Medical Physiology, 3rd Edition

Synthesis of Thyroid Hormones

T4 and T3, made by iodination of tyrosine residues on thyroglobulin, are stored as part of thyroglobulin molecules in thyroid follicles

The structures of T4 and T3, the two active thyroid hormones, are shown in Figure 49-2. T3 is far more active than T4. Also shown is reverse T3 (rT3), which has no known biological activity. It has two iodines on its outer benzyl ring, rather than two on its inner ring, as is the case for T3. All three compounds derive from the ether linkage of a tyrosine molecule to the benzyl group of a second tyrosine molecule; one or two iodine atoms are attached to each benzyl group. The bottom panel of Figure 49-2 shows T4 as part of the thyroglobulin molecule.

image

FIGURE 49-2 Structure of T4, T3, and rT3. T4, T3, and rT3 all are products of the coupling of two iodinated tyrosine derivatives. Only T4 and T3 are biologically active, and T3 is far more active than T4 because of a higher affinity for thyroid hormone receptors. rT3forms as an iodine is removed from the inner benzyl ring of T4 (labeled “A”); rT3 is present in approximately equal molar amounts with T3. However, rT3 is essentially devoid of biological activity. As shown in the bottom panel, T4 is part of the peptide backbone of the thyroglobulin molecule, as are T3 and rT3. Cleavage of the two indicated peptide bonds would release T4.

The synthesis of thyroid hormones begins with the trapping of iodine by the thyroid gland. Iodine is essential for the formation of thyroid hormones. It exists in nature as a trace element in soil and is incorporated into many foods. The iodide anion (I) is rapidly absorbed by the gastrointestinal (GI) tract and is actively taken up via a specialized Na/I cotransporter or symporter (NIS or SLC5A5; see p. 122), located at the basolateral membrane (i.e., facing the blood) of the thyroid follicular cell (Fig. 49-3). NIS is a 65-kDa integral membrane protein that is believed to have 13 membrane-spanning segments. NIS moves two Na+ and one I into the follicular cell against the I electrochemical gradient, fueled by the energy of the Na+ electrochemical gradient (see pp. 120–123). Several other anions (e.g., perchlorate, pertechnetate, and thiocyanate) can compete with I for uptake by the thyroid. Iodide leaves the follicular cell and enters the lumen of the follicle across the apical membrane. The Cl-I exchanger pendrin (SLC26A4; see p. 125imageN49-1 is present on the apical membrane, where it contributes to I secretion. Mutations in this protein can lead to a congenital syndrome typically characterized by a large thyroid gland (goiter) and hearing loss. The thyroid enlarges because of deficient I delivery to the follicular colloid, just as it would with an I-deficient diet (Box 49-1).

image

FIGURE 49-3 The follicular cell and its role in the synthesis of T4 and T3. The synthesis and release of T4 and T3 occurs in seven steps. Inside the follicular cell, a deiodinase converts some of the T4 to T3. TSH stimulates each of these steps except step 2. In addition, TSH exerts a growth factor or hyperplastic effect on the follicular cells. MCT8, monocarboxylate transporter 8.

Box 49-1

Iodine Deficiency

In areas where soil is relatively iodine deficient, human iodine deficiency is common. Because seawater and seafood contain large amounts of iodide, iodine deficiency is more common in inland areas, particularly in locales that rely on locally grown foods. For example, in inland areas of South America along the Andes Mountains, in central Africa, and in highland regions of Southeast Asia, iodine deficiency is common. In the early 1900s, investigators first recognized that iodide is present in high concentrations in the thyroid and that iodine deficiency promotes goiter formation. These observations led to efforts to supplement dietary iodine. Iodine deficiency causes thyroid hormone deficiency. The pituitary responds to this deficit by increasing the synthesis of TSH (see p. 1014), which in turn increases the activity of the iodine-trapping mechanism in the follicular cell in an effort to overcome the deficiency. The increased TSH also exerts a trophic effect that increases the size of the thyroid gland. If this trophic effect persists for sufficient time, the result is an iodine-deficiency goiter. Goiter is the generic term for an enlarged thyroid. If this effort at compensation is not successful (i.e., if insufficient thyroid hormone levels persist), the person will develop signs and symptoms of goitrous hypothyroidism. When iodine deficiency occurs at critical developmental times in infancy, the effects on the CNS are particularly devastating and produce the syndrome known as cretinism (see p. 1013). Persons so affected have a characteristic facial appearance and body habitus, as well as severe mental retardation. Dietary supplementation of iodine in salt and bread has all but eliminated iodine deficiency in North America. In many nations, especially in mountainous and landlocked regions of developing countries, iodine deficiency remains a major cause of preventable illness.

