacylglycerol (see Figure 3-5). IP3 binds to a receptor in endoplasmic reticulum that releases calcium from membrane stores. The release of stored calcium raises the [Ca2+]i rapidly and is followed by a sustained influx of extracellular calcium, through channels to produce a rise and a sustained plateau in [Ca2+]i. Increased intracellular calcium may be sufficient for inhibition of PTH release, but it is unclear whether calcium release from intracellular stores or sustained calcium influx from the cell exterior is most important. The other product of phospholipase C action is the lipid diacylglycerol, an activator of the calcium- and phospholipid-sensitive protein kinase, protein kinase C. The effects of protein kinase C on the release of PTH from the gland are complex. Calcium receptors also couple to the inhibition of cAMP generation, which also may play a role in setting the response of parathyroid cells to ambient calcium levels.
The initial effect of high extracellular calcium is to inhibit the secretion of preformed PTH from storage granules in the gland by blocking the fusion of storage granules with the cell membrane and release of their contents. In most cells, stimulation of exocytosis (“stimulus-secretion coupling”) is a calcium-requiring
process, which is inhibited by depletion of calcium. The parathyroid cell is necessarily an exception to this rule, because this cell must increase secretion of PTH when the calcium level is low. In the parathyroids, intracellular magnesium appears to serve the role in stimulus-secretion coupling that calcium does in other cells. As discussed below in the section on hypoparathyroidism, depletion of magnesium stores can paralyze the secretion of PTH, leading to reversible hypoparathyroidism.
Besides calcium, there are several regulators of PTH secretion. Hypermagnesemia inhibits PTH, and during the treatment of premature labor with infusions of magnesium sulfate a reduction in the PTH level and occasionally hypocalcemia are observed. Conversely, moderate hypomagnesemia can stimulate PTH secretion, even though prolonged depletion of magnesium will paralyze it. On a molar basis, magnesium is less potent in controlling the secretion than calcium. Catecholamines, acting through β-adrenergic receptors and cAMP, stimulate the secretion of PTH. This effect does not appear to be clinically significant. The hypercalcemia sometimes observed in patients with pheochromocytoma usually has another basis—secretion of parathyroid hormone-related protein (PTHrP) by the tumor.
Not only do changes in serum calcium regulate the secretion of PTH—they also regulate the synthesis of PTH at the level of stabilizing preproPTH mRNA levels and possibly enhancing gene transcription. It is estimated that glandular stores of PTH are sufficient to maintain maximal rates of secretion for no more than 1.5 hours, so increased synthesis is required to meet sustained hypocalcemic challenges.
Transcription of the PTH gene is also regulated by vitamin D: high levels of 1,25-dihydroxyvitamin D inhibit PTH gene transcription. This is one of many ways that the calciotropic hormones cooperatively regulate calcium homeostasis, and it has therapeutic implications. Vitamin D analogs are used to treat secondary hyperparathyroidism in dialysis patients with renal osteodystrophy.
Synthesis and Processing of Parathyroid Hormone
PTH is an 84-amino-acid peptide with a molecular weight of 9300. Its gene is located on chromosome 11. The gene encodes a precursor called preproPTH with a 29-amino-acid extension added at the amino terminal of the mature PTH peptide (Figure 8-3). This extension includes a 23-amino-acid signal sequence (the “pre” sequence) and a six-residue prohormone sequence. The signal sequence in preproPTH functions precisely as it does in most other secreted protein molecules, to allow recognition of the peptide by a signal recognition particle, which binds to nascent peptide chains as they emerge from the ribosome and guides them to the endoplasmic reticulum, where they are inserted through the membrane into the lumen (Figure 8-4). The process is discussed in detail in Chapter 2.
In the lumen of the endoplasmic reticulum, a signal peptidase cleaves the signal sequence from preproPTH to leave proPTH, which exits the endoplasmic reticulum and travels to the Golgi apparatus, where the “pro” sequence is cleaved from PTH by an enzyme called furin (Chapter 2). While preproPTH is evanescent, proPTH has a life span of about 15 minutes. The processing of proPTH is quite efficient, and proPTH, unlike other prohormones (eg, proinsulin), is not secreted. As it leaves the Golgi apparatus, PTH is repackaged into dense neuroendocrine-type secretory granules, where it is stored to await secretion.
Clearance & Metabolism of PTH
PTH secreted by the gland has a circulating half-life of 2–4 minutes. Intact PTH(1–84) is predominantly cleared in the liver and kidney. There, PTH is cleaved at the 33–34 and 36–37 positions to produce an amino terminal fragment and a carboxyl terminal fragment. Amino terminal fragments of PTH do circulate but not to the extent of carboxyl terminal fragments. The latter are cleared from blood by renal filtration, and they accumulate in chronic renal failure. Although the classic activities of PTH are encoded in the amino terminal portion of the molecule, mid region and carboxyl terminal fragments of the hormone may not be metabolically inert. Recent evidence suggests that they have their own receptors and may have their own biologic actions.
Assay of PTH
Modern assays of intact PTH(1–84) employ two-site immunoradiometric assay (IRMA) or immunochemiluminescent assay (ICMA) techniques, in which the normal range for PTH is 10–60 pg/mL (1–6 pmol/L). By utilizing antibodies to two determinants, one near the amino terminal of PTH and the other near the carboxyl terminal, these assays are designed to measure the intact, biologically active hormone specifically (Figure 8-5). In practice, such assays have sufficient sensitivity and specificity not only to detect increased levels of PTH in hyperparathyroid disorders but also to detect suppressed levels of PTH in patients with nonparathyroid hypercalcemia. The ability to detect suppression of
PTH makes these assays powerful tools for the differential diagnosis of hypercalcemia: If hypercalcemia results from some form of hyperparathyroidism, then the serum PTH level will be high; if hypercalcemia has a nonparathyroid basis, then PTH will be suppressed. Recently, intact PTH assays have been refined so that the amino terminal antibody requires more than just the immediate amino terminal of the PTH molecule for its interaction. This improvement was made as a result of the recognition that antibodies used in first-generation intact PTH assays could recognize small amino terminal PTH fragments which are generated in vivo and may circulate to a significant extent in chronic renal failure. Two-site assays of intact PTH have essentially replaced earlier techniques, many of which also detected inert carboxyl terminal fragments as well as intact PTH. These assays could not detect suppression of PTH in nonparathyroid disorders and gave very high levels of immunoreactive PTH in chronic renal failure.
Figure 8-3. Primary structure of human preparathyroid hormone. The arrows indicate sites of specific cleavages which occur in the sequence of biosynthesis and peripheral metabolism of the hormone. The biologically active sequence is enclosed in the center of the molecule. (Reproduced, with permission, from Felig P, Baxter JD, Frohman LA [editors]: Endocrinology and Metabolism, 3rd ed. McGraw-Hill, 1995.)
Biologic Effects of PTH
The function of PTH is to regulate serum calcium levels by concerted effects on three principal target organs: bone, intestinal mucosa, and kidney. The effect of PTH on intestinal calcium absorption is indirect, resulting from increased renal production of the intestinally active vitamin D metabolite 1,25-dihydroxyvitamin D. By its integrated effects on the kidney, gut, and bone, PTH acts to increase the inflow of calcium into
the extracellular fluid and thus defend against hypocalcemia. Removal of the parathyroid glands results in profound hypocalcemia and ultimately in tetany and death.
Figure 8-4. Biosynthetic events in the production of PTH within the parathyroid cell. PreproPTH gene is transcribed to its mRNA, which is translated on the ribosomes to preproPTH (amino acids -29 to +84). The pre- sequence is removed within the endoplasmic reticulum, yielding proPTH (-6 to +84). Mature PTH(1–84) released from the Golgi is packaged in secretory granules and released into the circulation in the presence of hypocalcemia. The calcium receptor senses changes in extracellular calcium that affect both the release of PTH and the transcription of the preproPTH gene. (Reproduced, with permission, from McPhee SJ et al [editors]: Pathophysiology of Disease: An Introduction to Clinical Medicine. Originally published by Appleton & Lange. Copyright Š 1995 by The McGraw-Hill Companies, Inc.)
In the kidney, PTH has direct effects on the tubular reabsorption of calcium, phosphate, and bicarbonate. Although the bulk of calcium is resorbed from tubule fluid together with sodium in the proximal convoluted tubule, the fine tuning of calcium excretion occurs in the distal nephron. There, PTH markedly increases the reabsorption of calcium, predominantly in the distal convoluted tubule. Although calcium is actively transported against an electrochemical gradient, the precise nature of the calcium transport process that is regulated
by PTH is controversial. However, from a physiologic standpoint, the ability to limit renal losses of calcium is one important means by which PTH protects the serum calcium level.
Figure 8-5. Schematic representation of the principle of the two-site assay for intact PTH. The label may be a luminescent probe or 125I in the immunochemiluminescent or immunoradiometric assay, respectively. Two different region-specific antibodies are used (Ab1 and Ab2). Only the hormone species containing both immunodeterminants is counted in the assay. (Reproduced, with permission, from McPhee SJ et al [editors]: Pathophysiology of Disease: An Introduction to Clinical Medicine. Originally published by Appleton & Lange. Copyright Š 1995 by The McGraw-Hill Companies, Inc.)
PTH inhibits the reabsorption of phosphate in the renal proximal tubule. In this nephron segment, phosphate is transported across the apical membrane of the tubule cell by a specific sodium-phosphate cotransporter, with phosphate influx driven by the energy of the sodium gradient. The transport protein has been identified by molecular cloning. It is suspected that PTH may inhibit sodium-phosphate reabsorption by reducing the rate of insertion of transporters from a sequestered cytoplasmic pool into the apical membrane. In any case, the phosphaturic effect of PTH is profound. It is best quantified by calculating the tubule reabsorption of phosphate (TRP) from the clearances of phosphate and creatinine (TRP = 1 - CP/Ccreat, normal range 80–97%), or by calculating the renal phosphate threshold (TmP/GFR) from a standard nomogram. Because it is primarily the renal phosphate threshold that sets the level of serum phosphorus, the phosphaturic effect of PTH is mirrored in the serum phosphorus level, eg, hypophosphatemia in hyperparathyroidism. Hyperparathyroid states may also be characterized by impaired bicarbonate reabsorption and a mild hyperchloremic metabolic acidosis, because of inhibition of Na+-H+ antiporter activity by PTH.
Though the hypocalciuric effect of PTH is readily understood as part of the concerted actions of the hormone to protect the serum calcium level, the utility of the phosphaturic effect of PTH is less obvious. One consideration is that the phosphaturic effect tends to prevent an increase in serum phosphate, which would otherwise result from the obligatory release of phosphate with calcium during bone resorption and would tend to dampen the homeostatic increase in serum calcium by complexing calcium in blood. An example is renal osteodystrophy. When phosphate clearance is impaired by renal failure, the hypocalcemic effect of phosphate released during bone remodeling is an important contributor to progressive secondary hyperparathyroidism as part of a positive feedback loop—the more that bone resorption is stimulated and phosphate released, the more hyperparathyroidism is induced.
Mechanism of Action of Parathyroid Hormone
There are two mammalian receptors for PTH. The first receptor to be identified recognizes PTH and PTH-related protein (PTHrP) and is designated the PTH and PTHrP receptor, or the PTH-1 receptor. The PTH-2 receptor is activated by PTH only. These receptors also differ in their tissue distribution. The PTH receptor-1 in kidney and bone is an 80,000-MW glycoprotein member of the G protein receptor superfamily. It has the canonical architecture of such receptors, with a large first extracellular domain, seven consecutive membrane-spanning domains, and a cytoplasmic tail (see Figure 3-2). PTH binds to sites in the large extracellular domain of the receptor. The hormone-bound form of the receptor then activates associated G proteins via several determinants in the intracellular loops. PTH receptors are only remotely related in sequence to most other G protein coupled receptors, but they are more closely related to a small subfamily of peptide hormone receptors, which includes those for secretin, VIP, ACTH, and calcitonin.
PTH itself has a close structural resemblance to a sister protein, PTHrP, and also resembles the peptides secretin, VIP, calcitonin, and ACTH. As noted above, the receptors for these related ligands are themselves
members of a special family. These peptide hormones are characterized by an amino terminal α-helical domain, thought to be directly involved in receptor activation, and an adjacent α-helical domain which seems to be the primary receptor-binding domain. In the case of PTH, residues 1–6 are required for activation of the receptor; truncated analogs without these residues (eg, PTH[7–34]) can bind the receptor but cannot efficiently activate it and thus serve as competitive antagonists of PTH action. The primary receptor-binding domain consists of PTH(18–34). Although the intact form of PTH is an 84-amino-acid peptide, PTH (35–84) does not seem to have any important role in binding to the bone-kidney receptor. However, a separate PTH(35–84) receptor may exist; the carboxyl terminal PTH receptor could mediate an entirely new set of actions of PTH.
The PTH-1 receptor binds PTH and its sister hormone PTHrP with equivalent affinity. Inheritance of a constitutively active form of this receptor produces lifelong hypercalcemia as part of a rare heritable form of short-limbed dwarfism disorder called Jansen-type metaphysial chondrodysplasia. Homozygous inactivating mutations of the PTH-1 receptor gene are responsible for congenital Blomstrand's chondrodysplasia, a lethal developmental syndrome.
Physiologic activation of the receptor by binding of either PTH or PTHrP induces the active, GTP-bound state of two receptor-associated G proteins. Gs couples the receptor to the effector adenylyl cyclase and thereby to the generation of cAMP as a cellular second messenger. Gqcouples the receptor to a separate effector system, phospholipase C, and thereby to an increase in [Ca2+]i and to activation of protein kinase C (Figure 3-5). Although it is not clear which of the cellular messengers, cAMP or intracellular calcium, is responsible for each of the various cellular effects of PTH, there is evidence from an experiment of nature that cAMP is the intracellular second messenger for calcium homeostasis and renal phosphate excretion. The experiment is pseudohypoparathyroidism, in which null mutations in one allele of the stimulatory G protein subunit Gsα cause hypocalcemia and unresponsiveness of renal phosphate excretion to PTH.
When secreted in abundance by malignant tumors, PTHrP produces severe hypercalcemia by activating the PTH/PTHrP-1 receptor. However, the physiologic role of PTHrP is quite different from that of PTH. PTHrP is produced in many fetal and adult tissues. Based on gene knockout experiments and overexpression of PTHrP in individual tissues, we now know that PTHrP is required for normal development as a regulator of the proliferation and mineralization of chondrocytes and as a regulator of placental calcium transport. In postnatal life, PTHrP appears to regulate the epithelial-mesenchymal interactions that are critical for development of the mammary gland, skin, and hair follicle. In most physiologic circumstances, PTHrP carries out local rather than systemic actions. PTHrP is discussed more fully in Chapter 21.
Calcitonin is a 32-amino-acid peptide whose principal function is to inhibit osteoclast-mediated bone resorption. Calcitonin is secreted by parafollicular C cells of the thyroid. These are neuroendocrine cells derived from the ultimobranchial body, which fuses with the posterior lobes of the thyroid to become C cells. C cells make up only about 0.1% of the mass of the thyroid.
The secretion of calcitonin is under the control of the serum [Ca2+]. The C cell uses the same calcium receptor as the parathyroid cell to sense changes in the ambient calcium concentration, but the C cell increases secretion of calcitonin in response to hypercalcemia and shuts off hormone secretion during hypocalcemia.
The calcitonin gene is composed of six exons, and, through alternative exon splicing, it encodes two entirely different peptide products (Figure 8-6). In the thyroid C cell, the predominant splicing choice generates mature calcitonin (Figure 8-7), which is incorporated within a 141-amino-acid precursor. In other tissues, especially neurons of the central nervous system, a peptide called calcitonin gene-related peptide (CGRP) is produced from a 128-amino-acid precursor. CGRP is a 37-amino-acid peptide with considerable homology to calcitonin. The amino terminals of both peptides incorporates a seven-member disulfide-bonded ring (Figure 8-7). Acting through its own receptor, CGRP is among the most potent vasodilator substances known. There are two distinct genes encoding calcitonin and CGRP (CALC1 andCALC2). CALC1 can produce calcitonin and CGRP I, while CALC2 encodes CGRP II. This peptide differs from CGRP I by only three amino acids.
When administered intravenously, calcitonin produces a rapid and dramatic decline in levels of serum calcium and phosphorus, primarily through actions on bone. The major effect of the hormone is to inhibit osteoclastic bone resorption. After exposure to calcitonin, the morphology of the osteoclast changes rapidly. Within minutes, the cell withdraws its processes, shrinks in size, and retracts the ruffled border, the organelle of bone resorption, from the bone surface. Osteoclasts and cells of the proximal renal tubule express a
calcitonin receptor. Like the PTH receptor, this is a serpentine G protein-coupled receptor with seven membrane-spanning regions which is coupled by Gs to adenylyl cyclase and thereby to the generation of cAMP in target cells. Calcitonin also has renal effects. At the kidney, calcitonin inhibits the reabsorption of phosphate, thus promoting renal phosphate excretion. Calcitonin also induces a mild natriuresis and increases the renal excretion of calcium. The renal effects of calcitonin are not essential for its acute effect on serum calcium levels, which results from blockade of bone resorption.
Figure 8-6. Alternative processing of the human calcitonin gene. Calcitonin is produced by thyroid C cells; CGRP is produced in the brain. (Modified and reproduced, with permission, from Rosenfeld MG et al: Production of a novel neuropeptide encoded by the calcitonin gene via tissue-specific RNA processing. Nature 1983;304:129.)
Figure 8-7. Amino acid sequence of human calcitonin, demonstrating its biochemical features, including an amino terminal disulfide bridge and carboxyl terminal prolinamide. (Reproduced, with permission, from McPhee SJ et al [editors]: Pathophysiology of Disease: An Introduction to Clinical Medicine. Originally published by Appleton & Lange. Copyright Š 1995 by The McGraw-Hill Companies, Inc.)
Although its secretory control by calcium and its antiresorptive actions enable calcitonin to counter PTH in the control of calcium homeostasis, thus engendering bihormonal regulation, it is actually unlikely that calcitonin plays an essential physiologic role in humans and other terrestrial animals. This surprising conclusion is supported by two lines of evidence. First, removal of the thyroid gland—the only known source of calcitonin in mammals—has no perceptible impact on calcium handling or bone metabolism. Second, secretion of extremely high calcitonin levels by medullary thyroid carcinoma, a malignancy of the C cell, likewise has no apparent effect on mineral homeostasis. Thus, in humans, calcitonin is a hormone in search of a function. It plays a much more obvious homeostatic role in salt-water fish, in which the major challenge is maintenance of blood calcium levels in the sea, where the ambient calcium concentration of sea water is very high.
Calcitonin is of clinical interest for two reasons. First, calcitonin is important as a tumor marker in medullary thyroid carcinoma. Second, calcitonin has found several therapeutic uses as an inhibitor of osteoclastic bone resorption. Calcitonin can be administered either parenterally or as a nasal spray and is used in the treatment of Paget's disease of bone, hypercalcemia, and osteoporosis.
The term vitamin D (calciferol) refers to two secosteroids: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) (Figure 8-8). Both are produced by photolysis from naturally occurring sterol precursors. Vitamin D2 is the principal form of vitamin D available for pharmaceutical purposes other than as dietary supplements (Table 8-2). Vitamin D3 is produced from 7-dehydrocholesterol, a precursor of cholesterol found in high concentration in the skin. Vitamins D2 and D3 are nearly equipotent in humans, so the term vitamin D, unless otherwise qualified, will be understood here to denote both. Vitamins D2 and D3 differ in their side chains: vitamin D2 has a methyl group at C24 and a double bond at C22 to C23. These features alter the metabolism of vitamin D2 compared with vitamin D3; however, both are converted to 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D3.
In the process of forming vitamins D2 and D3, the B ring of the sterol precursor is cleaved, and the A ring is rotated around the C5 to C6 double bond so that the 3β-hydroxyl group is positioned below the plane of the A-ring. By convention, this hydroxyl group retains its designation as 3β, and the 1-position above the plane of the A-ring is designated 1α. Hydroxylations in the side-chain can lead to stereoisomers designated R and S. The natural position for the C24 hydroxyl group is R. Both the R and the S positions can be hydroxylated at C25 in the formation of 25,26(OH)2D.
Since vitamin D can be formed in vivo (in the epidermis) in the presence of adequate amounts of ultraviolet light, it is more properly considered a hormone (or prohormone) than a vitamin. To be biologically active, vitamin D must be metabolized further. The liver metabolizes vitamin D to its principal circulating form, 25(OH)D. The kidney and other tissues metabolize 25(OH)D to a variety of other metabolites, the most important of which are 1,25(OH)2D3 and, perhaps, 24,25(OH)2D. A large number of other metabolites have been identified, but their physiologic roles are unclear. They may only represent products destined for elimination. Normal circulating levels of the principal metabolites are listed in Table 8-3. The recommended daily allowance for vitamin D is 400 units. One unit equals 0.025 ľg vitamin D.
Cutaneous Synthesis of Vitamin D
Vitamin D3 is formed in the skin from 7-dehydrocholesterol, which is distributed throughout the epidermis and dermis but has its highest concentration in the lower layers of the epidermis, the stratum spinosum and stratum basale. These epidermal layers also account for the highest production of vitamin D. The cleavage of the B ring of 7-dehydrocholesterol to form previtamin D3 (Figure 8-8) requires ultraviolet light. Following cleavage of the B ring, the previtamin D3 undergoes thermal isomerization to vitamin D3 but also to the biologically inactive compounds lumisterol and tachysterol. Formation of pre-D3 is rapid and reaches a maximum within hours during exposure to solar or ultraviolet irradiation. The degree of epidermal pigmentation, the age of the skin, and the intensity of exposure all affect the time required to reach the maximum pre-D3 concentration but do not alter that maximum. Continued ultraviolet light exposure then results in continued formation of the inactive compounds from pre-D3. The formation of lumisterol is reversible,
so lumisterol can be converted back to pre-D3 as pre-D3 levels fall. Short exposure to sunlight causes prolonged release of vitamin D3 from the exposed skin because of the slow conversion of pre-D3 to vitamin D3 and the conversion of lumisterol to pre-D3. Prolonged exposure to sunlight does not produce toxic quantities of vitamin D3 because of the photoconversion of pre-D3 to lumisterol and tachysterol.
Figure 8-8. The photolysis of ergosterol and 7-dehydrocholesterol to vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), respectively. An intermediate is formed after photolysis, which undergoes a thermal-activated isomerization to the final form of vitamin D. The rotation of the A-ring puts the 3β-hydroxyl group into a different orientation with respect to the plane of the A-ring during production of vitamin D.
Table 8-2. Commonly used vitamin D metabolites and analogs.
Table 8-3. Vitamin D and its metabolites.1
Vitamin D3 transport from the skin into the circulation has not been thoroughly studied. Vitamin D3 is carried in the bloodstream primarily bound to vitamin D-binding protein (DBP), an α-globulin produced in the liver. DBP has a lower affinity for vitamin D3 than other vitamin D metabolites such as 25(OH)D and 24,25(OH)2D. 7-Dehydrocholesterol, pre-D3, lumisterol, and tachysterol bind to DBP even less well. Therefore, vitamin D3 could be selectively removed from skin by the gradient established by selective binding to DBP. Since the deepest levels of the epidermis make the most vitamin D3 when the skin is irradiated, the distance over which vitamin D3 must diffuse to reach the circulation is short. However, simple diffusion is an unlikely means for so hydrophobic a molecule to enter the bloodstream. Epidermal lipoproteins may play a role in transport, but this remains to be established.
