Ad R. Hermus
Dyde A. Huysmans
Nontoxic goiter may be defined as a diffuse or nodular enlargement of the thyroid gland that is not associated with thyrotoxicosis and that does not result from an autoimmune or inflammatory process. The term “endemic goiter” is used when goiter prevalence in children 6 to 12 years of age within a population is more than 10%. Goiter is called “sporadic” when this prevalence is 10% or less.
Both environmental and genetic factors play a role in the pathogenesis of endemic and sporadic goiter. Worldwide, the most important environmental factor contributing to both endemic and sporadic goiter is iodine deficiency (see section Iodine Deficiency in Chapter 11). In some iodine-deficient areas, ingestion of certain plants that contain goitrogenic substances may contribute to the growth of goiter, but there is little evidence they do so in the absence of iodine deficiency. Other suggested risk factors for the development and growth of nontoxic goiter are cigarette smoking (1) and goitrogenic drugs, such as lithium and aminoglutethimide (see section Effects of Drugs and Other Substances on Thyroid Hormone Synthesis and Metabolism in Chapter 11 and see Chapter 50). Constitutional factors like sex are also important in the etiology of nontoxic goiter—nontoxic goiters are 5 to 10 times more common in women than men. Various thyroid enzymatic disorders, when partial, may cause familial goitrogenesis (see Chapter 48). Higher concordance rates for nontoxic goiter in female monozygotic twins than in dizygotic twins have provided strong evidence of a genetic component in the etiology of nontoxic goiter both in endemic (2,3) and in nonendemic (4) areas (see Chapter 20). Two regions of interest, one on chromosome 14 (5,6) and one on the X chromosome (7), have been linked with sporadic goiter, but it is unlikely that genes from these regions play a major role in the pathogenesis of these goiters (8).
NATURAL HISTORY OF NONTOXIC GOITER
In the early phase of goitrogenesis, nontoxic goiters are diffuse goiters. In areas of iodine deficiency with a high prevalence of goiter, many prepubertal children have diffuse goiters, although these goiters may regress in early adolescence. In areas with a lower prevalence of goiter, goitrogenesis usually starts at an older age. With time, diffuse goiters tend not only to grow but also to become nodular. Concomitantly, thyroid function often becomes autonomous, that is, thyroid hormone secretion becomes independent of thyrotropin (TSH) secretion, and therefore the patients gradually develop subclinical thyrotoxicosis and eventually overt thyrotoxicosis. The rate of progression from euthyroidism to thyrotoxicosis in patients with a nodular goiter has not been studied extensively, but in two studies the incidence of overt thyrotoxicosis was approximately 10% during 7 to 12 years of follow-up (9,10).
In a cross-sectional study of patients with nontoxic goiter, evidence for thyroid growth, nodule formation, and development of functional autonomy with age was found (11). Thyroid volume was positively correlated with age and duration of goiter. The patients with a multinodular goiter were older and had a larger thyroid volume and significantly lower serum TSH concentrations than those with a diffuse or uninodular goiter. In another cross-sectional study of patients with nontoxic goiter, there was a significant negative correlation between the number of nodules as identified by ultrasonography and serum TSH concentrations (12).
Thus, the natural history of nontoxic goiter is characterized clinically by gradual thyroid growth, nodule formation, and the development of functional autonomy. Another well-known characteristic is increasing functional heterogeneity within the thyroid gland during goitrogenesis. In this chapter, the pathogenic basis of these four typical features of nontoxic goiter will be discussed.
The increased thyroid mass of a nontoxic goiter is mainly caused by excessive cell replication, as demonstrated by the finding of a highly significant positive correlation between the total DNA content of nodular goiters and their weight (13). Histologically, the newly generated cells appear to be mainly thyroid follicular cells, and increases in interstitial tissue and colloid contribute little to goiter growth (13) (see Chapter 21).
Thyroid Growth-Stimulating Factors
Several thyroid growth-stimulating factors are thought to be of importance for the increased follicular cell replication that leads to the formation of nontoxic goiters. These factors may be derived either from the systemic circulation (endocrine action) or from thyroid follicular or stromal cells (autocrine or paracrine action).
Thyrotropin (TSH) is the main extrathyroidal thyroid growth-stimulating factor (14,15). TSH stimulation plays an important role in the pathogenesis of iodine deficiency goiter. A decrease in iodine intake leads to a decrease in the synthesis and secretion of thyroxine (T4) and triiodothyronine (T3). As a result, serum TSH concentrations increase, stimulating thyroid growth (see section Biological Actions of Thyrotropin in Chapter 10). The increases must be relatively short lived because most patients with nontoxic goiter have normal serum TSH concentrations. Furthermore, nontoxic goiters may grow despite administration of T4in doses that reduce serum TSH concentrations to well below normal. These findings suggest that other thyroid growth-stimulating factors are involved in goiter growth (16).
Growth factors such as insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), and fibroblast growth factor (FGF) may be important for stimulation of thyroid growth (14,15,17). In vitro, IGF-1, EGF, and FGF stimulate proliferation of thyroid follicular cells (14,15,16,17,18,19). In vivo, intravenous administration of FGF in rats resulted in an increase in thyroid weight (20). The expression of thyroid growth-stimulating factors such as IGF-1 and FGF is increased in nodular goiters in humans (21,22). A role for IGF-1 in goitrogenesis in humans is also supported by the finding that serum IGF-1 concentrations are positively correlated with thyroid volume in patients with acromegaly (23). Growth factors like vascular endothelial growth factor (VEGF) are probably important for expansion of the thyroid bed during goitrogenesis (17).
In contrast, transforming growth factor (TGF) seems to inhibit thyroid growth (24,25,26). TGF acts as an autocrine growth inhibitor on thyroid follicular cells by inhibiting the growth-stimulating action of TSH, IGF-1, and EGF (24). Tissue concentrations of TGF messenger RNA are lower in iodine-deficient nontoxic goiters than in normal thyroid tissue (24). In addition to decreased production of TGF, resistance to TGF action may be present in nontoxic goiters, as demonstrated in primary cell cultures prepared from human multinodular goiters (25).
Some studies suggested that patients with nontoxic goiters have thyroid growth-stimulating antibodies in their serum (27), but most subsequent studies revealed no evidence for these antibodies. The methods used to separate these substances from TSH and to assess thyroid growth have many limitations (28), and therefore the existence of thyroid growth-stimulating antibodies in patients with nontoxic goiters should be viewed as unproven.
In addition to and possibly modulated by extracellular stimulators of thyroid growth, some alterations in intracellular mechanisms related to the control of cell replication (e.g., increased expression of protooncogenes such as the ras protooncogene) may contribute to the growth of nontoxic goiters (29). Also, intrathyroidal iodine depletion may stimulate follicular growth irrespective of the serum TSH concentration (30).
Heterogeneity in Growth: the Basis of Nodule Formation
Whatever thyroid growth-stimulating factors may be present, the formation of nodules in nontoxic goiters can only be explained by a constitutive heterogeneity of the growth responses of individual thyroid follicular cells (31,32,33,34).
In the normal thyroid gland, the sensitivity of individual cells within the same follicle to the growth-stimulating action of TSH varies widely (31,32,33,34,35,36). A few cells have the capacity to replicate autonomously (i.e., in the absence of TSH). However, most cells replicate only when TSH is present, and the amount of TSH needed for replication varies among cells. Upon weak stimulation, cells with a high sensitivity to TSH start to proliferate. With increasing intensity and duration of the growth stimulus, the percentage of replicating cells increases gradually, a phenomenon known as recruitment. Only with strong stimulation will the large majority of follicular cells start to proliferate.
It is assumed that during formation of a nontoxic goiter the stimulation of follicular cells by TSH or other thyroid growth-stimulating factors is relatively weak (31,32,33,34). Therefore, only a small fraction of follicular cells, namely those with a high growth potential, will enter the mitotic cycle and contribute to the formation of new follicles. These cells transfer their high growth potential to their daughter cells (35). Thus, during goiter growth, the number of replicating cells increases progressively. Follicular cells with a high growth potential are not evenly distributed within the thyroid gland, and after replication their daughter cells remain clustered (35). Therefore, nontoxic goiters become increasingly nodular with time.