Dietary Sources & Intestinal Absorption
Dietary sources of vitamin D are clinically important because exposure to ultraviolet light may not be sufficient to maintain adequate production of vitamin D in the skin. The farther away from the equator one lives, the shorter the period of the year during which the intensity of sunlight is sufficient to produce vitamin D3. Most dairy products in the United States are supplemented with vitamin D. Unfortified dairy products contain little or no vitamin D. Although plants contain ergosterol, their content of vitamin D2 is limited unless they are irradiated with ultraviolet light during processing. Vitamin D is found in high concentrations in fish oils, fish liver, and eggs. Vitamin D is absorbed from the diet in the small intestine with the help of bile salts. Drugs that bind bile salts such as colestipol will reduce vitamin D absorption. Most of the vitamin D passes into the lymph in chylomicrons, but a significant amount is absorbed directly into the portal system. The presence of fat in the lumen decreases vitamin D absorption. 25(OH)D is preferentially absorbed into the portal system and is less influenced by the amount of fat in the lumen. Biliary conjugates of the vitamin D metabolites have been identified, and an enterohepatic circulation of these metabolites has been established. Vitamin D is taken up rapidly by the liver and is metabolized to 25(OH)D. 25(OH)D is also transported in the blood bound to DBP. Little vitamin D is stored in the liver. Excess vitamin D is stored in adipose tissue and muscle.
Binding Proteins for Vitamin D Metabolites
As noted above, vitamin D metabolites are transported in the blood bound principally to DBP (85%) and albumin (15%). DBP binds 25(OH)D and 24,25 (OH)2D with approximately 30 times greater affinity than it binds 1,25(OH)2D3 and vitamin D. DBP circulates at a concentration (5 × 10-6 mol/L) approximately
50 times greater than the total concentrations of the vitamin D metabolites. DBP levels are reduced in liver disease and in the nephrotic syndrome and increased during pregnancy and with estrogen administration but are not altered by states of vitamin D deficiency or excess. The high affinity of DBP for the vitamin D metabolites and the large excess binding capacity maintain the free and presumed biologically active concentrations of the vitamin D metabolites at very low levels—approximately 0.03% and 0.4% of the total 25(OH)D and 1,25(OH)2D3levels, respectively. Liver disease reduces the total level of the vitamin D metabolites commensurate with the reduction in DBP and albumin levels, but the free concentrations of the vitamin D metabolites remain normal in most subjects. This fact must be borne in mind when evaluating a patient with liver disease and determining whether or not such a patient is truly vitamin D-deficient. Pregnancy, on the other hand, increases both the free and total concentrations by increasing DBP levels, altering the binding of the metabolites to DBP, and increasing the production of 1,25(OH)2D3. The binding of the vitamin D metabolites to DBP appears to occur at the same site, and saturation of this site by one metabolite can displace the other metabolites. This is an important consideration in vitamin D toxicity because the very high levels of 25(OH)D that mark this condition displace the often normal levels of 1,25(OH)2D3, leading to elevated free and biologically active concentrations of 1,25(OH)2D3. This phenomenon at least partially explains the hypercalcemia and hypercalciuria that mark vitamin D intoxication even in patients with normal total 1,25(OH)2D3 levels. At present, it is not clear whether DBP functions just to maintain a circulating reservoir of vitamin D metabolites or whether DBP participates in transporting the vitamin D metabolites to and within their target tissues. There is little evidence for a DBP receptor on cells. Independently of vitamin D, DBP binds to actin and appears to activate macrophages, suggesting that DBP has functions other than vitamin D metabolite transport.
The conversion of vitamin D to 25(OH)D occurs principally in the liver (Figure 8-9). Both mitochondria and microsomes have the capacity to produce 25(OH)D, but with different kinetics and probably with different enzymes. The mitochondrial enzyme has been identified as the 5β-cholestane-3α,7α,12α-triol-27(26)-hydroxylase (CYP27), a cytochrome P450 mixed-function oxidase, and its cDNA has been isolated. Regulation of 25(OH)D production is difficult to demonstrate. Drugs such as phenytoin and phenobarbital reduce serum 25(OH)D levels primarily by increased catabolism of 25(OH)D and vitamin D. Liver disease leads to reduced serum 25(OH)D levels, primarily because of reduced DBP synthesis and not reduced synthesis of 25(OH)D. Vitamin D deficiency is marked by low blood levels of 25(OH)D primarily because of lack of substrate for the 25-hydroxylase. Vitamin D intoxication, on the other hand, leads to increased levels of 25(OH)D because of the lack of feedback-inhibition.
Control of vitamin D metabolism is exerted principally in the kidney (Figure 8-9). 1,25(OH)2D3 and 24,25(OH)2D are produced by cytochrome mixed-function oxidases in mitochondria of the proximal tubules. cDNAs encoding these enzymes have both been identified and show substantial homology to each other and to the 25-hydroxylase (CYP27). Their homology to other mitochondrial steroid hydroxylases is considerable, but not as high. Although the 24-hydroxylase is widely distributed, the 1-hydroxylase is more limited, being found only in the epidermis, placenta, bone, macrophages, and prostate in addition to kidney.
The kidney remains the principal source for circulating 1,25(OH)2D3. Production of 1,25(OH)2D3 in the kidney is stimulated by PTH and is inhibited by high blood levels of calcium and phosphate. Calcium and phosphate have both direct and indirect effects on 1,25(OH)2D3production. In addition to direct actions on the 1-hydroxylase, calcium alters PTH secretion, and phosphate alters factors (possibly growth hormone) from the pituitary gland which regulate 1,25(OH)2D3 production as well. 1,25(OH)2D3 inhibits its own production and stimulates the production of 24,25 (OH)2D. Other hormones such as prolactin, growth hormone, and calcitonin may regulate 1,25(OH)2D3 production, but their roles under normal physiologic conditions in humans have not been demonstrated clearly.
Extrarenal production of 1,25(OH)2D3 is regulated differently in a cell-specific fashion. Cytokines such as γ-interferon and tumor necrosis factor-α stimulate 1,25(OH)2D3 production in macrophages and keratinocytes. 1,25(OH)2D3 and calcium are less inhibitory of 1,25(OH)2D3production in macrophages. This is important in understanding the pathophysiology of hypercalcemia and the increased 1,25(OH)2D3 levels in patients with sarcoidosis, lymphomas, and other granulomatous diseases.
Mechanisms of Action
The main function of vitamin D metabolites is the regulation of calcium and phosphate homeostasis, which occurs in conjunction with PTH. The gut, kidney, and bone are the principal target tissues for this regulation. The major pathologic complication of vitamin D deficiency
is rickets (in children with open epiphyses) or osteomalacia (in adults), which in part results from a deficiency in calcium and phosphate required for bone mineralization. 1,25(OH)2D3 is the most biologically active if not the only vitamin D metabolite involved in maintaining calcium and phosphate homeostasis.
Figure 8-9. The metabolism of vitamin D. The liver converts vitamin D to 25(OH)D. The kidney converts 25(OH)D to 1,25(OH)2D3 and 24,25(OH)2D. Control of metabolism is exerted primarily at the level of the kidney, where low serum phosphorus, low serum calcium, and high parathyroid hormone (PTH) levels favor production of 1,25(OH)2D3.
Most of the cellular processes regulated by 1,25(OH)2D3 involve the nuclear vitamin D receptor (VDR), a 50-kDa protein related by structural and functional homology to a large class of nuclear hormone receptors including the steroid hormone receptors, thyroid hormone receptors (TRs), retinoic acid and retinoid X receptors (RARs, RXRs), and endogenous metabolite receptors, including peroxisome proliferation activator receptors (PPARs), liver X receptors (LXRs), and farnesoid X receptors (FXRs). These receptors are transcription factors. The VDR, as is the case with TRs, RARs, PPARs, LXRs, and FXRs, generally acts by forming heterodimers with RXRs. The VDR-RXR complex then binds to specific regions within the regulatory portions of the genes whose expression is controlled by 1,25(OH)2D3 (Figure 8-10). The regulatory regions are called vitamin D response elements (VDREs) and are generally composed of two stretches of six nucleotides (each called a half-site) with a specific and nearly identical sequence separated by three nucleotides with a less specific sequence (a direct repeat with three base-pair separation; DR3). Exceptions to this rule can be found such as a VDR-RAR complex binding to a DR6 (direct repeat with six base-pair separation). Other nuclear hormone receptors bind to similar elements but with different spacing or orientation of the half-sites. The binding of the VDR-RXR complex to the VDRE then attracts a number of other proteins called coactivators which are thought to bridge the gap between the VDRE and the initiation complex (TATA box proteins) to signal the beginning of transcription. Many of these coactivators have histone acetyltransferase activity, which leads to an unraveling of the histone-chromatin complex encompassing the gene, thus
allowing it to be activated. Not all actions of 1,25(OH)2D3 can be explained by changes in gene expression. An area of active investigation involves efforts to identify a membrane receptor for 1,25(OH)2D3 that may mediate the rapid effects of 1,25(OH)2D3 on calcium influx and protein kinase C activity observed in a number of tissues.
Figure 8-10. 1,25(OH)2D3-initiated gene transcription. 1,25(OH)2D3 enters the target cell and binds to its receptor, VDR. The VDR then heterodimerizes with the retinoid X receptor, RXR. This increases the affinity of the VDR/RXR complex for the vitamin D response element (VDRE), a specific sequence of nucleotides in the promoter region of the vitamin D responsive gene. Binding of the VDR/RXR complex to the VDRE attracts a complex of proteins termed coactivators to the VDR/RXR complex which span the gap between the VDRE and RNA polymerase II and other proteins in the initiation complex centered at or around the TATA box (or other transcription regulatory elements). Transcription of the gene is initiated to produce the corresponding mRNA, which leaves the nucleus to be translated to the corresponding protein.
- INTESTINAL CALCIUM TRANSPORT
Intestinal calcium transport is the best-understood target tissue response of 1,25(OH)2D3. Calcium transport through the intestinal epithelium proceeds by at least three distinct steps: (1) entrance into the cell from the lumen across the brush border membrane down a steep electrochemical gradient; (2) passage through the cytosol, probably within subcellular organelles such as mitochondria and dense bodies; and (3) removal from the cell against a steep electrochemical gradient at the basolateral membrane. Each of these steps is regulated by 1,25(OH)2D3. At the brush border, 1,25(OH)2D3 induces a change in the binding of calmodulin to brush border myosin 1, a unique form of myosin found only in the intestine, where it resides primarily in the microvillus bound to actin and to the plasma membrane. The calmodulin-myosin 1 complex may provide the mechanism for removing calcium from the brush border after it crosses the membrane into the cell. Changes in the phospholipid composition of the brush border may explain the increased flux of calcium across this membrane after calcitriol administration. None of these changes require new protein synthesis. However, recent evidence indicates that a newly described calcium channel (CaT1) in the brush border membrane may be induced by 1,25(OH)2D. This channel may be the major mechanism by which calcium enters the intestinal epithelial cell. The transport of calcium through the cytosol requires a vitamin D-inducible protein called calbindin. Calbindin exists either in a 28-kDa or 9-kDa form, depending on the species. Calbindin has a high affinity for calcium. If its synthesis is blocked, the calcium content of the cytosol and mitochondria increases, and efficiency of transport is reduced. At the basolateral membrane, calcium is removed from the cell by an ATP-driven pump, the Ca2+-ATPase, a protein also induced by 1,25(OH)2D3. In mice in which the VDR is rendered nonfunctional (VDR “knockout”), intestinal calcium transport is markedly impaired, indicating the importance of VDR function—presumably
regulation of gene expression—in governing intestinal calcium transport. The requirement for a functional VDR is less clear for other target tissues.
- ACTIONS OF VITAMIN D IN BONE
The critical role of 1,25(OH)2D3 in regulating bone formation and resorption is evidenced by the development of rickets in children who lack the ability to produce 1,25(OH)2D3 (vitamin D-dependent rickets type 1 or pseudovitamin D-deficient rickets) or who lack a functioning VDR (vitamin D-dependent rickets type 2 or hereditary 1,25(OH)2D3-resistant rickets). The appearance of bone in these conditions is quite similar to that in patients with vitamin D deficiency. The former have inactivating mutations in the 1-hydroxylase gene and can be treated successfully with calcitriol (but not with vitamin D), whereas the latter are resistant to calcitriol as well as to vitamin D. These experiments of nature, however, do not exclude the possibility that other vitamin D metabolites, in combination with 1,25(OH)2D3, are essential for normal bone metabolism.
In vitamin D deficiency, all vitamin D metabolites are reduced, but 25(OH)D and 24,25(OH)2D tend to be reduced out of proportion to 1,25(OH)2D3, which may be maintained in the low normal range despite clear abnormalities in bone mineralization. Thus, it is reasonable to ask whether 1,25(OH)2D3 is the only vitamin D metabolite of consequence for the regulation of skeletal homeostasis. In fact, a number of studies point to unique actions of 24,25(OH)2D that cannot be replicated by 1,25(OH)2D3, especially in cartilage, although this remains controversial. Studies in mice in which the 24-hydroxylase gene has been inactivated (“knocked out”) support the concept that 24,25 (OH)2D has a role in bone formation that is not replaced by 1,25(OH)2D3. This is an important concept in the treatment of vitamin D deficiency, which is best done with vitamin D or 25(OH)D rather than calcitriol alone, as the former serve as precursors for both 1,25(OH)2D3 and 24,25(OH)2D.
The mechanisms by which 1,25(OH)2D3 regulates skeletal homeostasis remain uncertain. Provision of adequate calcium and phosphate for mineralization is clearly important. The rickets of patients with defective VDRs (vitamin D-dependent rickets type 2) can be cured with calcium and phosphate infusions. Similarly, in mice in which the VDR is rendered nonfunctional (“knocked out”), rickets could be prevented with diets high in calcium and phosphate. Thus, even though the osteoblast, the cell responsible for forming new bone, contains a VDR and the transcription of a number of proteins in bone is transcriptionally regulated by 1,25(OH)2D3, it is not clear how essential the direct actions of 1,25(OH)2D3 on bone are for bone formation and mineralization.
In organ cultures of bone, the best established action of 1,25(OH)2D3 is bone resorption. This is accompanied by an increase in osteoclast number and activity and decreased collagen synthesis. The increase in osteoclast activity is now known to be mediated by the production in osteoblasts of a membrane-bound protein called receptor activator of NF-κB ligand (RANKL), which acts on its receptor in osteoclasts and their precursors to stimulate osteoclast differentiation and activity. 1,25(OH)2D3 is one of several hormones (PTH and selected cytokines are others) that stimulate RANKL production. Although vitamin D deficiency is not marked by decreased bone resorption (probably due to the elevated PTH levels, which could compensate for the lack of 1,25(OH)2D3 at the level of RANKL production), the clinical evidence that the bones of patients with vitamin D deficiency are partially resistant to the actions of PTH may be explained by this mechanism.
1,25(OH)2D3 also promotes the differentiation of osteoblasts. This action is less well delineated than the promotion of osteoclast differentiation. Osteoblasts pass through a well-defined sequence of biochemical processes as they differentiate from proliferating osteoprogenitor cells to cells capable of producing and mineralizing matrix. The effect of 1,25(OH)2D3 on the osteoblast depends on the stage of differentiation at which the 1,25(OH)2D3 is administered. Early in the differentiation process (or in bone cells characterized as immature osteoblast-like cells), 1,25(OH)2D3 stimulates collagen production and alkaline phosphatase activity. These functions are inhibited in more mature osteoblasts. On the other hand, osteocalcin production is stimulated by 1,25(OH)2D3 only in mature osteoblasts. Experiments in vivo demonstrating that excessive 1,25(OH)2D3 can inhibit normal bone mineralization—leading to the paradoxical appearance of osteomalacia—can be understood in the context of these differential effects of 1,25(OH)2D3 on the osteoblast as it differentiates. Too much 1,25(OH)2D3 can disrupt the differentiation pathway.
- ACTIONS OF VITAMIN D IN KIDNEY
The kidney expresses VDR, and 1,25(OH)2D3 stimulates calbindin and Ca2+-ATPase in the distal tubule as well as 24,25(OH)2D production in the proximal tubule. However, the role of 1,25(OH)2D3 in regulating calcium and phosphate transport across the renal epithelium remains controversial. 25(OH)D may be more important than 1,25(OH)2D3 in acutely stimulating calcium and phosphate reabsorption by the kidney tubules. In vivo studies are complicated by the effect of 1,25(OH)2D3 on other hormones, particularly PTH, which appears to be more important than the vitamin D metabolites in regulating calcium and phosphate handling by the kidney.
- ACTIONS OF VITAMIN D IN OTHER TISSUES
An exciting recent discovery has been that VDRs are found in a large number of tissues beyond the classic target tissues—gut, bone, and kidney—and these tissues respond to 1,25(OH)2D3. These tissues include elements of the hematopoietic and immune systems; cardiac, skeletal, and smooth muscle; brain, liver, breast, endothelium, skin (keratinocytes, melanocytes, and fibroblasts), and endocrine glands (pituitary, parathyroid, pancreatic islets [B cells], adrenal cortex and medulla, thyroid, ovary, and testis). Furthermore, malignancies developing in these tissues often contain VDRs and respond to the antiproliferative actions of 1,25(OH)2D3.
The responses of these tissues to 1,25(OH)2D3 are as varied as the tissues themselves. 1,25(OH)2D3 regulates hormone production and secretion, including insulin from the pancreas, prolactin from the pituitary, and PTH from the parathyroid gland, just as it regulates cytokine production and secretion of interleukin-2 from lymphocytes and tumor necrosis factor from monocytes. Myocardial contractility and vascular tone are modulated by 1,25(OH)2D3, as is liver regeneration. 1,25(OH)2D3 reduces the rate of proliferation of many cell lines, including normal keratinocytes, fibroblasts, lymphocytes, and thymocytes as well as abnormal cells of mammary, skeletal, intestinal, lymphatic, and myeloid origin. Differentiation of numerous normal cell types, including keratinocytes, lymphocytes, hematopoietic cells, intestinal epithelial cells, osteoblasts, and osteoclasts as well as abnormal cells of the same lineage is enhanced by 1,25(OH)2D3. Thus, the potential for manipulating a vast array of physiologic and pathologic processes with calcitriol and its analogs is enormous. This promise is starting to be realized in that vitamin D analogs are being used to treat psoriasis, uremic hyperparathyroidism, and osteoporosis. Trials of vitamin D analogs in the treatment of a variety of cancers are also being conducted. Thus, although regulation of bone mineral homeostasis remains the major physiologic function of vitamin D, clinical applications for these compounds are being found outside the classic target tissues.
INTEGRATED CONTROL OF MINERAL HOMEOSTASIS
Consider a person who switches from a high normal to a low normal intake of calcium and phosphate—from 1200 per day to 300 mg per day of calcium (the equivalent of leaving three glasses of milk out of the daily diet). The net absorption of calcium falls sharply, causing a transient decrease in the serum calcium level. The homeostatic response to this transient hypocalcemia is led by an increase in PTH, which stimulates the release of calcium and phosphate from bone and the retention of calcium by the kidney. The phosphaturic effect of PTH allows elimination of phosphate, which is resorbed from bone together with calcium. In addition, the increase in PTH, along with the fall in serum calcium and serum phosphorus, activates renal 1,25 (OH)2D3 synthesis. In its turn, 1,25(OH)2D3 increases the fractional absorption of calcium and further increases bone resorption. External calcium balance is thus restored by increased fractional absorption of calcium and increased bone resorption at the expense of increased steady state levels of PTH and 1,25(OH)2D3.
MEDULLARY THYROID CARCINOMA
Medullary carcinoma, a neoplasm of thyroidal C cells, accounts for 5–10% of all thyroid malignancies. Approximately 75% of medullary carcinomas are sporadic. The remainder are familial and associated with one of three heritable syndromes: familial isolated medullary carcinoma; multiple endocrine neoplasia 2A (MEN 2A), consisting of medullary carcinoma, pheochromocytoma, and primary hyperparathyroidism; or multiple endocrine neoplasia 2B (MEN 2B), consisting of medullary carcinoma, pheochromocytoma, multiple mucosal neuromas, and, rarely, primary hyperparathyroidism (Table 8-4). The MEN syndromes are more extensively discussed in Chapter 22.
Understanding of the pathogenesis of medullary carcinoma has been greatly enhanced by the identification of causative mutations in the RETproto-oncogene located on chromosome 10q11.2. The RET gene encodes a membrane tyrosine kinase receptor for whose ligands are in the glial cell line-derived neurotrophic factor (GDNF) family. This receptor is expressed developmentally in migrating neural crest cells that will give rise to hormone-secreting neuroendocrine cells (eg, C cells and adrenal medullary cells) and to the parasympathetic and sympathetic ganglia of the peripheral nervous system. Remarkably, different mutations in RET can produce five distinct diseases. Inheritance of certain activating mutations is responsible for MEN 2A and familial medullary thyroid carcinoma. Inheritance of a different set of activating mutations causes MEN 2B. In over half of sporadic medullary thyroid carcinomas, the tumor has a clonal somatic mutation (present in the tumor but not in genomic DNA), which is identical to one of the mutations that is responsible for the familial forms of medullary carcinoma. Almost certainly, such somatic mutations cause sporadic medullary carcinoma.
In addition to its role in medullary carcinoma, where the RET gene product is activated by point mutations, the RET gene is often rearranged in papillary carcinoma of the thyroid, giving rise to RET chimeric genes. Transgenic experiments indicate that the rearranged RET gene is sufficient to cause papillary thyroid carcinoma in mice. Finally, mutations that inactivate the RET gene produce Hirschsprung's disease, a congenital absence of the enteric parasympathetic ganglia, in which intestinal motility is disturbed, resulting in megacolon.
Table 8-4. Clinical features of multiple endocrine neoplasia syndromes.
Medullary carcinoma is usually located in the middle or upper portions of the thyroid lobes. It is typically unilateral in sporadic cases but often multicentric and bilateral in familial forms of medullary carcinoma. Pathologically, medullary thyroid carcinoma was originally distinguished from other thyroid cancers by the presence of amyloid, eosinophilic material which stains with Congo red. Molecular studies have shown that amyloid consists of dense fibrillar deposits of protein in a β-pleated sheet structure. In the case of medullary carcinoma, the protein deposited as amyloid is procalcitonin or calcitonin itself. Thus, the pathologic diagnosis of medullary carcinoma can now be made by immunohistochemical staining for calcitonin.
The natural history of medullary carcinoma is variable. Sporadic tumors may be quite aggressive or very indolent; the mean five-year survival rate is about 50%. The behavior of familial forms varies among syndromes. MEN 2B has the most aggressive form of medullary carcinoma, with a 2-year survival of about 50%; MEN 2A has a course similar to that of sporadic medullary carcinoma, and familial medullary carcinoma has the most indolent course of all. The tumor may spread to regional lymph nodes or undergo hematogenous spread to the lungs and other viscera. When metastatic, medullary thyroid carcinoma is sometimes associated with a chronic diarrhea syndrome. The pathogenesis of the diarrhea is unclear. In addition to calcitonin, these tumors secrete a variety of other bioactive products, including prostaglandins, serotonin, histaminase, and peptide hormones (ACTH, somatostatin, CRH). In some cases the associated diarrhea responds dramatically to treatment with long-acting somatostatin analogs such as octreotide, which block secretion of these bioactive products.
Calcitonin is a tumor marker for medullary thyroid carcinoma. It is most sensitive for this purpose when secretion is stimulated with provocative agents. The standard provocative tests used pentagastrin (0.5 ľg/kg intravenously over 5 seconds) or a rapid infusion of calcium gluconate (2 mg calcium/kg over 1 minute). Blood samples are obtained at baseline and 1, 2, and 5 minutes after the stimulus. For maximal sensitivity both tests are usually combined, with the calcium infusion immediately followed by administration of pentagastrin. Although basal calcitonin levels are often normal in early tumors, calcitonin levels may be many times higher than normal in patients with disseminated medullary carcinoma. Despite this, the patients are uniformly normocalcemic. Although tumors secrete larger-MW forms of calcitonin with decreased biologic activity, monomeric calcitonin levels are often high as well. RET oncogene analysis has replaced provocative testing in most cases, though basal calcitonin levels can still be used to follow disease activity.
Members of families that carry RET mutations must be individually screened for medullary thyroid carcinoma and for the associated tumors which occur in MEN 2A and MEN 2B (Chapter 22). In the case of medullary thyroid carcinoma, the presence of the RET mutation in an individual will lead generally to the recommendation of a total thyroidectomy prior to the development of frank malignancy or abnormal calcitonin levels. It is now recommended in a kindred with a known RET mutation that children be screened at birth using genomic DNA. Early thyroidectomy can be performed in carriers of the trait, and further testing can be discontinued in genetically normal family members. The timing of prophylactic thyroidectomy in asymptomatic carriers of RET mutations, however, remains uncertain. Most experts favor surgery in early childhood. Because a limited number of mutations in RET cause
over 95% of hereditary medullary thyroid carcinomas and up to 25% of sporadic disease, it is possible to screen many patients through commercial reference laboratories.
Apparently sporadic medullary thyroid carcinoma also calls for family studies, since up to 25% of new cases may actually be probands of families who harbor one of the familial syndromes. As in known familial cases, screening can be accomplished by provocative testing of the calcitonin response in first-degree relatives or by testing tumor and genomic DNA from the patient for RET mutations. Identification of a mutation that is present only in tumor tissue would establish the mutation as somatic and the tumor as a sporadic one. Identification of the same RET mutation in tumor and genomic DNA would make the diagnosis of a familial form of the disorder and would mandate careful screening of the family.
A number of symptoms and signs accompany the hypercalcemic state: central nervous system effects such as lethargy, depression, psychosis, ataxia, stupor, and coma; neuromuscular effects such as weakness, proximal myopathy, and hypertonia; cardiovascular effects such as hypertension, bradycardia (and eventually asystole), and a shortened QT interval; renal effects such as stones, decreased glomerular filtration, polyuria, hyperchloremic acidosis, and nephrocalcinosis; gastrointestinal effects such as nausea, vomiting, constipation, and anorexia; eye findings such as band keratopathy; and systemic metastatic calcification. This constellation of clinical findings has led to the mnemonic for recalling the signs and symptoms of hypercalcemia: “stones, bones, abdominal groans, and psychic moans” (Figure 8-11).
Although many disorders are associated with hypercalcemia (Table 8-5), they can produce hypercalcemia through only a limited number of mechanisms: (1) increased bone resorption, (2) increased gastrointestinal absorption of calcium, or (3) decreased renal excretion of calcium. While any of these mechanisms can be involved in a given patient, the common feature of virtually all hypercalcemic disorders is accelerated bone resorption. The only recognized hypercalcemic disorder in which bone resorption does not play a part is the milk-alkali syndrome.
Figure 8-11. Signs and symptoms of primary hyperparathyroidism.
The central feature of the defense against hypercalcemia is suppression of PTH secretion. This reduces bone resorption, reduces renal production of 1,25(OH)2D3 and thereby inhibits calcium absorption, and increases urinary calcium losses. The kidney plays a key role in the adaptive response to hypercalcemia as the only route of net calcium elimination, and the level of renal calcium excretion is markedly increased by the combined effects of an increased filtered load of calcium and the suppression of PTH. However, the patient who relies on the kidneys to excrete an increased calcium load is in precarious balance: Glomerular filtration is impaired by hypercalcemia; the urinary concentrating ability is diminished, predisposing to dehydration; poor mentation may interfere with access to fluids; and nausea and vomiting may further predispose to dehydration and renal azotemia. Renal insufficiency in turn compromises calcium clearance, leading to a downward spiral (Figure 8-12). Thus, once established, many hypercalcemic states are self-perpetuating or aggravated through the “vicious cycle of hypercalcemia.” The only alternative to the renal route for elimination of calcium from the extracellular fluid is deposition as calcium phosphate and other salts in bone and soft tissues. Soft tissue calcification is observed with massive
calcium loads, with massive phosphate loads (as in crush injuries and compartment syndromes), and when renal function is markedly impaired.
Table 8-5. Causes of hypercalcemia.
The differential diagnosis is set forth in Table 8-5. As a practical matter, the categories can be divided into primary hyperparathyroidism and everything else. Hyperparathyroidism is by far the commonest cause of hypercalcemia and has distinctive pathophysiologic features. Thus, the first step in the differential diagnosis is determination of PTH, using an assay for intact PTH (Figure 8-13). If the PTH level is high and thus inappropriate for hypercalcemia, little further workup is required except to consider the variant forms of hyperparathyroidism which are discussed below. If the PTH level is suppressed, then a search for other entities must be conducted. Most other entities in Table 8-5 are readily diagnosed by their distinctive features, as discussed below.
Figure 8-12. Once established, hypercalcemia can be maintained or aggravated as depicted. (Reproduced, with permission, from Felig P, Baxter JD, Frohman LA [editors]: Endocrinology and Metabolism, 3rd ed. McGraw-Hill, 1995.)
DISORDERS CAUSING HYPERCALCEMIA
- Primary Hyperparathyroidism
Primary hyperparathyroidism is a hypercalcemic disorder that results from excessive secretion of PTH. With the advent of multiphasic screening of serum chemistries, we have come to recognize that primary hyperparathyroidism is a common and usually asymptomatic disorder. Its incidence is approximately 42 per 100,000, and its prevalence is up to 4 per 1000 in women over age 60. Primary hyperparathyroidism is approximately two to three times as common in women as in men.
Etiology & Pathogenesis
Primary hyperparathyroidism is caused by a single parathyroid adenoma in about 80% of cases and by primary hyperplasia of the parathyroids in about 15%. Parathyroid carcinoma is a rare cause of hyperparathyroidism, accounting for 1–2% of cases. Parathyroid carcinoma is often recognizable preoperatively because it presents with severe hypercalcemia or a palpable neck mass. Primary hyperparathyroidism can occur as part of at least three different familial endocrinopathies. All of them are autosomal dominant traits causing four-gland parathyroid hyperplasia. They include MEN 1, MEN 2A, and isolated familial hyperparathyroidism.
Thyroid adenomas have a clonal origin, indicating that they can be traced back to an oncogenic mutation in a single progenitor cell. A few of these mutations are identified or can tentatively be assigned a genomic locus. About 25% of sporadic parathyroid adenomas
have chromosomal deletions involving chromosome 11q12–13 that are thought to eliminate the putative tumor suppressor gene MENIN. As reviewed below and in Chapter 22, loss of the functioning menin protein, a tumor suppressor gene, at this locus is the cause of parathyroid, pituitary, and pancreatic tumors in the MEN 1 syndrome. An additional 40% of parathyroid adenomas display allelic loss on chromosome 1p (1p32pter).
Figure 8-13. Clinical utility of immunoradiometric assay for intact PTH. (Reproduced, with permission, from Endres DB et al: Measurement of parathyroid hormone. Endocrinol Metab Clin North Am 1989;18:611.)
Another important locus on chromosome 11 that has been implicated in approximately 4% of sporadic parathyroid adenomas involves thePRAD1 oncogene. In the initial elucidation of this pathogenetic mechanism, a chromosomal rearrangement was found. Inversion of a piece of chromosome 11 brings the PRAD1 cell cycle regulatory gene under the control of the PTH gene promoter. This rearrangement results in marked overexpression of PRAD1 in the parathyroid. The PRAD1 gene encodes a cell cycle regulatory protein, cyclin D, which is normally expressed at high levels in the G1 phase of the cell cycle and permits entry of cells into the mitotic phase of the cycle. Thus, a parathyroid-specific disorder of cell cycle regulation leads to abnormal cell proliferation and ultimately excessive PTH production.
Parathyroid hyperplasia occurs spontaneously, accounting for 12–15% of cases of primary hyperparathyroidism
and as part of three forms of familial hyperparathyroidism: MEN 1, MEN 2A, and isolated familial hyperparathyroidism. Parathyroid hyperplasia was traditionally viewed as an example of true hyperplasia, a polyclonal expansion of cell number. This occurs in other endocrine tissues when a trophic hormone is present in excess, eg, ACTH excess produces bilateral adrenal hyperplasia. Molecular analysis, however, has revised this view. MEN 1 is due to the inherited absence of one allele of the menin gene, which is a tumor suppressor gene. Somatic mutations in the MENIN gene that result in loss of the other allele's function produce tumors in endocrine tissues where the gene is expressed. In this view, multicentric somatic mutations would account for the occurrence of four-gland hyperparathyroidism. In MEN 2, the occurrence of parathyroid hyperplasia is presumably a consequence of expression of activating mutations of the RET gene in the four glands. Surprisingly, it has recently been shown that the majority of glands in spontaneous parathyroid hyperplasia are monoclonal, implying that they arose from a single progenitor cell, presumably as the result of a somatic mutation. Perhaps there is a parathyroid trophic hormone, eg, the RET receptor ligand, in some cases of parathyroid hyperplasia, and the increased mitotic rate in hyperplastic glands predisposes to clonal oncogenic mutations.
Parathyroid carcinomas frequently display loss of the retinoblastoma tumor-suppressor gene RB, another cell cycle regulator. Certain parathyroid carcinomas show loss of another tumor-suppressor, the P53 gene. The P53 and RB mutations do not commonly occur in parathyroid adenomas, whereas loss of one or both of these tumor suppressors is prevalent in many other kinds of carcinoma. It is thus likely that these abnormalities in parathyroid carcinoma account for its aggressiveness.
Several candidate parathyroid oncogenes have been eliminated by molecular analysis. Familial benign hypocalciuric hypocalcemia is caused typically by a germline inactivating mutation of one allele of the parathyroid calcium sensor. Somatic mutations of the parathyroid calcium sensor could theoretically produce isolated primary hyperparathyroidism, but such mutations appear to be very infrequent in sporadic hyperparathyroidism. Similarly, mutations in RET that cause MEN 2A are quite uncommon in sporadic parathyroid tumors.
- SYMPTOMS AND SIGNS
The typical clinical presentation of primary hyperparathyroidism has evolved considerably over the past two decades. As the disease is detected increasingly by multiphasic screening that includes determination of serum calcium levels, there has been a marked reduction in the frequency of the classic signs and symptoms of primary hyperparathyroidism, renal disease—renal stones, decreased renal function, and occasionally nephrocalcinosis—and the classic hyperparathyroid bone disease osteitis fibrosa cystica. In fact, about 85% of patients presenting today have neither bony nor renal manifestations of hyperparathyroidism and are regarded as asymptomatic or minimally symptomatic. At the same time, we have begun to recognize more subtle manifestations of hyperparathyroidism in some patients. This has presented a number of questions about the role of parathyroid surgery in primary hyperparathyroidism, which are discussed below (see Treatment).
- Hyperparathyroid bone disease—The classic bone disease of hyperparathyroidism is osteitis fibrosa cystica. Formerly common, this disorder now occurs in less than 10% of patients. Clinically, osteitis fibrosa cystica causes bone pain and sometimes pathologic fractures. The most common laboratory finding is an elevation of the alkaline phosphatase level, reflecting high bone turnover. Histologically, there is an increase in the number of bone-resorbing osteoclasts, marrow fibrosis, and cystic lesions that may contain fibrous tissue (brown tumors) or cyst fluid. The most sensitive and specific radiologic finding of osteitis fibrosa cystica is subperiosteal resorption of cortical bone, best seen in high-resolution films of the phalanges (Figure 8-14A). A similar process in the skull leads to a salt-and-pepper appearance (Figure 8-14B). Bone cysts or brown tumors may be evident as osteolytic lesions. Dental films may disclose loss of the lamina dura of the teeth, but this is a nonspecific finding also seen in periodontal disease.
The other important consequence of hyperparathyroidism is osteoporosis. Unlike other osteoporotic disorders, hyperparathyroidism typically results in predominant loss of cortical bone (Figure 8-15). In general, both the mass and the mechanical strength of trabecular bone are relatively well maintained. Patients who are followed medically for mild primary hyperparathyroidism often do not experience progressive bone loss even when they are osteoporotic at diagnosis. This may be due to the fact that PTH under certain circumstances has anabolic effects on the skeleton to increase bone mass. Although osteoporosis is generally considered to be an indication for surgical treatment of primary hyperparathyroidism, its impact on morbidity is hard to assess (see Treatment of Hypercalcemia).
- Hyperparathyroid kidney disease—Once common in primary hyperparathyroidism, kidney stones now occur in less than 15% of cases. These are usually
calcium oxalate stones. From the perspective of a stone clinic, only about 7% of calcium stone formers will prove to have primary hyperparathyroidism. They are difficult to manage medically, and stones constitute one of the agreed indications for parathyroidectomy. Clinically evident nephrocalcinosis rarely occurs, but a gradual loss of renal function is not uncommon. Renal function is stabilized after a successful parathyroidectomy, and otherwise unexplained renal insufficiency in the setting of primary hyperparathyroidism is also considered to be an indication for surgery because of the risk of progression. Chronic hypercalcemia can also compromise the renal concentrating ability, giving rise to polydipsia and polyuria.
Figure 8-14. A: Magnified x-ray of index finger on fine-grain industrial film showing classic subperiosteal resorption in a patient with severe primary hyperparathyroidism. Note the left (radial) surface of the distal phalanx, where the cortex is almost completely resorbed, leaving only fine wisps of cortical bone. B: Skull x-ray from a patient with severe secondary hyperparathyroidism due to end-stage renal disease. Extensive areas of demineralization alternate with areas of increased bone density, resulting in the “salt and pepper” skull x-ray. (Both films courtesy of H Genant.)
Figure 8-15. Bone density at several sites in primary hyperparathyroidism. (Reproduced, with permission, from Silverberg SJ, Shane E, Jacobs TP et al: Nephrolithiasis and bone involvement in primary hyperparathyroidism. Am J Med 1990;89:327.)
- 3. Nonspecific features of primary hyperparathyroidism—Although stupor and coma occur in severe hypercalcemia, the degree to which milder impairments of central nervous system function affect the typical patient with primary hyperparathyroidism is unclear. Lethargy, fatigue, depression, difficulty in concentrating,
and personality changes occur and in some patients appear to benefit from parathyroidectomy. Frank psychosis will also respond to surgery on occasion. Muscle weakness with characteristic electromyographic changes is also seen, and there is good evidence from controlled clinical trials that surgery can improve muscle strength. It was formerly thought that the incidence of hypertension was increased in primary hyperparathyroidism, but more recent evidence suggests that the appreciable incidence in this patient group is probably no greater than in age-matched controls, and parathyroidectomy appears to be of no benefit. Dyspepsia, nausea, and constipation all occur, probably as a consequence of hypercalcemia, but there is probably no increase in the incidence of peptic ulcer disease. The articular manifestations of primary hyperparathyroidism are several. Chondrocalcinosis occurs in up to 5% of patients, but acute attacks of pseudogout are less frequent.
- LABORATORY FINDINGS
Hypercalcemia is virtually universal in primary hyperparathyroidism, though the serum calcium sometimes fluctuates into the upper normal range. In patients with subtle hyperparathyroidism, repeated serum calcium measurements over a period of time may be required to establish the pattern of intermittent hypercalcemia. Both total and ionized calcium are elevated, and in most clinical instances there is no advantage to measuring the ionized calcium level. Patients with normocalcemic primary hyperparathyroidism, in whom the possibility of subtle forms of vitamin D deficiency has been eliminated, are being recognized more frequently. In patients with primary hyperparathyroidism, the serum phosphorus level is low-normal (< 3.5 mg/dL) or low (< 2.5 mg/dL) because of the phosphaturic effect of PTH. A mild hyperchloremic metabolic acidosis may be manifest as hyperchloremia.
The diagnosis of primary hyperparathyroidism in a hypercalcemic patient can be made by determining the intact PTH level with a two-site assay. As shown in Figure 8-13, an elevated or even upper-normal level of PTH is clearly inappropriate in a hypercalcemic patient and establishes the diagnosis of hyperparathyroidism (or one of its variants—familial benign hypercalciuric hypercalcemia or lithium-induced hypercalcemia). The reliability of the two-site intact PTH assay allows the diagnosis of primary hyperparathyroidism to be definitive. In a patient with a high PTH level, there is no need to screen for metastatic malignancy, sarcoidosis, etc. Determinations of renal function and a plain abdominal radiograph for renal stones are often obtained for prognostic reasons. A determination of urinary calcium concentration and urinary creatinine excretion should be obtained to exclude familial benign hypocalciuric hypercalcemia.
The definitive treatment of primary hyperparathyroidism is parathyroidectomy. The surgical strategy depends on the ability of localizing studies such as sestamibi scanning to identify one clearly abnormal gland and the availability of intraoperative PTH determinations to verify that the disease-producing lesion has been removed during surgery. If multiple enlarged glands are suspected, the likely diagnosis is parathyroid hyperplasia or double adenoma. In patients with hyperplasia, the preferred operation is a 3 1/2-gland parathyroidectomy, leaving a remnant sufficient to prevent hypocalcemia. Double parathyroid adenomas are both removed in affected patients. The pathologist is of little help in distinguishing among normal tissue, parathyroid adenoma, and parathyroid hyperplasia: these in essence are surgical diagnoses, based on the size and appearance of the glands. The recurrence rate of hypercalcemia is high in patients who have parathyroid hyperplasia—particularly in those with one of the MEN syndromes, because of the inherited propensity for tumor growth. In such cases, the parathyroid remnant can be removed from the neck and implanted in pieces in forearm muscles to allow for easy subsequent removal of some additional parathyroid tissue if hypercalcemia recurs.
In competent hands, the cure rate of parathyroid adenoma is over 95%. The success rate in primary parathyroid hyperplasia is somewhat lower, because of missed glands and recurrent hyperparathyroidism in patients with MEN syndromes. There is a 20% incidence of persistent or recurrent hypercalcemia. However, parathyroidectomy is difficult surgery: the normal parathyroid gland weighs only about 40 mg and may be located throughout the neck or upper mediastinum. It is mandatory not only to locate a parathyroid adenoma but also to find the other gland or glands and determine whether they are normal. Complications of surgery include damage to the recurrent laryngeal nerve, which passes close to the posterior thyroid capsule, and inadvertent removal or devitalization of all parathyroid tissue, producing permanent hypoparathyroidism. In skilled hands, the incidence of these complications is less than 1%. It is critical that parathyroid surgery be performed by someone with specialized skill and experience. (See Chapter 26.)
Localization studies of the parathyroid glands in patients with primary hyperparathyroidism and intraoperative PTH testing are critical components of the contemporary management of patients presenting for initial surgical procedures. If these studies clearly indicate a single abnormal gland and if intraoperative PTH testing is available, surgical management now typically consists of unilateral exploration and limited parathyroidectomy.
Localization studies continue to be essential in the management of patients with recurrent or persistent hyperparathyroidism. The most successful procedures are 99mTc-sestamibi scanning, computed tomography, MRI, and ultrasound. Individually, each has a sensitivity of 60–80% in experienced hands. Used in combination, they are successful in at least 80% of reoperated cases. Invasive studies, such as angiography and venous sampling, are rarely performed.
There is no definitive medical therapy for hyperparathyroidism. In postmenopausal women, estrogen replacement therapy in high doses (1.25 mg of conjugated estrogens or 30–50 ľg of ethinyl estradiol) will produce an average decrease of 0.5–1 mg/dL in the serum calcium and an increase in bone mineral density. The effect of estrogen treatment is on the bone response to PTH. PTH levels do not fall. Limited data are available on bisphosphonates and selective estrogen response modulators. Calcimimetic agents that activate the parathyroid calcium-sensing receptor, currently in clinical trials, may offer an alternative to surgery in the future.
The relatively asymptomatic state of most patients today presents a dilemma: Which of them should be subjected to surgery? To answer this question definitively, it would be necessary to know more than we presently do about the natural history of untreated primary hyperparathyroidism. However, in observational studies over as many as 10 years, it is clear that most patients are stable with regard to serum calcium, stone disease, and renal function. Recent data also indicate that osteoporosis, when present, is usually nonprogressive (Figure 8-16). On the other hand, surgery is usually curative. In experienced hands, surgery has a low morbidity rate. Although parathyroid surgery has a substantial initial cost, over the long term the cost-benefit
ratio may be favorable when compared with a lifetime of medical follow-up. Moreover, there is a marked improvement in bone density after surgery (Figure 8-16), with sustained increases over at least 5 years postoperatively. These changes remain stable over years. Some surgeons also argue that the nonspecific symptoms often present may improve with surgery, but no controlled trial has excluded the placebo effect of surgery as a possible explanation for these results.
A 1990 NIH Consensus Development Conference considered the issue of surgery in primary hyperparathyroidism and arrived at the following recommendations: Surgery should be recommended (1) if the serum calcium is markedly elevated (above 11.4–12 mg/dL [2.8–3 mmol/L]); (2) if there has been a previous episode of life-threatening hypercalcemia; (3) if the creatinine clearance is reduced below 70% of normal; (4) if a kidney stone is present; (5) if the urinary calcium is markedly elevated (> 400 mg/24 h); (6) if bone mass is substantially reduced (less than 2 SD below normal for age, sex, and race, ie, two Z-scores below normal); or (7) if the patient is young (under 50 years of age, particularly premenopausal women). In addition, medical surveillance is not considered suitable for patients who request surgery, patients who are unlikely to comply with long-term follow-up schedules, or patients with a coexisting illness complicating their management. These recommendations remain provisional and are currently being revised in consideration of minimally invasive parathyroid surgery and the beneficial skeletal effects of parathyroidectomy in patients with mild disease.
Variants of Primary Hyperparathyroidism
- FAMILIAL BENIGN HYPOCALCIURIC HYPERCALCEMIA (FBHH)
Inherited as an autosomal dominant trait, this disorder is responsible for lifelong asymptomatic hypercalcemia, first detectable in cord blood. Hypercalcemia is usually mild (10.5–12 mg/dL [2.7–3 mmol/L]) and is often accompanied by mild hypophosphatemia and hypermagnesemia. The PTH level is normal or slightly elevated, indicating that this is a PTH-dependent form of hypercalcemia. The parathyroid glands are normal in size or slightly enlarged. The most notable laboratory feature of the disorder is hypocalciuria. The urinary calcium level is usually less than 50 mg/24 h, and the calcium/creatinine clearance ratio is less than 0.01 and calculated as follows:
Hypocalciuria is an intrinsic renal trait, as it persists even in patients who have undergone total parathyroidectomy.
Since familial benign hypocalciuric hypercalcemia is an asymptomatic trait, the most important role of diagnosis is to distinguish it from primary hyperparathyroidism and avoid an unnecessary parathyroidectomy. If subtotal parathyroidectomy is performed, the serum calcium invariably returns shortly to preoperative levels; these persons resist attempts to lower the serum calcium. Unfortunate patients with this variant who have undergone total parathyroidectomy are rendered hypoparathyroid and are dependent on calcium and vitamin D.
The diagnosis must be considered in patients with asymptomatic mild hypercalcemia who are relatively hypocalciuric. However, an unequivocal diagnosis cannot be made biochemically because the serum and urinary calcium and PTH levels all overlap with typical primary hyperparathyroidism. Family studies are necessary to make the diagnosis. The penetrance of the phenotype is 100%, and affected family members are hypercalcemic throughout life, so if the proband has the disorder, each first-degree relative who is screened will have a 50% chance of being hypercalcemic.
Most cases of familial benign hypocalciuric hypercalcemia are caused by loss-of-function mutations in the parathyroid calcium receptor. Loss of one functional receptor allele shifts the set-point for inhibition of PTH release to the right, producing hypercalcemia. The same receptor is expressed in the kidney, where it regulates renal calcium excretion. A variety of point mutations in several exons of the calcium receptor produce the familial benign hypocalciuric hypercalcemia phenotype. Thus, simple molecular genetic testing is impractical for diagnosis at present.