Convincing evidence that rapidly replicating cells remain clustered during goitrogenesis is the demonstration by X chromosome inactivation analysis that some macroscopic nodules within a nontoxic goiter are monoclonal (37,38,39,40,41). Despite their monoclonal nature, these nodules may contain morphologically and functionally heterogeneous follicles. Therefore, monoclonal nodules may be indistinguishable from polyclonal nodules, which also may be present in the same goiter. Thus, although morphologic and functional homogeneity of a nodule usually indicates monoclonality, some monoclonal nodules have marked heterogeneity. The molecular mechanisms underlying the transformation of originally homogeneous clonal nodules into heterogeneous ones are unknown.
Other Mechanisms Contributing to Nodule Formation
During goiter growth, the number of cells with a high growth rate increases, resulting in an increasing growth rate of the goiter as a whole. The growing thyroid gland requires expansion of blood vessels. However, the newly formed capillary network is often fragile and unable to supply the growing thyroid tissue adequately. This may result in areas of hemorrhagic necrosis within the goiter. The necrotic areas are invaded by granulation tissue, ultimately resulting in fibrosis, scarring, and even calcification. The resulting network of inelastic strands of connective tissue interferes with smooth growth of the thyroid parenchyma and will further increase the formation of macroscopic nodules (34). Furthermore, markedly distended follicles may fuse to form colloid cysts, which are characteristic of nontoxic goiters.
INCREASING FUNCTIONAL HETEROGENEITY DURING GOITROGENESIS
In the normal thyroid gland, not only the growth potential but also the functional activity of individual cells within a single follicle varies widely. Some follicular cells have a high capacity to synthesize and iodinate thyroglobulin, whereas others lack this ability almost completely (31,32,34,35,42). Similarly, immunohistochemical studies have demonstrated that only a small fraction of thyroid follicular cells contain the sodium/iodine cotransporter (43,44), and there are large differences in endocytotic activity among cells within the same follicle (31,32,34,42).
Some follicular cells take up iodine, synthesize thyroglobulin (and T4 and T3), and have endocytotic activity, so that they secrete T4 and T3 in the presence of only low concentrations of TSH or even in the absence of TSH. With increasing concentrations of TSH, the fraction of cells that are responsive increases, and eventually most of the cells will respond (45). Thus, the thyroid contains subpopulations of cells with differing sensitivity to TSH with respect to iodine metabolism as well as growth.
Despite the heterogeneity in function between individual cells within normal follicles, the balance between thyroglobulin synthesis and endocytotic activity in the follicle as a whole is finely tuned, so that the size of most follicles is similar. In contrast, the follicles of nontoxic goiters vary much more widely in both functions (Fig. 68.1). Large, colloid-rich follicles and small follicles almost devoid of colloid coexist in most nontoxic goiters. This variation in size among follicles in a nontoxic goiter can be explained by a distortion of the balance between thyroglobulin synthesis and endocytotic activity (34).
FIGURE 68.1. Autoradiographs of two different areas of a nontoxic multinodular goiter showing heterogeneity of morphology and function. The goiter was excised after administration of radioiodine to the patient. Note the large differences in size, shape, and function (as indicated by the degree of darkening) among the individual follicles. Note also the lack of correlation between the size or any other morphologic feature of a single follicle and its iodine uptake. (From Studer H, Peter HJ, Gerber H. Natural heterogeneity of thyroid cells: the basis for understanding thyroid function and nodular goiter growth. Endocr Rev 1989; 10:125, with permission.)
The follicles in a nontoxic goiter also vary widely in their ability to take up and further metabolize iodine, as demonstrated by autoradiographic studies of nontoxic goiter tissue obtained from patients given radioiodine before the goiter was removed (31,32,34,42) (Fig. 68.1). The radioiodine content in the cells and the lumens of different thyroid follicles in this figure varies a great deal, as would be expected if each follicle was generated from one or a few mother cells with a high growth potential but with widely differing abilities to take up and further metabolize iodine. The growth potential and functional activity of individual mother cells and their progeny are independent properties of the cell, as demonstrated in studies of human nontoxic goiter tissue transplanted onto nude mice (Fig. 68.2). Thus, both iodine metabolism and growth rate of cells within newly formed follicles vary widely (Fig. 68.3).