Children of two parents with the disorder may inherit a mutant allele from each parent, producing neonatal severe hypercalcemia, a life-threatening disorder in which failure to sense extracellular calcium causes severe hyperparathyroidism, requiring total parathyroidectomy soon after birth.
- MEN SYNDROMES
As noted above, primary hyperparathyroidism is a feature of both MEN 1 and MEN 2A. The penetrance of primary hyperparathyroidism in MEN 1 is over 90% by age 40. Patients with the MEN 1 syndrome are thought to inherit a loss-of-function germline mutation of the tumor suppressor gene MENIN on chromosome 11q12–13. Menin is a nuclear protein which appears to interact with JunD, a member of the AP1 family of transcription factors. A gene deletion during mitosis of a parathyroid cell that resulted in loss of the remaining allele in that cell would abrogate the cell's growth control
mechanism and permit clonal expansion of its progeny to generate the parathyroid tumor. A similar mechanism appears also to operate in a small fraction of sporadic parathyroid adenomas with 11q12–13 deletions.
The penetrance of primary hyperparathyroidism in MEN 2A is about 30%. As discussed in the section on medullary thyroid carcinoma, the disorder is caused by activating mutations in the RET gene, a tyrosine kinase growth factor receptor. Evidently, the RET gene product is less important for growth of the parathyroids than for thyroid C cells, since the penetrance of primary hyperparathyroidism is fairly low and since in MEN 2B a separate class of activating RET mutations produces medullary carcinoma and pheochromocytoma and very rarely primary hyperparathyroidism. The treatment for parathyroid hyperplasia in MEN 1 or MEN 2 is subtotal parathyroidectomy. The recurrence rate is higher than in sporadic parathyroid hyperplasia and may approach 50% in MEN 1.
- LITHIUM THERAPY
Both in patients and in isolated parathyroid cells, exposure to extracellular lithium shifts the set-point for inhibition of PTH secretion to the right. Clinically, this results in hypercalcemia and a detectable or elevated level of PTH. Lithium treatment also produces hypocalciuria and is thus a virtual phenocopy of familial benign hypocalciuric hypercalcemia. Most patients with therapeutic lithium levels for bipolar affective disorder will have a slight increase in the serum calcium level, and up to 10% become mildly hypercalcemic, with PTH levels that are high normal or slightly elevated. Lithium treatment can also unmask underlying primary hyperparathyroidism. It is difficult to diagnose primary hyperparathyroidism in a lithium-treated patient, particularly when temporary cessation of lithium therapy is deemed dangerous. However, the likelihood of underlying primary hyperparathyroidism is high when the serum calcium is greater than 11.5 mg/dL, and the decision to undertake surgery must be based on such clinical criteria. Unfortunately, surgical cure of hyperparathyroidism rarely ameliorates the underlying psychiatric condition.
- Malignancy-Associated Hypercalcemia
Malignancy-associated hypercalcemia is the second most common form of hypercalcemia, with an incidence of 15 cases per 100,000 per year—about one-half the incidence of primary hyperparathyroidism. It is, however, much less prevalent than primary hyperparathyroidism, because most patients have a very limited survival. Nonetheless, malignancy-associated hypercalcemia is the commonest cause of hypercalcemia in hospitalized patients. The clinical features and pathogenesis of malignancy-associated hypercalcemia are presented inChapter 23. The treatment of nonparathyroid hypercalcemia is presented below.
- Sarcoidosis & Other Granulomatous Disorders
Hypercalcemia is seen in up to 10% of subjects with sarcoidosis. A higher percentage have hypercalciuria. This is due to inappropriately elevated 1,25(OH)2D3 levels. Provocative testing with vitamin D or 25(OH)D increases serum or urine calcium into the abnormal range in an even larger percentage of subjects, demonstrating that most patients with sarcoidosis have abnormal vitamin D metabolism. Lymphoid tissue and pulmonary macrophages from affected individuals contain 25(OH)D 1-hydroxylase activity which is not seen in normal individuals. 1-Hydroxylase activity in these cells is not readily inhibited by calcium or 1,25(OH)2D3, indicating a lack of feedback-inhibition. This makes these subjects vulnerable to hypercalcemia or hypercalciuria during periods of increased vitamin D production (eg, summertime with increased sunlight exposure). In contrast, gamma interferon stimulates 1-hydroxylase activity in these cells, which makes such subjects more vulnerable to altered calcium homeostasis when their disease is active. Glucocorticoids, on the other hand, suppress the 1-hydroxylase activity, and this provides effective treatment for both the disease and this complication of it. The 1-hydroxylase enzyme, responsible for the overproduction of 1,25(OH)2D3 in sarcoidosis, is thought to be the same as that in the kidney. Thus, it is unclear what the basis is for differences in regulation of its activity in sarcoid tissue compared with kidney.
Other granulomatous diseases are associated with abnormal vitamin D metabolism resulting in hypercalcemia and or hypercalciuria. These disorders include tuberculosis, berylliosis, disseminated coccidioidomycosis, histoplasmosis, leprosy, and pulmonary eosinophilic granulomatosis. Furthermore, a substantial number of subjects with Hodgkin's or non-Hodgkin's lymphomas develop hypercalcemia associated with inappropriately elevated 1,25(OH)2D3 levels. Although most such patients are normocalcemic on presentation, they may be hypercalciuric, and this should be evaluated as part of the workup. Furthermore, hypercalcemia and hypercalciuria may not become apparent until situations such as increased sunlight exposure or vitamin D and calcium ingestion are experienced. Thus, one should remain alert to this complication even when the initial evaluation of serum calcium is within normal limits.
Mild hypercalcemia is found in about 10% of patients with thyrotoxicosis. The PTH is suppressed and the serum phosphorus is in the upper normal range. The serum alkaline phosphatase and urinary hydroxyproline may be mildly increased. Significant hypercalcemia is seen only in patients with severe thyrotoxicosis, particularly if they are temporarily immobilized. Thyroid hormone has direct bone-resorbing properties causing a high-turnover state, which often eventually progresses to mild osteoporosis.
Hypercalcemia can be a feature of acute adrenal crisis and responds rapidly to glucocorticoid therapy. Animal studies suggest that hemoconcentration is a critical factor. In experimental adrenal insufficiency, [Ca2+] is normal.
- Endocrine Tumors
Hypercalcemia in patients with pheochromocytoma is most often a manifestation of the MEN 2A syndrome, but hypercalcemia is found occasionally in uncomplicated pheochromocytoma, where it appears to result from secretion of PTHrP by the tumor. About 40% of tumors secreting vasoactive intestinal peptide (VIPomas) are associated with hypercalcemia. The cause is unknown. It is known, however, that at high levels VIP may activate the PTH/PTHrP receptor.
- Thiazide Diuretics
The administration of thiazides and related diuretics such as chlorthalidone, metolazone, and indapamide can produce an increase in the serum calcium which is not fully accounted for by hemoconcentration. Hypercalcemia is mild and usually transient, lasting for days or weeks, but occasionally it persists. Thiazide administration can also exacerbate the effects of underlying primary hyperparathyroidism; in fact, thiazide administration was formerly used as a provocative test for hyperparathyroidism in patients with borderline hypercalcemia. Most patients with persistent hypercalcemia while receiving thiazides will prove to have primary hyperparathyroidism.
- Vitamin D & Vitamin A
Hypercalcemia may occur in individuals ingesting large doses of vitamin D either therapeutically or accidentally (eg, irregularities in milk product supplementation with vitamin D have been reported). The initial signs and symptoms of vitamin D intoxication include weakness, lethargy, headaches, nausea, and polyuria and are attributable to the hypercalcemia and hypercalciuria. Ectopic calcification may occur, particularly in the kidneys, resulting in nephrolithiasis or nephrocalcinosis; other sites include blood vessels, heart, lungs, and skin. Infants appear to be quite susceptible to vitamin D intoxication and may develop disseminated atherosclerosis, supravalvular aortic stenosis, and renal acidosis.
Hypervitaminosis D is readily diagnosed by the very high serum levels of 25(OH)D, because the conversion of vitamin D to 25(OH)D is not tightly regulated. In contrast, 1,25(OH)2D3 levels are often normal, but not suppressed. This reflects the expected feedback regulation of 1,25(OH)2D3 production by the elevated calcium and reduced PTH levels. Levels of free 1,25(OH)2D3 when measured have been found to be increased. This is in part caused by the high levels of 25(OH)D that displace 1,25(OH)2D3 from DBP, raising the ratio of free:total 1,25(OH)2D3. The elevated free concentration of 1,25(OH)2D3, plus the intrinsic biologic effects of the elevated 25(OH)D concentration, combine to increase intestinal calcium absorption and bone resorption. The hypercalciuria, which is invariably seen, may lead to dehydration and coma as a result of hyposthenuria, prerenal azotemia, and worsening hypercalcemia.
The dose of vitamin D required to induce toxicity varies among patients, reflecting differences in absorption, storage, and subsequent metabolism of the vitamin as well as in target tissue response to the active metabolites. For example, an elderly patient is likely to have reduced intestinal calcium transport and renal production of 1,25(OH)2D3. Such an individual may be able to tolerate 50,000–100,000 units of vitamin D daily. However, patients with unsuspected hyperparathyroidism receiving such doses for the treatment of osteoporosis are more likely to experience hypercalcemia. Treatment consists of withdrawing the vitamin D, rehydration, reducing calcium intake, and administration of glucocorticoids, which antagonize the ability of 1,25(OH)2D3 to stimulate intestinal calcium absorption. Excess vitamin D is slowly cleared from the body (weeks to months), so treatment is prolonged.
Excessive ingestion of vitamin A, usually from self-medication with vitamin A preparations, causes a number of abnormalities, including gingivitis, cheilitis, erythema, desquamation, and hair loss. Bone resorption is increased, leading to osteoporosis and fractures, hypercalcemia, and hyperostosis. Excess vitamin A causes hepatosplenomegaly with hypertrophy of fat storage
cells, fibrosis, and sclerosis of central veins. Many of these effects can be attributed to the effects of vitamin A on cellular membranes. Under normal circumstances, such effects are prevented because vitamin A is bound to retinal-binding protein (RBP), and its release from the liver is regulated. In vitamin A toxicity, however, these protective mechanisms are overcome, and retinol and its retinyl esters appear in blood unbound to RBP. The mechanism by which vitamin A stimulates bone resorption is not clear.
- Milk-Alkali Syndrome
The ingestion of large quantities of calcium together with an absorbable alkali can produce hypercalcemia with alkalosis, renal impairment, and often nephrocalcinosis. The syndrome was more common when absorbable antacids were the standard treatment for peptic ulcer disease, but it is still seen occasionally. This is the only recognized example of pure absorptive hypercalcemia. The details of its pathogenesis are poorly understood.
- Miscellaneous Conditions
In immobilized patients there is a marked increase in bone resorption, which often produces hypercalciuria and occasionally hypercalcemia, mainly in individuals with a preexisting high bone turnover state, such as adolescents and patients with thyrotoxicosis or Paget's disease. Intact PTH and PTHrP levels are suppressed. The disorder remits with the restoration of activity. If acute treatment is required, bisphosphonates appear to be the treatment of choice.
Acute Renal Failure
Hypercalcemia is often seen when renal failure is precipitated by rhabdomyolysis and usually occurs during the early recovery stage, presumably as calcium deposits are mobilized from damaged muscle tissue. It typically resolves over a few weeks.
TREATMENT OF HYPERCALCEMIA
Initial management of hypercalcemia consists of assessing the hydration state of the patient and rehydrating as necessary with saline. The first goal is to restore renal function, which is often impaired in hypercalcemia because of reduced glomerular filtration and dehydration. Hypercalcemia impairs the urinary concentrating ability, leading to polyuria, and at the same time impairs the sensorium, diminishing the sense of thirst. Once renal function is restored, renal excretion of calcium can be further enhanced by inducing a saline diuresis. Because most of the filtered calcium is reabsorbed by bulk flow in the proximal tubule along with sodium chloride, a saline diuresis will markedly increase calcium excretion. However, a vigorous saline diuresis will also induce substantial urinary losses of potassium and magnesium, and these must be monitored and replaced as necessary.
After these initial steps, attention should be given to finding a suitable chronic therapy. It is important to start chronic therapy soon after hospitalization, as several of the most useful agents take up to 5 days to have their full effect. Intravenous bisphosphonates (pamidronate or zoledronic acid) are the first choice for most patients. Bisphosphonates act by inhibiting osteoclastic bone resorption. The initial dose of pamidronate is 60–90 mg by intravenous infusion over 4 hours, and the dose of zoledronic acid is 4 mg infused over 15 minutes. In two large trials, 88% and 70% of patients with malignancy-associated hypercalcemia normalized their serum calcium values after infusions of zoledronic acid (4 mg) and pamidronate (90 mg), respectively. Zoledronic acid produced a longer duration of response—32 days versus 18 days for pamidronate. The nadir of serum calcium does not occur until 4–5 days after administration of either agent. Re-treatment with either agent can be conducted after recurrence of hypercalcemia. Transient fever and myalgia occur in 20% of patients who undergo intravenous bisphosphonate therapy. Increased serum creatinine (≥ 0.5 mg/dL) occurs in about 15% of patients. Intravenous bisphosphonates should be used cautiously and at reduced doses when the baseline serum creatinine exceeds 2.5 mg/dL.
In patients with severe hypercalcemia and those with renal insufficiency that is refractory to rehydration, it may be necessary to use a second antiresorptive agent for a few days while awaiting the full therapeutic effect of bisphosphonates. For this purpose, synthetic salmon calcitonin may be administered at a dose of 4–8 IU/kg subcutaneously every 12 hours. This is a useful adjunct acutely, but most patients become totally refractory to calcitonin within a few days, so it is not suitable for chronic use.
The use of an antiresorptive agent together with saline diuresis provides for a two-pronged approach to hypercalcemia. Other agents besides bisphosphonates may be considered (eg, plicamycin or gallium nitrate), but their toxicity and lack of superior efficacy tend to discourage their use. Both agents act to inhibit osteoclastic bone resorption.
Glucocorticoid administration is first-line treatment for hypercalcemia in patients with multiple myeloma, lymphoma, sarcoidosis, or intoxication with vitamin D or vitamin A. Glucocorticoids are also beneficial in some patients with breast carcinoma. However, they are
of little use in most other patients with solid tumors and hypercalcemia.
Both PTH and 1,25(OH)2D3 function to maintain a normal serum calcium and are thus central to the defense against hypocalcemia. Hypocalcemic disorders are best understood as failures of the adaptive response. Thus, chronic hypocalcemia can result from a failure to secrete PTH, a failure to respond to PTH, a deficiency of vitamin D, or a failure to respond to vitamin D. Acute hypocalcemia is most often the consequence of an overwhelming challenge to the adaptive response such as rhabdomyolysis, in which a flood of phosphate from injured skeletal muscle inundates the extracellular fluid (Table 8-6).
Most of the symptoms and signs of hypocalcemia occur because of increased neuromuscular excitability (tetany, paresthesias, seizures, organic brain syndrome) or because of deposition of calcium in soft tissues (cataract, calcification of basal ganglia).
- NEUROMUSCULAR MANIFESTATIONS
Clinically, the hallmark of severe hypocalcemia is tetany. Tetany is a state of spontaneous tonic muscular contraction. Overt tetany is often heralded by tingling paresthesias in the fingers and about the mouth, but the classic muscular component of tetany is carpopedal spasm. This begins with adduction of the thumb, followed by flexion of the metacarpophalangeal joints, extension of the interphalangeal joints, and flexion of the wrists to produce the main d'accoucheur posture (Figure 8-17). These involuntary muscle contractions are painful. Although the hands are most typically involved, tetany can involve other muscle groups, including life-threatening spasm of laryngeal muscles. Electromyographically, tetany is typified by repetitive motor neuron action potentials, usually grouped as doublets. Tetany is not specific for hypocalcemia. It also occurs with hypomagnesemia and metabolic alkalosis, and the most common cause of tetany is respiratory alkalosis from hyperventilation.
Lesser degrees of neuromuscular excitability (eg, serum calcium 7–9 mg/dL) produce latent tetany, which can be elicited by testing for Chvostek's and Trousseau's signs. Chvostek's sign is elicited by tapping the facial nerve about 2 cm anterior to the earlobe, just below the zygoma. The response is a contraction of facial muscles ranging from twitching of the angle of the mouth to hemifacial contractions. The specificity of the test is low; about 25% of normal individuals have a mild Chvostek sign. Trousseau's sign is elicited by inflating a blood pressure cuff to about 20 mm Hg above systolic pressure for 3 minutes. A positive response is carpal spasm. Trousseau's sign is more specific than Chvostek's, but 1–4% of normals have positive Trousseau signs.
Table 8-6. Causes of hypocalcemia.
Hypocalcemia predisposes to focal or generalized seizures. Other central nervous system effects of hypocalcemia
include pseudotumor cerebri, papilledema, confusion, lassitude, and organic brain syndrome. Twenty percent of children with chronic hypocalcemia develop mental retardation. The basal ganglia are often calcified in patients with long-standing hypoparathyroidism or pseudohypoparathyroidism. This is usually asymptomatic but can produce a variety of movement disorders.
Figure 8-17. Position of fingers in carpal spasm due to hypocalcemic tetany. (Reproduced, with permission, from Ganong WF: Review of Medical Physiology, 16th ed. Originally published by Appleton & Lange. Copyright Š 1993 by The McGraw-Hill Companies, Inc.)
- OTHER MANIFESTATIONS OF HYPOCALCEMIA
- Cardiac effects—Repolarization is delayed, with prolongation of the QT interval. Excitation-contraction coupling may be impaired, and refractory congestive heart failure is sometimes observed, particularly in patients with underlying cardiac disease.
- Ophthalmologic effects—Subcapsular cataract is common in chronic hypocalcemia, and its severity is correlated with the duration and level of hypocalcemia.
- 3. Dermatologic effects—The skin is often dry and flaky and the nails brittle. A dermatitis known as impetigo herpetiformis or pustular psoriasis is peculiar to hypocalcemia.
CAUSES OF HYPOCALCEMIA
Hypoparathyroidism may be surgical, autoimmune, familial, or idiopathic. The signs and symptoms are those of chronic hypocalcemia. Biochemically, the hallmarks of hypoparathyroidism are hypocalcemia, hyperphosphatemia (because the phosphaturic effect of PTH is lost), and an inappropriately low or undetectable PTH level.
The most common cause of hypoparathyroidism is surgery on the neck, with removal or destruction of the parathyroid glands. The operations most often associated with hypoparathyroidism are cancer surgery, total thyroidectomy, and parathyroidectomy, but the skill and experience of the surgeon are more important predictors than the nature of the operation. Tetany ensues 1 or 2 days postoperatively, but about half of patients with postoperative tetany will recover sufficiently not to require long-term replacement therapy. In these cases, a devitalized parathyroid remnant has recovered its blood supply and resumes secretion of PTH. In some patients, hypocalcemia may not become evident until years after the procedure. Surgical hypoparathyroidism is the presumptive diagnosis for hypocalcemia in any patient with a surgical scar on the neck.
In patients with severe hyperparathyroid bone disease preoperatively, a syndrome of postoperative hypocalcemia can follow successful parathyroidectomy. This is the “hungry bones syndrome,” which results from such avid uptake of calcium and phosphate by the bones that the parathyroids, though intact, cannot compensate. The syndrome is usually seen in patients with an elevated preoperative serum alkaline phosphatase. It can usually be distinguished from surgical hypoparathyroidism by the serum phosphorus, which is low in the hungry bones syndrome because of skeletal avidity for phosphate, and high in hypoparathyroidism, and by the serum PTH, which will become appropriately elevated in the hungry bones syndrome.
Acquired hypoparathyroidism is sometimes seen in the setting of polyglandular endocrinopathies. Most commonly, it is associated with primary adrenal insufficiency and mucocutaneous candidiasis in the syndrome of pluriglandular autoimmune endocrinopathy, or type I polyglandular autoimmune syndrome (Chapter 4). The typical age at onset of hypoparathyroidism is 5–9 years. A similar form of hypoparathyroidism can occur as an isolated finding. The age at onset of idiopathic hypoparathyroidism is 2–10 years, and there is a preponderance of female cases. Circulating parathyroid antibodies are common in both the polyglandular syndrome and in isolated hypoparathyroidism. Up to one-third of patients with the latter syndrome have antibodies that recognize the parathyroid calcium sensor, though the pathogenetic role of these autoantibodies is not yet clarified. Mutations have been uncovered in a
protein termed AIRE (autoimmune regulator) that appears to be a transcription factor involved in endocrine and immune function.
Hypoparathyroidism can rarely present in a familial form, which may be transmitted as an autosomal dominant or an autosomal recessive trait. Two families with PTH gene mutations that interfere with the normal processing of PTH have been reported. Several families have also been shown to have point mutations in the parathyroid calcium-sensing receptor gene, which renders the protein constitutively active. This property enables the receptor to mediate suppression of PTH secretion at normal and subnormal serum calcium levels. Affected individuals have mild hypoparathyroidism which may require replacement therapy. The set-point for calcium-induced suppression of PTH secretion in these patients is shifted to the left. Thus, this syndrome is the mirror image of familial benign hypocalciuric hypercalcemia.
Other Causes of Hypoparathyroidism
Neonatal hypoparathyroidism can be part of the DiGeorge syndrome (dysmorphia, cardiac defects, immune deficiency, and hypoparathyroidism) due to a microdeletion on chromosome 22q11.2; the HDR syndrome (hypoparathyroidism, sensorineural deafness, and renal anomalies) due to the loss of a copy of the GATA3 transcription factor; and other rare conditions. Transfusion-dependent individuals with thalassemia or red cell aplasia who survive into the third decade of life are susceptible to hypoparathyroidism as the result of iron deposition in the glands. Copper deposition can cause hypoparathyroidism in Wilson's disease. Aluminum deposition in dialysis patients blunts the parathyroid reserve. Infiltration with metastatic carcinoma is a rare cause of hypoparathyroidism.
Severe magnesium depletion temporarily paralyzes the parathyroid glands, preventing secretion of PTH. Magnesium depletion also blunts the actions of PTH to counteract the hypocalcemia. This is seen with magnesium losses due to gastrointestinal and renal disorders and alcoholism. The syndrome responds immediately to infusion of magnesium. As discussed above in the section on regulation of parathyroid hormone secretion, magnesium is probably required for stimulus-secretion coupling in the parathyroids.
Pseudohypoparathyroidism is a heritable disorder of target-organ unresponsiveness to parathyroid hormone. Biochemically, it mimics hormone-deficient forms of hypoparathyroidism, with hypocalcemia and hyperphosphatemia, but the PTH level is elevated and there is a markedly blunted response to the administration of PTH (see Diagnosis, below).
Two distinct forms of pseudohypoparathyroidism are recognized. Pseudohypoparathyroidism type 1B is a disorder of isolated resistance to PTH, which presents with the biochemical features of hypocalcemia, hyperphosphatemia and secondary hyperparathyroidism. Pseudohypoparathyroidism type 1A has, in addition to these biochemical features, a characteristic somatic phenotype known as Albright's hereditary osteodystrophy. This consists of short stature, a round face, short neck, brachydactyly (short digits), and subcutaneous ossifications. Because of shortening of the metacarpal bones—most often the fourth and fifth metacarpals—affected digits have a dimple, instead of a knuckle, when a fist is made (Figure 8-18). In addition, primary hypothyroidism occurs commonly, and many patients have abnormalities of reproductive function—oligomenorrhea in females and infertility in males. Interestingly, certain individuals in families with pseudohypoparathyroidism inherit the somatic phenotype of Albright's hereditary osteodystrophy without any disorder of calcium metabolism; this state, which mimics pseudohypoparathyroidism, is called pseudopseudohypoparathyroidism.
Pseudohypoparathyroidism type 1A is caused by loss of one functional allele of the gene encoding the G protein subunit Gsα. This is predicted to produce a 50% deficiency of the alpha subunits of the heterotrimeric Gs, which couples the PTH receptor to adenylyl cyclase. Patients with pseudohypoparathyroidism type 1A have a markedly blunted response of urinary cAMP to administration of PTH. Since Gs also couples many other receptors to adenylyl cyclase, the expected result of this mutation would be a generalized disorder of hormonal unresponsiveness. The high prevalence of primary hypothyroidism and primary hypogonadism indicates that in fact resistance to TSH, LH, and FSH are commonly present, but the response to other hormones (eg, ACTH, glucagon) is fairly normal. Thus, a 50% loss of the αsprotein produces resistance to some hormones but not others. Gsα is also deficient in individuals with pseudopseudohypoparathyroidism, who have Albright's hereditary osteodystrophy but normal responsiveness to PTH. Thus, the mutation in the Gsα gene invariably produces Albright's hereditary osteodystrophy but only sometimes produces resistance to PTH, suggesting that the occurrence of resistance may be determined by other factors (Table 8-7).
Figure 8-18. Hands of a patient with pseudohypoparathyroidism. A: Note the short fourth finger. B: Note the “absent” fourth knuckle. C:Film shows the short fourth metacarpal. (Reproduced, with permission, from Potts JT: Pseudohypoparathyroidism: Clinical features; signs and symptoms; diagnosis and differential diagnosis. In: The Metabolic Basis of Inherited Disease, 4th ed. Stanbury JB, Wyngaarden JB, Fredrickson DS [editors]. McGraw-Hill, 1978.)
Table 8-7. Features of pseudohypoparathyroidism (PHP).
In pseudohypoparathyroidism type 1B, where there is resistance to PTH but no somatic phenotype, levels of Gsα protein in red blood cell or fibroblast membranes are normal. The disorder, however, has also been linked to the GNAS1 locus, but it does not involve mutations in the coding region of Gsα. It is likely that abnormal methylation of GNAS1 regulatory sequences is involved in the pathogenesis of PHP1b.
Pseudohypoparathyroidism type 1A is inherited as an autosomal dominant trait. Few cases of male-to-male transmission are recognized, but this is probably because of male infertility in affected individuals. Individuals who have acquired the trait from their fathers almost always present with pseudopseudohypoparathyroidism and lack hormone resistance. When inheritance is from the mother, pseudohypoparathyroidism with hormone resistance is almost always present. These features
suggest genomic imprinting, where the maternal allele is preferentially expressed in the kidney.
Several disorders present with hypocalcemia and secondary hyperparathyroidism (eg, vitamin D deficiency), but when these features occur together with hyperphosphatemia or Albright's hereditary osteodystrophy, this suggests the diagnosis of pseudohypoparathyroidism. To confirm that resistance to PTH is present, the patient is challenged with PTH (the Ellsworth-Howard test). For this purpose, synthetic human PTH(1–34) (teriparatide acetate, 3 IU/kg body weight) is infused intravenously over 10 minutes during a water diuresis, and urine is collected during the hour preceding the infusion, during the half-hour following the infusion, 30–60 minutes after the infusion, and 1–2 hours after the infusion and assayed for cAMP and creatinine. Data are expressed as nanomoles of cAMP per liter of glomerular filtrate, based on creatinine measurements. Normally, there is an increase in urinary cAMP of > 300 nmol/L glomerular filtrate after administration of PTH. The use of the urinary phosphate response as a gauge of PTH responsiveness is much less reliable.
- Vitamin D Deficiency
Vitamin D deficiency results from one or a combination of three causes: inadequate sunlight exposure, inadequate nutrition, and malabsorption. In addition, drugs that activate the catabolism of vitamin D and its metabolites, such as phenytoin and phenobarbital, can precipitate vitamin D deficiency in subjects with marginal vitamin D status. Although the human skin is capable of producing sufficient amounts of vitamin D if exposed to sunlight of adequate intensity, institutionalized patients frequently do not get adequate exposure. Furthermore, the fear of skin cancer has led many to avoid sunlight exposure or to apply protective agents that block the UV portion of sunlight from reaching the lower reaches of the epidermis where most of the vitamin D is produced. Heavily pigmented and elderly individuals have less efficient production of vitamin D for a given exposure to UV irradiation. The intensity of sunlight is an important factor that limits effective vitamin D production as a function of season (summer greater than winter) and latitude (less intense the higher the latitude). The supplementation of dairy products has reduced the incidence of vitamin D deficiency in the United States, but a number of countries do not follow this practice. Even in the United States, vitamin D deficiency may occur in children of vegetarian mothers who avoid milk products (and presumably have reduced vitamin D stores) and in children who are not weaned to vitamin D-supplemented milk by age 2. Breast milk contains little vitamin D. Elderly people who avoid dairy products as well as sunlight are likewise at risk. Individuals with a variety of small bowel diseases, partial gastrectomy, pancreatic diseases, and biliary tract diseases have reduced capacity to absorb the vitamin D in the diet.
The clinical features of individuals with vitamin D deficiency will be discussed more thoroughly in the section on osteomalacia and rickets. Vitamin D deficiency should be suspected in individuals complaining of lethargy, proximal muscle weakness, and bone pain who on routine biochemical evaluation have low or low normal serum calcium and phosphate and low urine calcium. A low serum 25(OH)D level is diagnostic in this setting. 1,25(OH)2D3 levels are often normal and reflect the increased 1-hydroxylase activity in these subjects which is responding appropriately to the increased PTH levels as well as the low serum calcium and phosphate levels.
The goal in treating vitamin D deficiency is to normalize the clinical, biochemical, and radiologic abnormalities without producing hypercalcemia, hyperphosphatemia, hypercalciuria, nephrolithiasis, or ectopic calcification. To realize this goal, patients must be followed carefully. As the bone lesions heal or the underlying disease improves, the dosage of vitamin D, calcium, or phosphate needs to be adjusted to avoid such complications. Simple nutritional vitamin D deficiency responds to oral doses of 2000–4000 units of vitamin D per day taken for several months and then followed by replacement doses of up to 800 units/d. Patients with malabsorption may respond to larger amounts of vitamin D (25,000–100,000 units/d or one to three times per week). 25(OH)D (50–100 ľg/d) is better-absorbed than vitamin D and may be used if malabsorption of vitamin D is a limiting factor. Calcitriol is not appropriate therapy for patients with vitamin D deficiency because of the likely requirement for vitamin D metabolites other than 1,25(OH)2D3 in the healing of rachitic bone. Vitamin D therapy should be supplemented with 1–3 g of elemental calcium per day. Care must be taken in managing patients with vitamin D deficiency who also have elevated PTH levels as long-standing vitamin D deficiency may produce a degree of autonomy in the parathyroid glands such that rapid replacement
with calcium and vitamin D could result in hypercalcemia or hypercalciuria. A listing of available vitamin D metabolites and analogs with their main indications for clinical use is set forth in Table 8-2.
- Vitamin D-Dependent Rickets Type I
Vitamin D-dependent rickets type I, also known as pseudovitamin D deficiency, is a rare autosomal recessive disease in which there is a low level of 1,25(OH)2D3 but normal levels of 25(OH)D and rickets. The disease is due to mutation in the 25(OH)D 1-hydroxylase gene which renders it nonfunctional. Both alleles need to be defective in order for the disease to be manifest. Although affected patients do not respond to doses of vitamin D that are effective in subjects with vitamin D deficiency, they can respond to pharmacologic doses of vitamin D and to physiologic doses of calcitriol, which is the preferred treatment.
- Vitamin D-Dependent Rickets Type II
Vitamin D-dependent rickets type II, also known as hereditary 1,25(OH)2D3-resistant rickets, is a rare autosomal recessive disease that presents in childhood with rickets similar to that seen in patients with vitamin D deficiency. Many of these patients also have alopecia, which is not characteristic of vitamin D deficiency. The biochemical changes are similar to those reported in subjects with vitamin D deficiency except that the 1,25(OH)2D3 levels are generally very high. The disease is caused by inactivating mutations in the VDR gene. The location of the mutation can affect the severity of the disease. These patients are treated with large doses of calcitriol and dietary calcium, and may show partial or complete remission as they grow older. An animal model of this disease (inactivation of the VDR by homologous recombination or “knockout”) demonstrates that the bone disease can be corrected with high dietary intake of calcium and phosphate, although the alopecia is not altered. This disease points to a role for the VDR in epidermis and hair development that is independent of its activity in bone.
- Other Hypocalcemic Disorders
Hypoalbuminemia produces a low total serum calcium concentration because of a reduction in the bound fraction of calcium, but the ionized calcium is normal. The ionized calcium level can be determined directly, or the effect of hypoalbuminemia can be roughly corrected using the following formula:
Thus, in a patient with a serum calcium of 7.8 mg/dL and a serum albumin of 2 mg/dL, the corrected serum calcium is 7.8 + (0.8)(4 - 2) = 9.4 mg/dL.
Several disorders produce acute hypocalcemia despite intact homeostasis, simply because they overwhelm the adaptive mechanisms. Acute hyperphosphatemia that results from rhabdomyolysis or tumor lysis, often in the setting of renal insufficiency, may produce severe symptomatic hypocalcemia. Transfusion of citrated blood causes acute hypocalcemia by complexation of calcium as calcium citrate. In this instance, total calcium may be normal but the ionized fraction is reduced. In acute pancreatitis, hypocalcemia is an ominous prognostic sign. The mechanism of hypocalcemia is sequestration of calcium by saponification with fatty acids, which are produced in the retroperitoneum by the action of pancreatic lipases. Skeletal mineralization, when very rapid, can cause hypocalcemia. This is seen in the “hungry bones syndrome,” which was discussed above in the section on surgical hypoparathyroidism, and occasionally with widespread osteoblastic metastases from prostatic carcinoma.
TREATMENT OF HYPOCALCEMIA
Patients with tetany should receive intravenous calcium as calcium chloride (272 mg calcium per 10 mL), calcium gluconate (90 mg calcium per 10 mL), or calcium gluceptate (90 mg calcium per 10 mL). Approximately 200 mg of elemental calcium can be given over several minutes. The patient must be observed for stridor and the airway secured if necessary. Oral calcium and a rapidly acting preparation of vitamin D should be started. If necessary, calcium can be infused in doses of 400–1000 mg/24 h until oral therapy has taken effect. Intravenous calcium is irritating to the veins. Caution must be exercised in patients taking digitalis, since they are predisposed to toxicity by infusion of calcium.
The objective of chronic therapy is to keep the patient free of symptoms and to maintain a serum calcium of approximately 8.5–9.2 mg/dL. With lower serum calcium levels, the patient may not only experience symptoms but may be predisposed over time to cataracts. With serum calcium concentrations in the upper normal range, there may be marked hypercalciuria, which occurs because the hypocalciuric effect of PTH has been lost. This may predispose to nephrolithiasis, nephrocalcinosis, and chronic renal insufficiency. In addition, the patient with a borderline elevated calcium is at increased risk of overshooting the therapeutic goal, with symptomatic hypercalcemia.
The mainstays of treatment are calcium and vitamin D. Oral calcium can be given in a dose of 1.5–3 g of elemental calcium per day. These large doses of calcium reduce the necessary dose of vitamin D and allow for rapid normalization of calcium if vitamin D intoxication subsequently occurs. Numerous preparations of calcium are available. A short-acting preparation of vitamin D (calcitriol) and the very long-acting preparations such as vitamin D2 (ergocalciferol) are available (Table 8-2). By far the most inexpensive regimens are those that use ergocalciferol. In addition to economy, they have the advantage of rather easy maintenance in most patients. The disadvantage is that ergocalciferol can slowly accumulate and produce delayed and prolonged vitamin D intoxication. Caution must be exercised in the introduction of other drugs that influence calcium metabolism. For example, thiazide diuretics have a hypocalciuric effect. By reducing urinary calcium excretion in treated patients, whose other adaptive mechanisms, PTH and 1,25(OH)2D3, are nonoperative and who are thus absolutely dependent on renal excretion of calcium to maintain the serum calcium level, thiazides may produce severe hypercalcemia. In a similar way, intercurrent illnesses that compromise renal function may produce dangerous hypercalcemia in the patient who is maintained on large doses of vitamin D. Short-acting preparations are less prone to some of these effects but may require more frequent titration and are much more expensive than vitamin D2.
BONE ANATOMY & REMODELING
FUNCTIONS OF BONE
Bone has three major functions. (1) It provides rigid support to extremities and body cavities containing vital organs. In disease situations in which bone is weak or defective, erect posture may be impossible, and vital organ function may be compromised. An example is the cardiopulmonary dysfunction that occurs in patients with severe kyphosis due to vertebral collapse. (2) Bones are crucial to locomotion in that they provide efficient levers and sites of attachment for muscles. With bony deformity, these levers become defective, and severe abnormalities of gait develop. (3) Bone provides a large reservoir of ions, such as calcium, phosphorus, magnesium, and sodium, that are critical for life and can be mobilized when the external environment fails to provide them.
STRUCTURE OF BONE
Bone is not only rigid and resists forces that would ordinarily break brittle materials but is also light enough to be moved by muscle contractions. Cortical bone, composed of densely packed layers of mineralized collagen, provides rigidity and is the major component of tubular bones (Figure 8-19). Trabecular (cancellous) bone is spongy in appearance, provides strength and elasticity, and constitutes the major portion of the axial skeleton. Disorders in which cortical bone is defective or scanty lead to fractures of the long bones, whereas disorders in which trabecular bone is defective or scanty lead preferentially to vertebral fractures. Fractures of long bones may also occur because normal trabecular bone reinforcement is lost.
Two-thirds of the weight of bone is due to mineral; the remainder is due to water and type I collagen. Minor organic components such as proteoglycans, lipids, acidic proteins containing γ-carboxyglutamic acid, osteonectin, osteopontin, and growth factors are probably important, but their functions are poorly understood.
The mineral of bone is present in two forms. The major form consists of hydroxyapatite in crystals of varying maturity. The remainder is amorphous calcium phosphate, which lacks a coherent x-ray diffraction pattern, has a lower calcium-to-phosphate ratio than pure hydroxyapatite, occurs in regions of active bone formation, and is present in larger quantities in young bone.
Bone is composed of three types of cells: the osteoblast, the osteocyte, and the osteoclast.
The osteoblast is the principal bone-forming cell. It arises from a pool of mesenchymal precursor cells in the bone marrow which, as they differentiate, acquire a set of characteristics including PTH and vitamin D receptors; surface expression of alkaline phosphatase; and expression of bone matrix protein genes—type I collagen, osteocalcin, etc. Differentiated osteoblasts are directed to the bone surface, where they line regions of new bone formation, laying down bone matrix (osteoid) in orderly lamellae and inducing its mineralization (Figure 8-20). In the mineralization process, hydroxyapatite crystals are deposited on the collagen layers to produce lamellar bone. Mineralization requires an adequate supply of extracellular calcium and phosphate as well as the enzyme
alkaline phosphatase, which is secreted in large amounts by active osteoblasts. The fate of senescent osteoblasts is not well defined. Some probably become flattened, inactive lining cells on trabecular bone surfaces, and some are buried in cortical bone as osteocytes.
Figure 8-19. Diagram of some of the measures of the microstructure of mature bone seen in both transverse (top) and longitudinal section. Areas of cortical (compact) and trabecular (cancellous) bone are included. (Reproduced, with permission, from Gray's Anatomy, 35th ed. Warwick R, Williams PL [editors]. Longman, 1973.)
Figure 8-20. The remodeling cycle. A: Resting trabecular surface. B: Multinucleated osteoclasts dig a cavity of approximately 20 microns. C: Completion of resorption to 60 microns by mononuclear phagocytes. D: Recruitment of osteoblast precursors to the base of the resorption cavity. E: Secretion of new matrix (gray shading) by osteoblasts. F: Continued secretion of matrix, with initiation of calcification (black areas). G: Completion of mineralization of new matrix. Bone has returned to quiescent state, but a small deficit in bone mass persists.
Osteoblasts that are trapped in cortical bone during the remodeling process become osteocytes. Protein synthetic activity decreases markedly, and the cells develop multiple processes that reach out through lacunae in bone tissue to communicate with nutrient capillaries, with processes of other osteocytes within a unit of bone (osteon) and also with the cell processes of surface osteoblasts (Figure 8-19). The physiologic importance of osteocytes is controversial, but they are believed to act as a cellular syncytium that permits translocation of mineral in and out of regions of bone removed from surfaces.
The osteoclast is a multinucleated giant cell that is specialized for resorption of bone. Osteoclasts are terminally differentiated cells that arise continuously from hematopoietic precursors in the monocyte lineage and do not divide. In a process that requires hematopoietic growth factors such as macrophage colony-stimulating factor (M-CSF, also called CSF-1) and is accelerated by cytokines such as interleukin-6 and by the systemic calciotropic hormones PTH and vitamin D, osteoclast precursors gradually mature, acquire the capacity to produce osteoclast-specific enzymes, and finally fuse to produce the mature multinucleate cell (Figure 8-21).
To resorb bone, the motile osteoclast alights on a bone surface and seals off an area by forming an adhesive ring. Having isolated an area of bone surface, the osteoclast develops above the surface an elaborately invaginated plasma membrane structure called the ruffled border(Figure 8-22). The ruffled border is a distinctive organelle, but it acts essentially as a huge lysosome that dissolves bone mineral by secreting acid onto the isolated bone surface and simultaneously breaks down bone matrix by secretion of catheptic proteases. The resulting collagen peptides have pyridinoline structures that can be assayed in urine as a measure of bone resorption rates. Bone resorption can be controlled in two ways: by regulating the formation of osteoclasts to change their number or by regulating the activity of the mature osteoclast. The mature osteoclast has receptors for calcitonin but does not appear to have PTH or vitamin D receptors.
Bone remodeling is a continuous process of breakdown and renewal that occurs throughout life. During childhood and adolescence, remodeling proceeds at a vigorous rate but is quantitatively overwhelmed by the concomitant occurrence of bone modeling and linear growth. Once peak bone mass has been established, remodeling supervenes as the common mechanism by which bone mass is modified for the remainder of a person's life. Each remodeling event is carried out by individual “bone remodeling units” (BMUs) on bone surfaces throughout the skeleton (Figure 8-20). Normally, about 90% of these surfaces lie dormant, covered by a thin layer of lining cells. Following physical or biochemical signals, precursor cells from the bone marrow migrate to specific loci on the bone surface, where they fuse into multinucleated bone-resorbing cells—osteoclasts—that dig a cavity into the bone.
Cortical bone is remodeled from within by cutting cones, groups of osteoclasts that cut tunnels through the compact bone (Figure 8-23). They are followed by trailing osteoblasts that line the tunnels with a cylinder of new bone which progressively narrows the tunnels until all that remains are the tiny haversian canals by which the cells left behind as resident osteocytes are fed. The packet of new bone formed by a single cutting cone is called an osteon (Figure 8-19).
Figure 8-21. Working model of cells in the osteoclast lineage and the sites of action of various growth and differentiation factors. (Reproduced, with permission, from Favus MJ [editor]: Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 2nd ed. Raven Press, 1993.)
By contrast, trabecular resorption creates scalloped areas of the bone surface called Howship's lacunae. Two to 3 months after initiation, the resorption phase reaches completion, having created a cavity about 60 ľm deep. This is accompanied by ingress from marrow stroma into the base of the resorption cavity of precursors for bone-forming osteoblasts. These cells develop an osteoblastic phenotype, expressing characteristic bone-specific proteins such as alkaline phosphatase, osteopontin, and osteocalcin, and begin to replace the resorbed bone by elaborating new bone matrix. Once the newly formed osteoid reaches a thickness of about 20 ľm, mineralization begins. Completion of a full remodeling cycle normally lasts about 6 months (Figure 8-20).
How do osteoclasts and osteoblasts communicate to achieve the coupling that ensures bone balance? It appears that the important signals are local, not systemic. Although complete understanding of this process has not been achieved, evidence suggests that two proteins derived from osteoblasts comprise an effective bone turnover regulatory system: The first component of this system is an osteoclast differentiation factor called RANK ligand (RANKL) that binds to its receptor, RANK receptor, which activates NF-κB on the surface of osteoclast precursors to directly stimulate osteoclast production. The second component, osteoprotegerin (OPG), is a soluble protein that binds to RANKL and thereby inhibits its ability to interact with RANK. According to current views of this system, when peripheral signals instruct the osteoblast to increase bone remodeling, RANKL is secreted and binds to RANK, its natural receptor, thereby initiating the proliferation of new osteoclasts. When circumstances favor decreasing the rate of remodeling, RANKL production abates and increased OPG binds to residual RANKL, thereby minimizing the binding of RANKL to RANK and down-regulating osteoclast production.
Bone remodeling does not absolutely require systemic hormones except to maintain intestinal absorption of minerals and thus ensure an adequate supply of calcium and phosphorus. For example, bone is normal, aside from low turnover, in patients with hypoparathyroidism. However, systemic hormones use the bone pool as a source of minerals for regulation of extracellular calcium homeostasis. When they do, the coupling mechanism ensures that bone is replenished. For example, when bone resorption is activated by PTH to provide calcium to correct hypocalcemia, bone formation will also increase, tending to replenish lost bone. One probable mechanism of coupling has to do with the apparent absence of PTH receptors and VDRs on osteoclasts. This means that other bone cells that have receptors for these hormones, such as osteoblasts, must receive the hormonal signal and pass it along to the osteoclast.
This would allow for bone formation to be activated along with bone resorption.
Figure 8-22. Osteoclast-mediated bone resorption. The osteoclast attaches to the bone surface via integrin-mediated binding to bone matrix bone proteins. When enough integrin binding has occurred, the osteoclast is anchored and a sealed space is formed. The repeatedly folded plasma membrane creates a“ruffled” border. Secreted into the sealed space are acid and enzymes forming an extracellular “lysosome.” (Reproduced, with permission, from Felig P, Baxter JD, Frohman LA [editors]: Endocrinology and Metabolism, 3rd ed. McGraw-Hill, 1995.)
If the replacement of resorbed bone matched the amount that was removed, remodeling would lead to no net change in bone mass. However, small bone deficits persist on completion of each cycle, reflecting inefficiency in remodeling dynamics. Consequently, lifelong accumulation of remodeling deficits underlies the well-documented phenomenon of age-related bone loss, a process that begins shortly after growth stops. Alterations in remodeling activity represent the final pathway through which diverse stimuli, such as dietary insufficiency, hormones, and drugs affect bone balance. A change in whole body remodeling rate can be brought about through distinct perturbations in remodeling dynamics. Changes in hormonal milieu often increase the activation of remodeling units. Examples include hyperthyroidism, hyperparathyroidism, and hypervitaminosis D. Other factors may impair osteoblastic functional adequacy, such as high doses of glucocorticoids or ethanol. Yet other perturbations, such as estrogen deficiency, may augment osteoclastic resorptive capacity. At any given time, a transient deficit in bone exists called the remodeling space, representing sites of bone resorption that have not yet filled in. In response to any stimulus that alters the birth rate of new remodeling units, the remodeling space will either increase or decrease accordingly until a new steady state is established, and this adjustment will be seen as an increase or decrease in bone mass.
Figure 8-23. Schematic representation of the four principal stages involved in the formation of a new basic structural unit in cortical bone. (Reproduced, with permission, from Felig P, Baxter JD, Frohman LA [editors]: Endocrinology and Metabolism, 3rd ed. McGraw-Hill, 1995.)