FIGURE 68.2. Two autoradiographs of the same section of a human nontoxic goiter after transplantation into a nude mouse treated with a high dose of thyroxine to inhibit TSH secretion. The mouse was injected with 3H-thymidine three times daily for 2 weeks before sacrifice and with iodine 131I 1 hour before sacrifice. Top: Autoradiograph immediately after sacrifice showing organification of 131I. Bottom: Autoradiograph 4 months after sacrifice (i.e., after decay of 131I), showing 3H-thymidine incorporation, indicating proliferative activity. Follicle A has the highest capacity for 131I organification but contains only a few proliferating cells. In follicle B, the 131I-labeled thyroglobulin is concentrated in the upper left part of the lumen of the follicle, and only a few of the cells containing 131I in this area have incorporated the 3H-thymidine label. In contrast, most of the cells containing lesser amounts of 131I in this follicle (those near the dotted line) are labeled with 3H-thymidine. This experiment shows that autonomy of function and autonomy of growth of the follicular epithelial cells are independent properties of the cells. (From Peter HJ, Gerber H, Studer H, et al. Pathogenesis of heterogeneity in human multinodular goiter: a study on growth and function of thyroid tissue transplanted onto nude mice. J Clin Invest 1985;76:1992, with permission.)
FIGURE 68.3. Diagram of the pathogenesis of functional and morphologic heterogeneity of nontoxic goiter. The mother follicle is composed of cells with either high (black) or low (gray) peroxidase activity at the apical cell membrane. Moreover, two families of cells, one black and the other gray, have a high intrinsic growth capacity that is passed on to the offspring. Therefore, they have generated two daughter follicles with widely differing iodinating activity. Iodinating activity may be replaced by other cell traits such as thyroglobulin synthesis, endocytotic capacity, or iodide transport to illustrate the mechanisms generating the characteristic functional and morphologic heterogeneity among the follicles in nontoxic goiters. (Courtesy of Professor H. Studer, Bern, Switzerland.)
If a group of follicles generated in this way grows large enough, it may become visible as a hot or cold area on thyroid scintigraphy, depending on the degree of activity of its iodine metabolism. The iodine metabolism of particular areas within a nontoxic goiter and their growth behavior are not necessarily parallel. Therefore, areas of low radioiodine uptake detected by radioiodine imaging in patients with multinodular goiters may have the same growth potential as areas of high uptake. Furthermore, the areas of increased or decreased iodine uptake do not necessarily correspond to thyroid nodules as detected by physical examination or ultrasonography (31,32) (see Chapter 16).
DEVELOPMENT OF FUNCTIONAL AUTONOMY
Some normal thyroid follicular cells take up and organify iodine in the absence of TSH, as noted above (31,32,34). During goitrogenesis, the number of cells with functional autonomy increases, especially when the cells also have a high replicating capacity. The increase in the total mass of follicular cells with autonomous iodine metabolism during goiter growth explains why a patient with a nontoxic goiter can eventually develop subclinical and then overt thyrotoxicosis, either spontaneously or, particularly in iodine-deficient areas, after exposure to large amounts of iodine (46,47,48).
In recent years, activating mutations in the TSH receptor such as those found in some autonomously functioning thyroid adenomas and in the thyroid glands of patients with autosomal-dominant nonautoimmune hyperthyroidism (49,50) have been identified in hyperfunctioning nodules of toxic multinodular goiters (51,52,53,54,55,56). In some goiters, different activating mutations were found in different nodules. The mutations have been detected not only in histologically true adenomas, but also in hyperplastic, nonencapsulated nodules (54,55). In contrast, mutations were not detected in nonfunctioning nodules, whether true adenomas or hyperplastic nodules, or perinodular tissue in the same multinodular goiters (51,53,55). The pathogenic importance of these mutations in the TSH receptor in multinodular goiters is not known, but the mutations probably contribute to the development of functional autonomy in multinodular goiters (see Chapter 25).
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