Osteoporosis is a condition of low bone mass and microarchitectural disruption that results in fractures with minimal trauma. Osteoporotic fractures are a major public health problem for older women and men in Western society, and billions of dollars are spent annually for acute hospital care of hip fracture alone. The consequences of vertebral deformity are less readily measured, but chronic pain, inability to conduct daily activities, and psychologic depression may be devastating.
The term “primary osteoporosis” denotes reduced bone mass and fractures in postmenopausal women (postmenopausal osteoporosis) or in older men and women (“senile” osteoporosis). “Secondary osteoporosis” is bone loss resulting from specific clinical disorders, such as thyrotoxicosis or hyperadrenocorticism (Table 8-8). States of estrogen-dependent bone loss, such as exercise-related amenorrhea or prolactin-secreting tumors, are conventionally treated as special cases of primary osteoporosis.
Fractures constitute the only clinically relevant consequence to having a fragile skeleton. At any age, women experience twice as many osteoporosis-related fractures as men do, reflecting gender-related differences in skeletal properties as well as the almost universal loss of bone at menopause. Currently increasing in prevalence, osteoporotic fractures in older men should not be considered of trivial importance (Figure 8-24). Fractures attributed to bone fragility are those due to trauma equal to or less than a fall from a standing position. Common sites of fragility-related fractures include vertebral bodies, the distal forearm, and the proximal femur, but since the skeletons of patients with osteoporosis are diffusely fragile, other sites, such as ribs and long bones, also fracture with high frequency. Vertebral compression fractures are the most common fragility-related fractures. Pain sufficient to require medical attention occurs in approximately one-third of vertebral fractures, the majority being detected only as height loss or spinal deformity (kyphosis) occurs. Both skeletal and extraskeletal factors determine fracture risk (Table 8-9).
Table 8-8. Representative examples of “secondary” osteoporosis.
Gain, Maintenance, & Loss of Bone
The amount of bone mineral present at any time in adult life represents that which has been gained at skeletal maturity (peak bone mass) minus that which has been subsequently lost. Bone acquisition is almost complete by 17 years in girls and by 20 years in boys. Heredity accounts for most of the variance in bone acquisition. Specific genes implicated in bone acquisition include those affecting body size, hormone responsiveness, and bone-specific proteins (Table 8-10). Other factors include circulating gonadal steroids, physical activity, and nutrient intake.
Bone gained during adolescence accounts for about 60% of final adult bone mass. Assuming adequate exposure to key nutrients, physical activity, and reproductive hormones, adolescent growth supports acquisition of the maximum bone mass permitted by genetic endowment. Estradiol plays a decisive role in the initiation of adolescent growth and bone acquisition. Rare examples have been reported of young men bearing
mutations in the estradiol receptor or in aromatase, the enzyme that converts androgen to estrogen. Although these men had normal or increased circulating concentrations of testosterone, estradiol was either absent or totally ineffective. In all cases, severe deficits in bone density were observed and the patients had not undergone the anticipated acceleration in linear growth at the time of puberty. In the patients with aromatase deficiency, substantial gains in bone were observed shortly after initiating estrogen therapy.
Figure 8-24. Incidence rates for the three common osteoporotic fractures (Colles', hip, and vertebral) in men and women, plotted as a function of age at the time of the fracture. (Reproduced, with permission, from Cooper C, Melton LJ III: Epidemiology of osteoporosis. Trends Endocrinol Metab 1992;314:224.)
Adolescent bone acquisition falters in the face of inadequacies in diet, physical activity, or reproductive function, resulting in a lower peak bone mass (acquisitional osteopenia) and less reserve to accommodate future losses. Recent trends in habitual physical activity and calcium intake for North American teenagers, particularly girls, offer little encouragement in this regard. A list of representative states of acquisitional osteopenia is presented in Table 8-11.
Once peak bone mass is achieved, bone mass remains fairly stable until about age 50 (Figure 8-25). Successful bone maintenance requires continued attention to the same “hygienic” factors that influenced bone acquisition: diet, physical activity, and reproductive status. Maintenance of bone requires sufficiency in all areas, and deficiency in one is not compensated by the others. For example, amenorrheic athletes lose bone despite frequent high-intensity physical activity and supplemental calcium intake. Successful bone maintenance is also jeopardized by known toxic exposures such as smoking, alcohol excess, and immobility as well as by systemic illnesses and many medications. Based on the above considerations, a rational osteoporosis prevention strategy can be formulated (Table 8-12).
Table 8-9. Factors affecting fracture risk.
Bone Loss Associated with Estrogen Deficiency
Estrogen deprivation promotes bone remodeling by releasing constraints on osteoblastic production of skeletally active cytokines which, in turn, stimulate the proliferation of osteoclast precursors. The primary cytokine involved in this process appears to be interleukin-6. Estradiol suppresses IL-6 secretion by marrow stromal osteoblastic cells, and treatment of mice with neutralizing IL-6 antibodies suppresses osteoclast production after oophorectomy. Estradiol directly suppresses IL-6 production in human osteoblasts, and IL-6 gene knockout
prevents bone loss in oophorectomized mice. Thus, a strong case implicates IL-6 as a critical molecule by which osteoblasts signal increased bone remodeling and suggests that this link is a major site for estrogen action in bone. Subsidiary roles for IL-1 and IL-11 have also been described. As discussed above, the RANK-RANKL-OPG system acts as a bone regulatory system through which osteoblast signals stimulate the production of osteoclasts. With estrogen deficiency, OPG secretion is low, permitting a robust response of osteoclast precursors to RANKL. With estrogen sufficiency, increased local concentrations of OPG bind to RANKL, reduce osteoclast production, and thereby decrease bone turnover.
Table 8-10. Genetic polymorphisms implicated in the acquisition of bone and regulation of bone mass in humans.
Table 8-11. Examples of disorders associated wtih acquisitional osteopenia.
Figure 8-25. Mean bone mineral density by DXA in white males (upper curve) and females (lower curve) aged 5–85 years. (The data are from Southard RN et al: Bone mass in healthy children: Measurement with quantitative DXA. Radiology 1991;179:735; and from Kelly TL: Bone mineral reference databases for American men and women. J Bone Miner Res 1990;5[Suppl 2]:702. Courtesy of Hologic, Inc.)
Table 8-12. A strategy to prevent osteoporosis at all ages.
With accelerated bone turnover, delivery of calcium from bone to the circulation increases, and the resultant subtle increase in plasma calcium concentration suppresses the secretion of PTH, thereby enhancing calciuria, suppressing renal production of 1,25 dihydroxyvitamin D, and reducing intestinal calcium absorption
efficiency. At menopause, loss of endogenous estrogen promotes an increase in daily calcium loss from 20 mg to about 60 mg, reflecting a relative increase in bone resorption over formation activity. Although small, this magnitude of change in mineral balance would account after a decade for about 13% of an original whole body calcium mass of 1000 g, equivalent to a standard deviation in bone mineral density (BMD), and would lead to a two- to threefold increase in the risk for fracture (Figure 8-25). In contrast, women who receive estrogen replacement as they enter menopause show calcium balance and rates of mineral turnover similar to those of premenopausal women. To accommodate menopausal changes in calcium economy by dietary means alone, a rise in daily calcium intake from 1000 mg to about 1500 mg would be necessary.
Bone Loss in Later Life
Progressive deficits in renal and intestinal function impair whole body calcium economy during normal human aging. These deficits include progressive inefficiency of vitamin D production by the skin as well as declining ability to convert 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D in the kidney. Consequently, intestinal calcium absorption becomes less efficient, leading to modest reductions in plasma ionized calcium activity and compensatory hypersecretion of PTH. PTH maintains blood calcium concentrations by activating new bone remodeling units, though as a result of its inherent inefficiency, increased bone remodeling leads to accelerated bone loss. Little can be done to counteract remodeling inefficiency, but the impact of these physiologic deficits can be minimized by suppressing the stimulus for PTH secretion by consuming adequate amounts of dietary calcium (about 1500 mg/d) and vitamin D (about 400–800 units/d).
Diagnosis of Osteoporosis
Diagnosis may be obvious in patients who have sustained fragility fractures (Figure 8-26), but noninvasive methods to estimate bone mineral density may be required to identify high-risk patients who have not yet sustained a fracture. Several techniques have been developed for this purpose, but dual-energy x-ray absorptiometry (DXA) currently offers the most precise measurements at multiple skeletal sites for the least amount of radiation exposure.
Bone mineral density (BMD) is a highly significant predictor of fracture risk. Each standard deviation below age-predicted mean values confers a two- to threefold increased risk for fracture over time (Figure 8-27). A World Health Organization panel offered an absolute BMD standard to make the diagnosis of osteoporosis. By this criterion, a person whose BMD value falls more than 2.5 SD below the average value for a 25-year-old Caucasian woman is stated to have osteoporosis, and individuals whose BMDs fall between -1.0 and -2.5 SD below the standard are said to have osteopenia. Although useful, this concept still presents difficulties, particularly with respect to its validity for men and to non-Caucasian populations. Absolute reliance on a BMD standard for diagnosis fails to account for the fact that BMD predicts fracture risk along a continuous, progressive relationship and ignores contributions of other factors to bone fragility. The latter include bone size and geometry as well as qualitative abnormalities of the matrix and mineral of osteoporotic bone (Table 8-13). Thus, the primary diagnostic value of densitometry is not to confer a specific diagnosis of osteoporosis but to predict an individual's long-term fracture risk.
TREATMENT OF OSTEOPOROSIS
Drugs used for prevention and treatment of osteoporosis act either by decreasing the rate of bone resorption, thereby slowing the rate of bone loss, or by increasing bone formation. All drugs currently approved in the United States for this indication inhibit resorption (Table 8-14). Because of the coupled nature of bone resorption and formation, these agents ultimately decrease the rate of bone formation. Thus, increases in BMD, representing a reduction of the remodeling space to a new steady-state level, are commonly observed during the first year or two of therapy, after which BMD values reach a plateau.
Specific Antiresorptive Agents
In childhood and adolescence, adequate substrate is required for bone accretion. Clinical trials indicate that supplemental calcium promotes adolescent bone acquisition, but its impact on peak bone mass is not known. In the seventh decade and beyond, supplemental calcium suppresses bone turnover, increases bone mass, and has been shown in clinical trials to decrease fracture incidence. Studies confirming the clinical efficacy of various pharmacologic agents to increase BMD or decrease fracture have been done in the setting of calcium sufficiency. Thus, adequate intake of calcium should be considered a basic approach for prevention and treatment of osteoporosis in all patients (Table 8-15).
Patients who are unable or unwilling to increase dietary calcium by diet alone may choose from many
palatable, low-cost calcium preparations, the most frequently prescribed being the carbonate salt (Table 8-16). Others include the lactate, gluconate, and citrate salts and hydroxyapatite. Absorption of most commonly prescribed calcium products is reasonable. For many patients, cost and palatability outweigh modest differences in efficacy. The usual dose of calcium is about 1000 mg/d—approximately the amount contained in a quart of vitamin D-supplemented milk. Added to the 500–600 mg of dietary calcium present in the typical diet of elderly men and women, this provides a total daily intake of about 1500 mg. Calcium carbonate requires an acid environment for optimal absorption. For older individuals who may have hypo- or achlorhydria, taking calcium carbonate tablets with meals generally provides adequate acidity for this requirement.
Figure 8-26. A: Magnified x-rays of thoracic vertebrae from a woman with osteoporosis. Note the relative prominence of vertical trabeculae and the absence of horizontal trabeculae. B: Lateral x-ray of the lumbar spine of a woman with postmenopausal osteoporosis. Note the increased density of the superior and inferior cortical margins of vertebrae, the marked demineralization of vertebral bodies, and the central compression of articular surfaces of vertebral bodies by intervertebral disks. (Courtesy of G Gordan.)
- VITAMIN D AND CALCITRIOL
In recent years, it has become necessary to reassess our concepts of vitamin D sufficiency. Although circulating 25(OH)D concentrations above 8 ng/mL appear adequate for preventing osteomalacia, a strong case can now be made that values below 25–30 ng/mL are associated with hypersecretion of PTH and increased bone turnover. In some regions, healthy adults show average 25(OH)D concentrations of 30 ng/mL or above, but in others, such as New England or in states farther north than Missouri, the ultraviolet component of sunlight fails to contain sufficient amounts at the critical region
of approximately 270–310 nm to support cutaneous vitamin D synthesis. In those areas, vitamin D insufficiency is very common, particularly in frail or hospitalized patients. Vitamin D supplementation (400–800 IU/d) improves intestinal calcium absorption, suppresses PTH and bone remodeling, and increases bone mass in individuals with marginal or deficient vitamin D status.
Figure 8-27. Bone density of (A) the lumbar spine (L2–4) and (B) femoral neck. The bold line represents the mean of 650 women with no overt bone disease; the thinner lines represent 1 SD and 2 SD above and below the mean. (Courtesy of Hologic, Inc.)
The use of potent vitamin D analogs or metabolites—such as calcitriol—to treat osteoporosis is distinct from ensuring vitamin D nutritional adequacy. In the treatment of osteoporosis, the rationale is to exploit the ability of these compounds to interact with the vitamin D receptor in parathyroid cells to suppress PTH secretion directly and reduce bone turnover. Clinical experience with calcitriol remains mixed. Higher doses impose an added risk of hypercalciuria and hypercalcemia but appear more likely to improve bone mass. Potent vitamin D metabolites and analogs are still considered experimental and should be preferably used in the context of a clinical trial.
Table 8-13. Qualitative abnormalities in osteoporotic bone.
Clinical trials clearly establish that timely replacement of estrogen conserves bone mass. Although such conservation is likely to confer protection against fracture, estimates of the antifracture efficacy of estrogen are based largely on observational data rather than on clinical trials. The minimum daily estrogen dose for skeletal protection of women within 10 years of menopause appears to be 0.625 mg/d of conjugated equine estrogens or its equivalent. This dose, given for 3 years, can be expected to increase BMD about 5% at the lumbar spine and 2.5% at the proximal femur. Long-term treatment at this dosage prevents bone loss in about 95% of women who adhere to therapy. In older women who
also receive adequate calcium supplementation, 0.3 mg of conjugated estrogens has been shown to be effective. Both oral and transdermal estrogen offer skeletal protection. Estrogen cessation leads rapidly to bone loss, and protection against fracture dissipates within a few years of stopping even after prolonged treatment. Thus, for persistent skeletal benefit, estrogen must be considered lifelong therapy. Standard practice recommends cyclic or continuous administration of progestational drugs to women with an intact uterus, and the skeletal response to estrogen appears not to be affected by progestin use. For women without a uterus, estrogen therapy is continuous and does not require adding a progestin. The optimal time to institute estrogen replacement is early menopause, when bone turnover accelerates. However, beneficial skeletal effects of estrogen are observed when estrogen is started even after age 65. Many older women will not accept cyclic bleeding or other anticipated side effects of estrogen, so the decision to initiate estrogen in elderly women must be individualized. Some women find the decision easy; for others, a careful weighing of risks and benefits may be required. Bone densitometry is of use in advising undecided women (Figure 8-28). Since women whose BMD is in the lower range of normal have a substantially increased risk of future fractures, they can be encouraged to consider replacement therapy. Conversely, women whose BMD is in the upper normal range can be encouraged to make their decision based on other issues, such as their experience of hot flushes and the risk for cardiovascular disease and breast cancer.
Table 8-14. Pharmacologic approaches to osteoporosis.
Table 8-15. Calcium nutrition and osteoporosis.
Table 8-16. Calcium preparations in common use.
- SELECTIVE ESTROGEN RESPONSE MODULATORS (SERMS)
The past decade has witnessed the design and development of several molecules that act as estrogens on some tissues but as antiestrogens on others. The first of these to reach clinical practice was tamoxifen, which is a full estrogen agonist on bone, liver, and uterus but an antiestrogen at the breast and brain. More recently, raloxifene has been introduced for skeletal protection. This compound is an estrogen agonist at bone and liver, resulting in conservation of BMD and lowering of LDL cholesterol concentrations, but it is inert at the endometrium and a potent antiestrogen at the breast. Raloxifene has been shown to decrease the vertebral fracture incidence in older osteoporotic women and has received FDA approval for both prevention and treatment of osteoporosis in postmenopausal women.
An inhibitor of osteoclastic bone resorption, calcitonin increases spine BMD in osteoporotic patients by 10–15%. Although this maximal calcitonin effect is accomplished with injectable salmon calcitonin, this agent has been largely supplanted by a more convenient nasal spray that has been approved for osteoporosis treatment and which, although it produces only modest changes in BMD, has been shown to reduce vertebral fracture incidence. In menopausal women who cannot or will not accept estrogen replacement or raloxifene, calcitonin affords reasonable conservation of bone mass.
Alendronate, a potent antiresorptive drug, has been shown to increase BMD, reduce by 50% the incidence of vertebral and cortical bone fractures—including hip fracture—and diminish height loss in postmenopausal women with established osteoporosis. For older women
with low bone mass who have not sustained fractures, alendronate also decreases the incidence of vertebral fractures. Alendronate and a related bisphosphonate, risedronate, are both approved for prevention and treatment of osteoporosis, including that associated with glucocorticoid use. These drugs are generally well tolerated, though they have been associated with esophagitis in some patients. For adequate absorption, they must be taken only with water after an overnight fast and at least 30 minutes before consuming other food, liquids, or medications. The patient should remain upright after swallowing the tablet to minimize the chance of esophageal distress.
Figure 8-28. Algorithm for making decisions about the use of estrogen replacement therapy to prevent osteoporosis in postmenopausal women. (Reproduced, with permission, from Riggs BL, Melton LJ III: The prevention and treatment of osteoporosis. N Engl J Med 1992;327:620.)
Although fluoride salts definitely increase BMD, particularly at the spine, considerable doubt remains about their ability to reduce fracture. A recent trial of sustained-release fluoride, which is associated with lower blood fluoride levels, showed a dramatic reduction in new vertebral fractures, but a European trial of monofluorophosphate showed no benefit. The status of slow-release fluoride remains under FDA review.
Testosterone increases bone mass of hypogonadal men. Androgens also improve bone mass in osteoporotic women, but therapy is frequently limited by virilizing side effects. Nandrolone decanoate, 50 mg intramuscularly every 3 weeks, and the androgenic progestin norethisterone acetate increased BMD in osteoporotic women without bothersome side effects. Fracture data do not yet permit a conclusion about the clinical utility of these agents.
- PARATHYROID HORMONE
Fractures, skeletal deformity, and bone pain are well-described manifestations of bone disease associated with severe primary hyperparathyroidism. It may seem counterintuitive, therefore, to suggest that administration of PTH to individuals with low bone mass may not only improve BMD but also reduce the risk for fractures—yet that is what experience over the past 2 decades suggests. The decisive element that determines the effects of PTH to be destructive or therapeutic is whether PTH concentrations in blood are elevated constantly or only intermittently. In severe hyperparathyroidism, PTH concentrations remain high, varying little throughout the day. In most cases of mild hyperparathyroidism
seen today, PTH concentrations are lower, sometimes within the normal range. In such patients, normal and even increased lumbar spine BMD are commonly observed. Administration of PTH as daily pulse injections has been shown to increase BMD in both animals and humans. Abundant evidence now indicates that PTH is truly an osteotropic agent, and several laboratories have shown increases in axial bone mineral density of patients with osteoporosis. In a large clinical trial conducted in postmenopausal women with osteoporosis, recombinant human PTH (1–34), given for approximately 1 1/2 years, reduced the incidence of vertebral fractures by over 50% and of nonvertebral fractures by 53%. During the course of this trial, a long-term carcinogenicity study showed that rats given near-lifetime daily injections of rhPTH (1–34) showed a dose-related incidence of osteosarcoma. Clinical studies with this agent were immediately suspended pending a thorough review of all relevant information by an independent group of cancer biologists, who concluded that the rat osteosarcoma finding was not likely to predict a relationship to osteosarcoma in humans. Consequently, human trials with rhPTH (1–34) were resumed. As of this writing, rhPTH (1–34) is awaiting FDA approval for treatment of patients with osteoporosis at high risk for fracture.
NONPHARMACOLOGIC ASPECTS OF OSTEOPOROSIS MANAGEMENT
Much of the day-to-day management of osteoporotic patients involves issues that are not specific for bone. These include depression, pain control, maintaining nutritional status, exercise, and assisting the patient to organize activities of daily living.
Minimal attention has been devoted to the role of exercise for patients with established osteoporosis. Patients and physicians often show reluctance to participate in exercise because of concerns for additional injury. However, activity avoidance aggravates bone loss and places the skeleton in even greater jeopardy, whereas improving muscle strength, particularly of the back extensor groups, constitutes a powerful means for reducing pain and increasing functional capacity.
Although exercise may improve bone density in patients with osteoporosis, gains have been small, and one may ask what the real benefits could be from such limited improvement. Many older individuals have such little proximal femur strength that even large increases in BMD would not permit a direct fall onto the trochanter without fracture. Since the great majority of hip fractures are the immediate result of a fall, strategies aimed at reduction of falls may be more effective in reducing the incidence of hip fracture than those aimed specifically at increasing bone mass. Muscle weakness is an important predictor of falls risk, and decreased muscle mass and strength are consequences of normal human aging. Resistance exercise, ie, weight training, promotes muscle strength even in very old men and women, and evidence suggests that increased lower extremity strength may reduce falls risk by improving postural stability. Thus, a widely disseminated program of leg-strengthening exercise could lower the risk of falling and reduce hip fracture incidence even if no changes in bone mineral were achieved.
Several additional measures may lower the risk of fracture for osteoporotic patients. Proper footwear and installation of safety features around the home may minimize the risk of falling. Such features include bathroom safety rails and night-lights, rails and lighting for stairways, and elimination of floor clutter. Assorted canes and walking aids can be recommended to patients with unsteady gait. Physicians may need to overcome substantial patient resistance, but these aids can be life-saving. Corsets and other support garments stabilize posture and relieve strain on paraspinous muscles. Metal braces may be indicated at times, but they are expensive, poorly accepted by most patients, and frequently end up unused. Patients should be encouraged to schedule daily rest periods. An hour or so of comfortable repose can restore sufficient energy to allow patients to participate in evening activities. Many patients are severely incapacitated by pain and depression, so that successful management requires timely and effective use of appropriate analgesic and antidepressant medications.
Few skeletal disorders prove as vexing to treat as glucocorticoid-induced osteoporosis. Recent years have witnessed important progress in understanding this condition as well as in achieving partial reversal of established disease. In children, chronic exposure to glucocorticoid impairs skeletal growth. Glucocorticoid-induced deficits in childhood bone mass reflect failure of normal skeletal acquisition occasionally compounded by bone loss. In adults, glucocorticoid-induced skeletal deficits exclusively reflect bone loss due to uncoupling of the normal relationship between the resorption and formation phases of bone remodeling. When adults are administered high doses of glucocorticoid for more than several days, rapid bone loss ensues, leading to BMD deficits within a few months. Bone loss in the axial (central) skeleton exceeds that in appendicular (peripheral) bone, reflecting greater investment of trabecular bone in axial regions. The magnitude of axial bone loss typically approaches 40% of initial values, which translates
into a high incidence of vertebral compression fractures. Peripheral bone loss is more likely to be about 20%. Skeletal fragility is not the only risk factor for fracture in steroid-treated patients, since glucocorticoids (and the illnesses for which they are prescribed) are also associated with muscle weakness, which may be profound and which itself creates instability and increases the risk for falling.
Glucocorticoid-induced bone loss generally follows sustained administration of systemic doses greater than 5 mg/d of prednisone or its equivalent. Skeletal consequences depend on the condition being treated. A week-long course at high dosage for treatment of poison oak dermatitis is not an important risk, but even low-dose steroids may place frail elders with rheumatic conditions in considerable jeopardy. Used according to manufacturer recommendations, most steroid inhalers available in the United States are of little concern since systemic absorption of glucocorticoids is insubstantial. However, excessive use of standard preparations may achieve systemic concentrations adequate to suppress the hypothalamic-pituitary axis and predictably lead to loss of bone. Patients receiving maintenance hydrocortisone dosages (eg, 20 mg/d) for adrenocortical insufficiency do not generally have an increased skeletal risk. However, based on accurate measurements of steroid production rates in normal humans, maintenance doses of hydrocortisone are now estimated to be lower (ie, 15mg/d) than have traditionally been recommended (about 30 mg/d). Patients who continue to be treated according to older recommendations may be at risk for excessive bone loss.
Glucocorticoids affect human mineral balance through functional alterations in the kidney, intestine, and skeleton.
- RENAL CALCIUM LOSSES
Within a few days after initiating therapy, direct inhibition of renal tubular calcium reabsorption leads to hypercalciuria, which is magnified by excessive dietary sodium and attenuated by thiazide diuretics.
- INTESTINAL CALCIUM LOSSES
Glucocorticoids given in high doses for several weeks directly inhibit intestinal calcium absorption, lowering plasma ionized calcium activity. This is thought to stimulate compensatory hypersecretion of PTH, which restores ionized calcium activity by increasing the efficiency of renal calcium reabsorption and by activating bone turnover to increase delivery of calcium to the circulation from bone. Thus, the dominant model for conceptualizing steroid-induced changes in bone homeostasis requires an increase in activity of the parathyroid axis. The operative term in this model is “activity,” because it has been very difficult to show with consistency that plasma PTH concentrations are actually elevated in steroid-treated patients. Evidence suggests that expression of PTH action in bone and kidney may be enhanced by glucocorticoids, so the described model does not require increased PTH concentrations for validity. Consistent effects of glucocorticoids on the production, clearance, and circulating concentrations of vitamin D metabolites have not been demonstrable, so it appears that the intestinal actions of glucocorticoids are independent of the vitamin D system.
- SKELETAL LOSSES
Glucocorticoids interact with the skeleton at multiple sites. In mice, they directly promote osteoclastic bone resorption, an effect that seems not to occur in humans. Glucocorticoids inhibit osteoblastic maturation and activity, and recent information indicates that glucocorticoids shift multipotent stem cell maturation away from osteoblastic lineage toward other cell lines, particularly adipocytes. Glucocorticoids also promote apoptosis in osteoblasts. In composite, the most coherent basis for glucocorticoid-associated bone loss is suppression of bone formation activity.
High-dose glucocorticoid administration suppresses gonadotropin secretion and creates a hypogonadal state in both men and women. Thus, a component of the bone loss in some steroid-treated patients is likely to be related to loss of gonadal function.
Prevention & Treatment of Glucocorticoid-Related Osteoporosis
- REDUCTION OF STEROID DOSE
Given the poor response of established glucocorticoid-related osteoporosis to available therapies, prevention remains the approach most likely to give a favorable outcome. The most important aspect of a sound preventive strategy is to limit exposure to glucocorticoids and to find alternative treatments if possible. Opportunities should be sought for initiating dose reduction or switching to nonsystemic forms of steroids. Unfortunately, the use of alternate-day steroids, which is known to protect growth in steroid-treated children, appears not to offer skeletal protection to steroid-treated adults. The possibility that a bone-sparing glucocorticoid might be developed has been under consideration for many years but no such agent has yet been approved in the United States. In patients who require immunosuppression following organ transplantation, widespread use of cyclosporine and tacrolimus has permitted reductions in glucocorticoid use. However, these newer drugs also promote bone loss, so their net effect on bone status may be just as worrisome.
- CALCIUM AND VITAMIN D
Providing supplemental calcium and vitamin D to glucocorticoid-treated patients will normalize plasma ionized calcium activity, suppress PTH secretion, and reduce bone remodeling, thereby reducing the number of remodeling units in action at a given time. So long as the patient continues to take glucocorticoids, however, suppression of osteoblast function is not reversed. Patients should receive 1500 mg/d of supplemental calcium. Vitamin D has been given at dosages of 400 IU/d to 50,000 IU three times per week. Periodic surveillance of glucocorticoid-treated patients for urinary calcium excretion is warranted, since they would be at higher risk for developing kidney stones as a consequence of hypercalciuria.
- PHYSICAL ACTIVITY
Skeletal immobilization precipitates bone loss, so, inactivity must be avoided in steroid-treated patients. Some conditions such as polymyalgia rheumatica respond exuberantly to steroid therapy, giving patients a rapid and complete recovery of function. These patients may have little difficulty maintaining a vigorous schedule of weight-bearing activities during treatment. Patients with other conditions may not regain full mobility and may have residual functional disabilities. It is particularly important for such patients to receive physical therapy or other appropriate assistance for maintaining and restoring functional capacity.
- HORMONE REPLACEMENT THERAPY
In some rheumatologic conditions (eg, systemic lupus erythematosus), estrogens have been traditionally avoided. If there are no contraindications to estrogen administration, replacement therapy will counteract the negative skeletal consequences of pituitary-gonadal suppression.
Pharmacologic Therapy of Glucocorticoid-Related Osteoporosis
Given that the primary skeletal abnormality induced by glucocorticoids is inhibition of bone formation, the potential benefits of antiresorptive therapy are not immediately obvious. Evidence from clinical trials of glucocorticoid-treated patients indicates clinical utility for calcitriol, with calcitonin being of only marginal value. However, enthusiasm for calcitriol is tempered by its significant risk for producing unacceptable degrees of hypercalciuria, if calcium intake is not restricted. Recent interest has centered around the use of bisphosphonates. Well-conducted trials have shown unequivocal benefit in terms of conserving and even increasing bone density. In a study of patients who had been treated with glucocorticoids for variable periods of time, 48 weeks of daily alendronate (5 mg/d or 10 mg/d) significantly increased BMD of the spine and hip compared with a control group, leading ultimately to a substantial reduction in the incidence of vertebral fracture. Similar results have been observed with risedronate, and both drugs are approved for prevention and treatment of glucocorticoid-induced osteoporosis.
Promoters of Bone Formation
It is reasonable to assume that an agent which directly promotes osteoblast function would be ideal for counteracting the effects of glucocorticoids. Several compounds have generated interest. There is strong evidence that PTH may exert a true bone anabolic effect. In a recent study, hPTH (1–34), an active fragment of human PTH, was given to estrogen-replaced postmenopausal women who were taking glucocorticoids, primarily for rheumatoid arthritis. Over 1 year of follow-up, lumbar spine BMD in the group receiving the PTH analog increased by 35% (measured by computed tomography) and by 11% (measured by DXA). By contrast, patients receiving hormone replacement therapy alone showed BMD changes of less than 2%. The differential response between CT and DXA reflects the fact that CT specifically measures trabecular bone, which appears to have been the primary responding tissue, whereas DXA measures both trabecular bone and the surrounding cortex. These results suggest that PTH may have great clinical utility in glucocorticoid-treated patients and emphasize the need for a longer-term controlled clinical trial of this agent.
OSTEOMALACIA & RICKETS
Osteomalacia and rickets are caused by the abnormal mineralization of bone and cartilage. Osteomalacia is a bone defect in which the epiphysial plates have closed (ie, in adults). Rickets occurs in growing bone (ie, in children). Abnormal mineralization in growing bone affects the transformation of cartilage into bone at the zone of provisional calcification. As a result, an enormous profusion of disorganized, nonmineralized, degenerating cartilage appears in this region, leading to widening of the epiphysial plate (observed radiologically as a widened radiolucent zone) with flaring or cupping and irregularity of the epiphysial-metaphysial junctions. This latter problem gives rise to the clinically obvious beaded swellings along the costochondral junctions known as the “rachitic rosary” and the swelling at the ends of the long bones. Growth is retarded by the failure to make new bone. Once bone growth has ceased (ie, after closure of the epiphysial plates), the clinical evidence for defective mineralization
becomes more subtle, and special diagnostic procedures may be required for its detection.
The best-known cause of abnormal bone mineralization is vitamin D deficiency (see above). Vitamin D, through its biologically active metabolites, ensures that the calcium and phosphate concentrations in the extracellular milieu are adequate for mineralization. Vitamin D may also permit osteoblasts to produce a bone matrix that can be mineralized and then allows them to mineralize that matrix in a normal fashion. Phosphate deficiency can also cause defective mineralization, as in diseases in which phosphate is lost in the urine or poorly absorbed in the intestine. Phosphate deficiency may act independently or in conjunction with other predisposing abnormalities since most hypophosphatemic disorders associated with osteomalacia or rickets also affect the vitamin D endocrine system. Dietary calcium deficiency has been shown to lead to rickets in children in the absence of vitamin D deficiency and may contribute to the osteomalacia of elderly adults who are also susceptible to vitamin D deficiency.
Osteomalacia or rickets may develop despite adequate levels of calcium, phosphate, and vitamin D if the bone matrix cannot undergo normal mineralization as a result of enzyme deficiencies such as decreased alkaline phosphatase in patients with hypophosphatasia or in the presence of inhibitors of mineralization such as aluminum, fluoride, and etidronate. Table 8-17 lists diseases associated with osteomalacia or rickets according to their presumed mechanism. Several diseases appear under several headings indicating that they contribute to the bone disease by several mechanisms.
The following discussion refers primarily to vitamin D deficiency. In children, the presentation of rickets is generally obvious from a combination of clinical and radiologic evidence. The diagnostic challenge is to determine the cause. In adults, the clinical, radiologic, and biochemical evidence for osteomalacia is often subtle. In situations in which osteomalacia should be suspected (malnutrition, malabsorption, unexpected osteopenia), the clinician must decide whether to obtain a bone biopsy for histomorphometric examination. This decision must rest on the availability of resources to take and examine the biopsy specimen, the index of suspicion coupled with the lack of certainty from other diagnostic procedures, and the degree to which the therapeutic approach will be altered by the additional information. In many cases, a clinical trial will suffice to establish the diagnosis without the need for bone biopsy.
Table 8-17. Causes of osteomalacia.
- SYMPTOMS AND SIGNS
The clinical presentation of rickets depends on the age of the patient and, to some extent, the cause of the syndrome (Figure 8-29). The affected infant or young child may be apathetic, listless, weak, hypotonic, and
growing poorly. A soft, somewhat misshapen head with widened sutures and frontal bossing may be observed. Eruption of teeth may be delayed, and teeth that do appear may be pitted and poorly mineralized. The enlargement and cupping of the costochondral junctions produce the “rachitic rosary” on the thorax. The tug of the diaphragm against the softened lower ribs may produce an indentation at the point of insertion of the diaphragm (Harrison's groove). Muscle hypotonia can result in a pronounced “potbelly” and a waddling gait. The limbs may become bowed, and joints may swell because of flaring at the ends of the long bones (including phalanges and metacarpals). Pathologic fractures may occur in patients with florid rickets. After the epiphyses have closed, the clinical signs of rickets or osteomalacia are subtle and cannot be relied upon to make the diagnosis. Patients with severe osteomalacia complain of bone pain and proximal muscle weakness. Difficulty climbing stairs or rising from chairs may be reported and should be looked for. Such individuals may have a history of fractures and be diagnosed as having osteoporosis. Distinguishing osteoporosis from osteomalacia on clinical grounds alone is at times quite difficult.
Figure 8-29. The clinical appearance of a young child with rickets. The most striking abnormalities are the bowing of the legs and protuberant abdomen. Flaring of the ends of the long bones can also be appreciated. (Photograph courtesy of S Arnaud.)
- LABORATORY FINDINGS
- Biochemistry—Vitamin D deficiency results in decreased intestinal absorption of calcium and phosphate. In conjunction with the resulting secondary hyperparathyroidism, vitamin D deficiency leads to an increase in bone resorption, increased excretion of urinary phosphate, and increased renal tubular reabsorption of calcium. The net result tends to be a low-normal serum calcium level, low serum phosphorus level, elevated serum alkaline phosphatase level, increased PTH level, decreased urinary calcium level, and increased urinary phosphate level. Finding a low 25(OH)D level, in combination with these other biochemical alterations, strengthens the diagnosis of vitamin D deficiency. The 1,25(OH)2D3level may be normal, making this determination less useful for the diagnosis of osteomalacia. Both 25(OH)D and 1,25(OH)2D3 levels may be reduced in patients with liver disease or nephrotic syndrome because the binding proteins for the vitamin D metabolites are low secondary to decreased production (liver disease) or increased renal losses (nephrotic syndrome; see below). Such individuals may have normal free concentrations of these metabolites and so are not vitamin D-deficient. Other factors, such as age and diet, must be considered. For example, serum phosphorus values are normally lower in adults than in children. Dietary history is important, since urinary phosphate excretion and, to a lesser extent, urinary calcium excretion reflect dietary phosphate and calcium content. Since phosphate excretion depends on the filtered load (the product of the glomerular filtration rate and plasma phosphate concentration), urinary phosphate levels may be reduced despite the presence of hyperparathyroidism, when serum phosphate levels are particularly low. Expressions of renal phosphate clearance that account for these variables (eg, renal threshold for phosphate, or TmP/GFR) are a better indicator of renal phosphate handling than is total phosphate excretion. The TmP/GFR can be calculated from a nomogram using measurements of a fasting serum and urine phosphorus concentration.
- Histologic examination—Transcortical bone biopsy is the definitive means of making the diagnosis of osteomalacia. A rib or the iliac crest is the site at
which biopsy is usually performed. To assess osteoid content and mineral appositional rate, the bone biopsy specimen is processed without decalcification. This requires special equipment. In osteomalacia, bone is mineralized poorly and slowly, resulting in wide osteoid seams (>12 ľm) and a large fraction of bone covered by unmineralized osteoid. States of high bone turnover (increased bone formation and resorption), such as hyperparathyroidism, can also cause wide osteoid seams and an increased osteoid surface, producing a superficial resemblance to osteomalacia. Therefore, the rate of bone turnover should be determined by labeling bone with tetracycline, which provides a fluorescent marker of the calcification front. When two doses of tetracycline are given at different times, the distance between the two labels divided by the time interval between the two doses equals the mineral appositional rate. The normal appositional rate is approximately 0.74 ľm/d. Mineralization lag time—the time required for newly formed osteoid to be mineralized—can be calculated by dividing osteoid seam width by the appositional rate corrected by the linear extent of mineralization of the calcification front (a measure of the bone surface that is undergoing active mineralization as measured by tetracycline incorporation). Mineralization lag time is normally about 20–25 days. Bone formation rate is calculated as the product of the appositional rate times the linear extent of mineralization of the calcification front. Depressed appositional rate, increased mineralization lag time, and reduced bone formation rate clearly distinguish osteomalacia from high-turnover states such as hyperparathyroidism. Low-turnover states, as can be seen in various forms of osteoporosis, also have low appositional and bone formation rates, but these conditions are distinguished from osteomalacia by normal or reduced osteoid surface and volume.
- IMAGING STUDIES
The radiologic features of rickets can be quite striking, especially in the young child. In growing bone, the radiolucent epiphyses are wide and flared, with irregular epiphysial-metaphysial junctions. Long bones may be bowed. The cortices of the long bones are often indistinct. Occasionally, evidence of secondary hyperparathyroidism—subperiosteal resorption in the phalanges and metacarpals and erosion of the distal ends of the clavicles—is observed. Pseudofractures (also known as Looser's zones or Milkman's fractures) are an uncommon but nearly pathognomonic feature of rickets and osteomalacia (Figure 8-30). These radiolucent lines are most often found along the concave side of the femoral neck, pubic rami, ribs, clavicles, and lateral aspects of the scapulae. Pseudofractures may result from unhealed microfractures at points of stress or at the entry point of blood vessels into bone. They may progress to complete fractures that go unrecognized and thus lead to substantial deformity and disability. Bone density is not a reliable indicator of osteomalacia, since bone density can be decreased in patients with vitamin D deficiency or increased in patients with chronic renal failure. In adults with normal renal function, radiologic evidence of a mineralization defect is often subtle and not readily distinguished from osteoporosis, with which it often coexists.
Figure 8-30. X-ray of the pelvis of an elderly woman with osteomalacia. Note marked bowing of both femoral necks, with pseudofractures of the medial aspect of the femoral necks and the superior aspect of the left pubic ramus (arrows). (Photograph courtesy of H Genant.)
The treatment of vitamin D deficiency is covered under that heading. The treatment of other diseases causing rickets/osteomalacia is found below under the specific diseases.
Nephrotic syndrome may lead to osteomalacia because of losses of vitamin D metabolites in the urine. Vitamin D metabolites are tightly bound to DBP, an α-globulin, and less tightly bound to albumin as described above. Patients with the nephrotic syndrome may lose large amounts of DBP and albumin in their urine and so deplete their vitamin D stores. Such patients can have very low levels of vitamin D metabolites in their serum, though the free concentrations are less affected. Thus, the measurement of total 25(OH)D or 1,25(OH)2D3 may be misleading with respect to the severity of the vitamin D deficiency. Although bone disease has been recognized as a complication of the
nephrotic syndrome, the prevalence of osteomalacia in this population is unknown. If vitamin D deficiency is suspected, treatment with vitamin D is indicated with the proviso that normal total levels of 25(OH)D and 1,25(OH)2D3 are not the goal. Blood levels of calcium, phosphorus, and PTH are a more reliable guide to treatment.
Hepatic osteodystrophy is the bone disease associated with liver failure. In the United States, patients with liver failure generally have osteoporosis, not osteomalacia. Both osteomalacia and osteoporosis are found in patients with liver disease in Great Britain, where vitamin D deficiency is more common. Although the liver is the site of the first step in the bioactivation of vitamin D—the conversion of vitamin D to 25(OH)D—this process is not tightly controlled, and until the liver disease is severe it is not rate-limiting. Neither cholestatic nor parenchymal liver disease has much effect on 25(OH)D production until the late stages of liver failure. The low levels of 25(OH)D and 1,25(OH)2D3associated with liver disease can usually be attributed to the reduced production of DBP and albumin, poor nutrition, or malabsorption rather than a deficiency in the vitamin D 25-hydroxylase. As in patients with the nephrotic syndrome, the low total levels of the vitamin D metabolites may be misleading, as they reflect a reduction in DBP and albumin rather than a reduction in the free concentrations of these metabolites.
Phenytoin and phenobarbital are anticonvulsants that induce drug-metabolizing enzymes in the liver which alter the hepatic metabolism of vitamin D and its metabolites. This action is not limited to anticonvulsants as the antituberculosis drug rifampin has been reported to do likewise. This effect may account for the lower circulating levels of 25(OH)D found in patients treated with such drugs. Levels of 1,25(OH)2D3 are less affected. Chronic anticonvulsant or antituberculosis therapy does not appear to lead to clinically significant bone disease except in subjects with other predisposing factors such as inadequate sunlight exposure (institutionalized patients) and poor nutrition. Children may be more vulnerable than adults. In animal studies, phenytoin has been noted to exert a direct inhibitory effect on bone mineralization, but the relevance of this observation to the human use of this drug is uncertain. The decrease in 25(OH)D levels in patients taking these drugs can be readily reversed with supplemental vitamin D administration.
Chronic hypophosphatemia may lead to rickets or osteomalacia independently of other predisposing abnormalities. The principal diseases in which hypophosphatemia is associated with osteomalacia or rickets, however, also include other abnormalities that can interfere with bone mineralization. Chronic phosphate depletion is caused by dietary deficiency (as in strict vegetarians), decreased intestinal absorption, or increased renal clearance (renal wasting). Acute hypophosphatemia can result from movement of phosphate into cells (eg, after infusion of insulin and glucose), but this condition is transient and does not result in bone disease.
Seventy to 90 percent of dietary phosphate is absorbed under normal conditions in the jejunum. This process is not tightly regulated, though 1,25(OH)2D3 stimulates phosphate absorption, a factor that needs to be considered when calcitriol is being used to treat other conditions. Meat and dairy products are the principal dietary sources of phosphate. The incidence of osteomalacia in vegetarians who avoid all meat and dairy products is unknown, but its occurrence has been reported. Intrinsic small bowel disease and small bowel surgery interfere with phosphate absorption and, if coupled with diarrhea or steatorrhea, can result in phosphate depletion. A number of widely used antacids (eg, Mylanta, Maalox, Basaljel, and Amphojel) contain aluminum hydroxide, which binds phosphate and prevents its absorption. Patients who ingest large amounts of these antacids may become phosphate-depleted. When this occurs in the setting of renal failure—in which these antacids are used to control serum phosphate—overzealous reduction in serum phosphate plus the added insult of aluminum intoxication in these individuals, who also have reduced 1,25(OH)2D3 production, can produce profound osteomalacia. Eighty-five to 90 percent of the phosphate filtered by the glomerulus is reabsorbed, primarily in the proximal tubule. This process is regulated by PTH, which reduces renal tubular phosphate reabsorption, and probably also by various vitamin D metabolites that appear to increase renal tubular phosphate reabsorption. Many diseases that affect renal handling of phosphate are associated with osteomalacia—especially those also associated with abnormalities in vitamin D metabolism, as discussed below.
Treatment of phosphate deficiency is generally geared to correction of the primary problem. Oral preparations of phosphate (and the amounts required to provide 1 g of elemental phosphorus) include Fleet Phospho-soda (6.12 mL) and Neutra-Phos (300 mL). These preparations are usually given in amounts that provide 1–3 g of phosphorus daily in divided doses,
though diarrhea may limit the dose. Careful attention to both serum calcium and serum phosphorus concentrations is required to avoid hypocalcemia or ectopic calcification.
X-Linked & Autosomal Dominant Hypophosphatemia
X-linked hypophosphatemia (formerly called vitamin D-resistant rickets) is characterized by renal phosphate wasting, hypophosphatemia, and decreased 1,25(OH)2 D3 production relative to the degree of hypophosphatemia. Clinical presentation is variable, but children often present with florid rickets. This dominant disorder affects males more severely than females. A similar but genetically distinct syndrome, autosomal dominant hypophosphatemic rickets, is less common, affects females to the same extent as males, and may present later in life (second to fourth decades). Animal studies have indicated that the cause of the renal phosphate wasting in X-linked hypophosphatemia is humoral rather than a structural defect in the phosphate transporter itself. Recent evidence strongly suggests that this humoral factor is FGF23. It is presumed to be responsible for both the reduction in phosphate reabsorption in the renal proximal tubule as well as the inadequate response of the proximal tubule to the ensuing hypophosphatemia with respect to 1,25(OH)2 D3 production. The gene responsible for X-linked hypophosphatemia has recently been identified and is called PHEX (originally called PEX) for phosphate-regulating gene with homologies to endopeptidases located on the X chromosome. As is implicit in this ambiguous name, the role of PHEX in renal phosphate transport is not clear. The prevailing hypothesis is that PHEX cleaves and so inactivates FGF23, thus relieving the inhibition of phosphate reabsorption and 1,25(OH)2 production. Autosomal dominant hypophosphatemic rickets appears to be due to a mutation in FGF23 that renders it resistant to cleavage by PHEX. Recent evidence suggests that the defects in X-linked hypophosphatemia and autosomal dominant hypophosphatemia may not be restricted to the proximal renal tubule but may affect osteoblast function as well, a cell in which PHEX is expressed. With the recent cloning of PHEX and FGF23, our understanding of this disease should rapidly increase. Treatment of X-linked hypophosphatemia generally requires a combination of phosphate (1–4 g/d in divided doses) combined with calcitriol (1–3 ľg/d) as tolerated. Vitamin D is less effective than calcitriol and should no longer be used for this condition. Appropriate therapy heals the rachitic lesions and increases growth velocity. A frequent complication of this treatment, however, is the development of hyperparathyroidism, which may become autonomous and require parathyroidectomy. Treatment with phosphate and calcitriol may also lead to ectopic calcification, including nephrocalcinosis. Thus, these patients need to be followed closely.
The association of osteomalacia and hypophosphatemia with tumors primarily of mesenchymal origin has been recognized for half a century. The cause of this syndrome has until recently remained obscure. Removal of the tumors, which are often small and hard to find, cures the disease. Although most implicated tumors are mesenchymal, including fibromas and osteoblastomas, other tumors, including breast, prostate, and lung carcinomas, multiple myeloma, and chronic lymphocytic leukemia, have been associated with this syndrome. The patient generally presents with bone pain, muscle weakness, and osteomalacia. Symptoms may occur for years before the diagnosis is made. Renal phosphate wasting, hypophosphatemia, and normal serum calcium and 25(OH)D levels but inappropriately low 1,25(OH)2D3 levels characterize the disease. Thus, it resembles X-linked hypophosphatemia or autosomal dominant hypophosphatemic rickets. Tumor extracts contain material—as yet not fully characterized—which inhibits renal phosphate transport and 1,25 (OH)2D3 production, suggesting that a phosphatonin-like substance is involved. Thus, it was of great interest to observe that these tumors overproduce FGF23. Recent evidence strongly indicates that production by the tumor of one species of fibroblast growth factor, FGF-23, underlies all of the metabolic abnormalities associated with hypophosphatemic syndrome.
The best treatment is removal of the tumor, but this is not always possible. Phosphate and calcitriol have been the mainstays of treatment but with limited success, and patients should be dosed to tolerance as in X-linked hypophosphatemia. Recently, the demonstration of somatostatin receptors on one such tumor led to successful correction of hypophosphatemia by use of the somatostatin analog octreotide.
De Toni-Debré-Fanconi Syndrome & Hereditary Hypophosphatemic Rickets with Hypercalciuria
The De Toni-Debré-Fanconi syndrome includes a heterogeneous group of disorders affecting the proximal tubule and leading to phosphaturia and hypophosphatemia, aminoaciduria, glycosuria, bicarbonaturia, and proximal renal tubular acidosis. Not all features must be present to make the diagnosis. Damage to the renal proximal tubule secondary to genetic or environmental causes is the underlying cause. This syndrome
may be divided into two types depending on whether vitamin D metabolism is also abnormal. In the more common type I, 1,25(OH)2D3production is reduced relative to the degree of hypophosphatemia. In type II disease, 1,25(OH)2D3 production is appropriately elevated in response to the hypophosphatemia, and this leads to hypercalciuria.
The type II Fanconi's syndrome shares the principal features of hereditary hypophosphatemic rickets with hypercalciuria. It is a rare disorder of uncertain genetic mode of transmission that is heterogeneous in clinical presentation but in severely affected individuals begins in childhood with bone pain and skeletal deformities. The syndrome is characterized by renal phosphate wasting and hypophosphatemia, hypercalciuria and normal serum calcium, and elevated 1,25(OH)2D3 levels. The osteomalacia associated with these renal phosphate-wasting syndromes is probably the result of the hypophosphatemia, with contributions from the acidosis and decreased 1,25(OH)2D3 levels in the type I syndrome.
The ideal treatment is correction of the underlying defect, which may or may not be identified and reversible. Otherwise, treatment includes phosphate supplementation in all cases, correction of the acidosis, and 1,25(OH)2D3 replacement in the type I syndrome.
Calcium deficiency may contribute to the mineralization defect that complicates gastrointestinal disease and proximal tubular disorders, but it is less well established as a cause of osteomalacia than is vitamin D or phosphate deficiency. In carefully performed studies of children who ingested a low-calcium diet in Africa, there was clinical, biochemical, and histologic evidence of osteomalacia. The serum phosphorus and 25(OH)D and 1,25(OH)2D3 levels were normal; serum alkaline phosphatase level was elevated; and serum and urine calcium levels were low. Since intestinal absorption of calcium decreases with age, the daily requirement for calcium increases from approximately 800 mg in young adults to 1400 mg in the elderly. Calcium deficiency can result not only from inadequate dietary intake but also from excessive fecal and urinary losses. Except in cases in which a renal leak of calcium plays an important role in the etiology of calcium deficiency (certain forms of idiopathic hypercalciuria or following glucocorticoid therapy for inflammatory diseases), urinary calcium excretion provides a useful means to determine the appropriate level of oral calcium replacement. Because of its low cost and high percentage of elemental calcium, calcium carbonate is the treatment of choice.
PRIMARY DISORDERS OF THE BONE MATRIX
Several diseases lead to abnormalities in bone mineralization because of intrinsic defects in the matrix or in the osteoblast producing that matrix. With the exception of osteogenesis imperfecta, these tend to be rare. Examples are listed below.
Osteogenesis imperfecta is a hereditable disorder of connective tissue due to a qualitative or quantitative abnormality in type I collagen, the most abundant collagen in bone. The disease is typically transmitted in an autosomal dominant mode, though autosomal recessive inheritance has also been described. Clinical expression is highly variable, depending on the location and type of mutation observed. Although skeletal deformity and fracture are the hallmarks of this disease, other tissues are often affected, including the teeth, skin, ligaments, and scleras. Routine biochemical studies of bone and mineral metabolism are generally normal, though elevated serum alkaline phosphatase and increased urine excretion of hydroxyproline and calcium are often found. Bone histology shows abundant disorganized, poorly mineralized matrix but with a high rate of bone turnover as reflected by increased tetracycline labeling.
Treatment is supportive and includes orthopedic, rehabilitative, and dental intervention as appropriate. Bisphosphonates have recently been shown to be of use.
Hypophosphatasia, transmitted in an autosomal recessive or dominant pattern, has considerable variability in clinical expression ranging from a severe form of rickets in children to a predisposition to fractures in adults. The biochemical hallmarks are low serum and tissue levels of alkaline phosphatase (liver-bone-kidney but not intestine-placenta) and increased urinary levels of phosphoethanolamine. Serum calcium and phosphorus levels may be high, and many patients have hypercalciuria. Levels of vitamin D metabolites and PTH are generally normal. In some families, a mutation in the alkaline phosphatase gene has been identified. Radiologic examination shows osteopenia, with the frequent occurrence of stress fractures and chondrocalcinosis. Bone biopsy specimens show osteomalacia. It is not clear why these patients develop osteomalacia or rickets. Skeletal alkaline phosphatase cleaves pyrophosphate, an inhibitor of bone mineralization. Thus, patients deficient in alkaline phosphatase may be unable to hydrolyze this inhibitor and so develop a mineralization defect.
There is no established medical therapy. Use of vitamin D or its metabolites may further increase the already elevated serum calcium and phosphorus and should not be used.
Fibrogenesis Imperfecta Ossium
Fibrogenesis imperfecta ossium is a rare, painful disorder that affects middle-aged subjects in what appears to be a sporadic fashion. Serum alkaline phosphatase activity is increased. The bones have a dense, amorphous mottled appearance radiologically and a disorganized arrangement of collagen with decreased birefringence histologically when viewed with polarized light microscopy. Presumably, the disorganized collagen matrix retards normal bone mineralization.
There is no specific therapy.
This rare disease involves only the axial skeleton and affects mostly middle aged or elderly males. Most cases are sporadic. It is not particularly painful. Alkaline phosphatase activity may be increased, but calcium, phosphorus, and vitamin D metabolite levels are normal. Histologically, the bone shows increased osteoid and reduced tetracycline incorporation. The reason for the mineralization disorder is uncertain.
No effective medical therapy has been established.
INHIBITORS OF MINERALIZATION
Several drugs are known to cause osteomalacia or rickets by inhibiting mineralization. However, the mechanisms by which this occurs are not fully understood.
Aluminum-induced bone disease is found primarily in patients with renal failure on chronic hemodialysis and in patients being treated with total parenteral nutrition (TPN). With the recognition of this complication and the switch to deionized water for dialysis, decreased use of aluminum-containing antacids, and the use of purified amino acids rather than casein hydrolysates (which contained large amounts of aluminum) for TPN, the incidence and prevalence of this complication has been markedly reduced. The problem, however, still exists. As described under renal osteodystrophy, the bone disease is osteomalacia and can be severe and painful. Aluminum deposits in the mineralization front and appears to inhibit the mineralization process.
Removal of the aluminum with deferoxamine chelation therapy is the treatment of choice.
This first-generation bisphosphonate possesses an unwelcome side effect: the ability to inhibit bone mineralization at doses greater than 5–10 mg/kg/d. This side effect is not found at clinically effective doses of newer bisphosphonates.
Fluoride is a potent stimulator of bone formation. If administered at high doses with inadequate calcium supplementation, the bone formed as a result is poorly mineralized. The mechanisms underlying the effects of fluoride on osteoblast function and bone mineralization remain unclear.
PAGET'S DISEASE OF BONE(OSTEITIS DEFORMANS)
Paget's disease is a focal disorder of bone remodeling that leads to greatly accelerated rates of bone turnover, disruption of the normal architecture of bone, and sometimes to gross deformities of bone. As a focal disorder it is not, strictly speaking, a metabolic bone disease. Paget's disease is highly prevalent in northern Europe, particularly in England and Germany, where up to 4% of people over age 40 are affected. It is also common in the United States but is unusual in Africa and Asia.
It has long been thought that Paget's disease, with its late onset and spotty involvement of the skeleton, might be due to a chronic slow virus infection of bone. Inclusion bodies that resemble paramyxovirus inclusions have been identified in pagetic osteoclasts, and the presence of measles virus transcripts has recently been detected by molecular biologic techniques. However, considerably more work would be required to prove the infectious etiology of Paget's disease. There are also familial clusters of the disease, with up to 20% of patients in some studies having afflicted first-degree relatives.
At the microscopic level, the disorder is characterized by highly vascular and cellular bone, consistent with its high metabolic activity. The osteoclasts are sometimes huge and bizarre, with up to 100 nuclei per cell. Because pagetic osteoclasts initiate the bone remodeling cycle in a chaotic fashion, the end result of remodeling is a mosaic
pattern of lamellar bone. Paget's disease (and other high turnover states) can also produce woven bone—bone that is laid down rapidly and in a disorganized fashion, without the normal lamellar architecture.
Abnormal osteoclastic bone resorption is the probable initiating event in Paget's disease. Not only are the osteoclasts very abnormal histologically and prone to unruly behavior, but some forms of the disease are marked by an early resorptive phase in which pure osteolysis occurs without an osteoblastic response. Additionally, Paget's disease responds dramatically to inhibitors of osteoclastic bone resorption.
The rate of bone resorption is often increased by as much as 10- to 20-fold, and this is reflected in biochemical indices of bone resorption, including urinary excretion of collagen metabolites like hydroxyproline and pyridinoline cross-linked peptides of collagen. Over the skeleton as a whole, osteoblastic new bone formation responds appropriately to this challenge. Even though local disparities in remodeling may result in areas with the radiographic appearance of osteolysis or dense new bone, there is a linear relationship between biochemical markers of bone resorption (eg, urinary hydroxyproline) and biochemical markers of bone formation (eg, alkaline phosphatase). Because this tight coupling is maintained in Paget's disease in the face of enormously increased skeletal turnover rates, systemic mineral homeostasis is usually unperturbed.
- SYMPTOMS AND SIGNS
Paget's disease may affect any of the bones, but the most common sites are the sacrum and spine (50% of patients), the femur (46%), the skull (28%), and the pelvis (22%). The clinical features of Paget's disease are pain, fractures, deformity, and manifestations of the neurologic, rheumatologic, or metabolic complications of the disease. However, at least two-thirds of patients are asymptomatic. Thus, Paget's disease is often discovered as an incidental radiologic finding or during the investigation of an elevated alkaline phosphatase level. On physical examination, enlargement of the skull, frontal bossing, or deafness may be evident. Involvement of the weight-bearing long bones of the lower extremity often results in bowing. The femur and tibia bow anteriorly and laterally, but the fibula is almost never affected by Paget's disease. Cutaneous erythema and warmth, as well as bone tenderness, may be evident over affected areas of the skeleton, reflecting greatly increased blood flow through pagetic bone. The findings of pain, warmth, and erythema led to the appellation osteitis deformans, though Paget's disease is not truly an inflammatory disorder. The most common fractures in Paget's disease are vertebral crush fractures and incomplete “fissure” fractures through the cortex, usually on the convex surface of the tibia or femur. Affected bones may fracture completely; when they do, healing is usually rapid and complete—the increased metabolic activity of pagetic bone seems to favor fracture healing.
- LABORATORY FINDINGS
The serum alkaline phosphatase activity and urinary hydroxyproline excretion are usually increased, sometimes to a very high degree. Levels of the newer biochemical markers of bone turnover are also high, but it is not clear that their determination offers any special benefit. The serum osteocalcin concentration is usually increased to a lesser extent than the alkaline phosphatase activity. The serum calcium and phosphorus concentrations and the urinary calcium excretion are normal, though if a patient sustains a fracture and becomes immobilized, hypercalciuria and hypercalcemia may occur.
- IMAGING STUDIES
The early stages of Paget's disease are often osteolytic. Examples are erosion of the temporal bone of the skull, osteoporosis circumscripta, and pagetic lesions in the extremities, which begin in the metaphysis and migrate down the shaft as a V-shaped resorptive front (Figure 8-31). Over years or even decades, the typical mixed picture of late Paget's disease evolves. Trabeculae are thickened and coarse. The bone may be enlarged or bowed. In the pelvis, the iliopectineal line or pelvic brim is often thickened (Figure 8-32). In the spine, osteoblastic lesions of the vertebral bodies may present a “picture-frame” appearance or a homogeneously increased density, the “ivory vertebra.” Associated osteoarthritis may present with narrowing of the joint space (Figure 8-32). Osteosarcoma may present with cortical destruction or a soft tissue mass (Figure 8-32). Radionuclide bone scanning with technetium-labeled bisphosphonates or other bone-seeking agents is uniformly positive in active Paget's disease and is useful for surveying the skeleton when a focus of Paget's disease has been found radiographically (Figure 8-33).
Complications of Paget's disease may be neurologic, rheumatologic, neoplastic, or cardiac (Table 8-18).
The brain, spinal cord, and peripheral nerves are all at risk. Sensorineural deafness occurs in up to 50% of patients
in whom the skull is involved, and compression of the other cranial nerves can also occur. At the base of the skull, Paget's disease can produce platybasia and basilar impression of the brain stem, with symptoms of brain stem compression, obstructive hydrocephalus, or vertebrobasilar insufficiency. Spinal stenosis is common in vertebral Paget's disease, in part because the pagetic vertebra can be enlarged, and may spread posteriorly when collapse occurs, but spinal stenosis responds well to medical treatment of the disease. Peripheral nerve entrapment syndromes include carpal and tarsal tunnel syndromes.
Figure 8-31. Lytic Paget's disease in the tibia before (left) and after (right) immobilization in a cast. The lytic area has a flame- or V-shaped leading edge (left). (Reproduced, with permission, from Strewler GJ: Paget's disease of bone. West J Med 1984;140:763.)
Osteoarthritis is common in Paget's disease. It may be an unrelated finding in elderly patients with the disorder,
or it may result directly from pagetic deformities and their effects on wear and tear in the joints. Arthritis presents a conundrum to the clinician attempting to relieve pain, as it may be difficult to determine whether the pain originates in pagetic bone or in the nearby joint. An association of osteitis deformans and gout was first noted by James Paget himself, and asymptomatic hyperuricemia is also common.
Figure 8-32. Paget's disease of the right femur and pelvis. The right femur displays cortical thickening and coarse trabeculation. The right ischium is enlarged, with sclerosis of ischial and pubic rami and the right ilium. Two complications of Paget's disease are present. There is concentric bilateral narrowing of the hip joint space, signifying osteoarthritis. The destructive lesion interrupting the cortex of the right ilium is an osteosarcoma. (Reproduced, with permission, from Strewler GJ: Paget's disease of bone. West J Med 1984;140:763.)
The most terrible complication of Paget's disease is development of bone sarcoma. The tumor arises in pagetic bone, typically in individuals with polyostotic involvement, and may present with soft tissue swelling, increased pain, or a rapidly increasing alkaline phosphatase. Osteosarcoma, chondrosarcoma, and giant cell tumors all occur in Paget's disease, with a combined incidence of about 1%. Because osteosarcoma is otherwise uncommon in the elderly, fully 30% of elderly patients with osteosarcoma have underlying Paget's disease.
High-output congestive heart failure occurs rarely and is due to markedly increased blood flow to bone, usually in patients with more than 50% involvement of the skeleton.
In a patient known to have Paget's disease, bone pain that is unresponsive to nonsteroidal anti-inflammatory
agents deserves a trial of specific therapy. As noted above, it may not be easy to differentiate pagetic bone pain from that due to osteoarthritis. The other indications for treatment of Paget's disease are controversial. Treatment has been advocated for neurologic compression syndromes, as preparation for surgery, and to prevent deformities. Neurologic deficits often respond to medical treatment, and a trial of treatment is often warranted. Pretreatment for 2–3 months before orthopedic surgery will prevent excessive bleeding and postoperative hypercalcemia, but satisfactory bone healing usually occurs without medical treatment. Paget's disease is sometimes treated in the hope of arresting the progress of deformities (eg, bowing of the extremities and resultant osteoarthritis), but it is not certain whether medical treatment can achieve this aim or whether it will arrest the progression of deafness in patients with skull involvement.
Figure 8-33. Bone scan of a patient with Paget's disease of the skull, spine, pelvis, right femur, and acetabulum. Note localization of bone-seeking isotope (99mTc-labeled bisphosphonate) in these areas.
Three classes of agents are used in the treatment of Paget's disease: the calcitonins, the bisphosphonates, and plicamycin. All are inhibitors of osteoclastic bone resorption.
Table 8-18. Complications of Paget's disease.
Calcitonin-salmon is administered initially at a dosage of 50–100 IU daily until symptoms are improved; thereafter, many patients can be maintained on 50 units three times a week. Improvement in pain is usually evident within 2–6 weeks. On average, the alkaline phosphatase and urinary hydroxyproline will fall by 50% within 3–6 months, with the alkaline phosphatase lagging slightly behind the hydroxyproline. Many patients will have a sustained response to treatment extending over years, and the biochemical parameters are often suppressed for 6 months to 1 year after treatment is discontinued. Up to 20% of patients receiving chronic calcitonin treatment will develop late resistance to calcitonin, which may be antibody-mediated. Human calcitonin therapy has been uniformly effective in these patients. A nasal spray preparation is now available, but data on its efficacy in Paget's disease are limited.
The bisphosphonate etidronate disodium has long been available for treatment of Paget's disease and was shown
to be beneficial in controlled clinical trials. It is administered in a dose of 5 mg/kg/d for 6 months, and about 60% of patients will show a biochemical response. However, some patients will experience worsening of bone pain or lytic bone lesions, and etidronate can impair bone mineralization, particularly at higher doses.
Three newer oral bisphosphonates are available for treatment of Paget's disease and appear to offer advantages over etidronate, both in terms of drug potency and in a decreased risk for impaired bone mineralization. Alendronate is administered at a dose of 40 mg daily for 6 months. On average, alkaline phosphatase activity is suppressed by about 80% on such treatment. Biochemical remissions are often prolonged for more than 1 year after the drug is stopped. The main side effect is significant gastrointestinal upset, requiring discontinuation of therapy in about 6% of patients. Two additional oral bisphosphonates have shown treatment efficacy and are approved for treatment of Paget's disease: tiludronate, given at a dosage of 400 mg/d for 6–12 months; and risedronate, given at a dosage of 30 mg/d for 2 months. For some patients, particularly those with gastrointestinal intolerance to oral bisphosphonates, it may be more convenient or more timely to administer an equivalent dose of bisphosphonate intravenously. Pamidronate, an amino-bisphosphonate closely related to alendronate, has been used for this purpose. In the United States, pamidronate is approved only for treatment of hypercalcemia. Intravenous infusions of pamidronate (60 or 90 mg) produce a high remission rate and a durable response. The principal side-effect observed with intravenous administration is an acute phase response, including fever and myalgia, that occurs in about 20% of patients and may last for several days after a dose.
Plicamycin is a cytotoxic antibiotic that has been used as a “last resort” in Paget's disease unresponsive to less toxic agents. With the advent of the newer bisphosphonates, there are few indications for treatment with plicamycin.
Metabolism of 25(OH)D to 1,25(OH)2D3 and 24,25 (OH)2D in the kidney is tightly regulated. Renal disease results in reduced circulating levels of both of these metabolites. With the reduction in 1,25(OH)2D3 levels, intestinal calcium absorption falls, and bone resorption appears to become less sensitive to PTH—a result that leads to hypocalcemia. Phosphate excretion by the diseased kidney is decreased, resulting in hyperphosphatemia, which amplifies the fall in serum calcium. The fall in serum calcium combined with the low levels of 1,25(OH)2D3 (which is an inhibitor of PTH secretion) results in hyperparathyroidism. The net effect of deficient 1,25(OH)2D3 and 24,25(OH)2D and excess PTH on bone is complex (Figure 8-34). Patients may have osteitis fibrosa (reflecting excessive PTH), osteomalacia (in part reflecting decreased vitamin D metabolites), or a combination of the two. One particularly debilitating form of renal osteodystrophy is found in a small percentage of patients on hemodialysis in whom only osteomalacia occurs. Some of these patients have increased aluminum content in their bones, particularly in the region where mineralization is occurring (calcification front), and this appears to inhibit mineralization. Other patients do not have aluminum excess but have low bone turnover. These patients are particularly prone to develop symptoms of bone pain, fractures, and muscle weakness.
Most patients with renal osteodystrophy have osteitis fibrosa alone or in combination with osteomalacia. If not well controlled, these patients will have a low serum level of calcium and high serum levels of phosphorus, alkaline phosphatase, and PTH. A few patients develop severe secondary hyperparathyroidism in which the PTH level increases dramatically and stays elevated even with restoration of the serum calcium to normal. Such patients are prone to developing hypercalcemia with the usual replacement doses of calcium (sometimes
referred to as tertiary hyperparathyroidism). Another subset of patients with renal osteodystrophy present with relatively normal levels of PTH and alkaline phosphatase. Their serum calcium levels are often elevated after treatment with small doses of calcitriol. These patients have pure osteomalacia on bone biopsy and generally suffer from aluminum intoxication (see above).
Figure 8-34. Schema for the pathogenesis of renal osteodystrophy.
Patients with renal osteodystrophy generally respond to calcitriol (0.5–1 ľg/d) or dihydrotachysterol (0.25–0.5 mg/d), calcium supplementation (1–3 g/d), and phosphate restriction. Because of the concern about aluminum intoxication, calcium carbonate is generally used as a first agent to block intestinal phosphate absorption, reserving aluminum hydroxide for situations in which calcium carbonate is not effective in maintaining normal serum phosphorus levels. The goal is to achieve and maintain normal serum levels of calcium, phosphorus, PTH, and alkaline phosphatase. This regimen treats osteitis fibrosa more effectively than osteomalacia. Patients with aluminum intoxication are very sensitive to and respond poorly to calcitriol and calcium, with rapid onset of hypercalcemia and little improvement in their bone disease. These patients may respond to chelation therapy with deferoxamine, a drug also used for iron chelation. Severe secondary hyperparathyroidism also creates management problems, as these patients also are prone to the development of hypercalcemia with calcium supplementation. Intravenous doses of calcitriol following hemodialysis may preferentially inhibit PTH secretion with less effect on raising serum calcium. Several analogs of calcitriol, which have less effect on raising serum calcium than calcitriol itself, are currently approved or in clinical trials for the management of such patients, as is a calcimimetic compound that inhibits PTH secretion by activating the calcium receptor of the parathyroid gland.
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