N49-1

Role of Pendrin in Apical Iodide Secretion by Thyroid Follicular Cells

Contributed by Emile Boulpaep, Walter Boron

Pendred syndrome—the combination of congenital hypothyroidism and acoustic nerve deafness—is caused by a mutation in SLC26A4, which encodes an anion exchanger called pendrin. The syndrome accounts for as much as 10% of the cases of inherited deafness and is also often associated with a goiter.

References

MedicineNet. Pendred syndrome.  http://www.medicinenet.com/pendred_syndrome/article.htm. Accessed June 12, 2015.

Mount DB, Romero MF. The SLC26 gene family of multifunctional anion exchangers. Pflugers Arch. 2004;447:710–721.

National Center for Biotechnology Information. Pendred syndrome.  http://www.ncbi.nlm.nih.gov/books/NBK22178/ [Accessed June 12, 2015].

In parallel with the secretion of I into the follicle lumen, the follicular cell secretes thyroglobulin (Tg) into the lumen; Tg contains the tyrosyl groups to which the I will ultimately attach. The Tg molecule is a glycoprotein synthesized within the follicular cell and exported to the follicular lumen via the secretory pathway (see p. 28). Tg is a very large protein (>600 kDa), and it accounts for approximately half of the protein content of the thyroid gland. It has relatively few tyrosyl residues (~100 per molecule of Tg), and only a few of these (<20) are subject to iodination. The secretory vesicles that contain Tg also carry the enzyme thyroid peroxidase (TPO)—an integral membrane protein, with the catalytic domain facing the vesicle lumen. As the secretory vesicles fuse with the apical membrane, the catalytic domain faces the follicular lumen and catalyzes the oxidation of I to I0. This reaction requires H2O, provided by another apical membrane protein, dual-oxidase 2 (DUOX2). As the Tg is entering the lumen of the thyroid follicle by the process of exocytosis, its tyrosyl groups react with I0.

One or two oxidized iodine atoms incorporate selectively into specific tyrosyl residues of Tg. TPO in the presence of H2O2 catalyzes the coupling of two iodinated tyrosyl residues within the Tg molecule to form a single iodothyronine as well as a remnant dehydroalanine. Both remain as part of the primary structure of the iodinated Tg until it is later degraded inside the follicular cell. This coupling of two tyrosines, catalyzed by TPO, does not occur unless they are iodinated. Because only a few tyrosyl groups become iodinated, something specific about the structure of the protein near these residues probably facilitates both iodination and conjugation. The thyroid hormones, although still part of the Tg molecule, are stored as colloid in the thyroid follicle.

Follicular cells take up iodinated thyroglobulin, hydrolyze it, and release T4 and T3 into the blood for binding to plasma proteins

Thyroid hormones, attached to Tg in the follicular lumen (see Fig. 49-1), remain inactive until the iodinated Tg is hydrolyzed (see Fig. 49-3). Before this proteolysis can begin, the follicular cells must resorb Tg from the follicular lumen by fluid-phase endocytosis (see pp. 41–42). As the endocytic vesicle containing the colloid droplet moves from the apical toward the basolateral membrane, it fuses with lysosomes to form a lysoendosome. Inside this vesicle, lysosomal enzymes hydrolyze the Tg and form T4 and T3, as well as diiodothyronine (DIT) and monoiodothyronine (MIT). The vesicle releases both T4 and T3 near the basolateral membrane, and these substances exit the cell into the blood by an unknown mechanism. Approximately 90% of the thyroid hormone secreted by the thyroid is released as T4, and 10% is released as T3. The thyroid releases very little rT3 into the blood. As discussed in the next section, nonthyroidal tissues metabolize the T4 released by the thyroid into T3 and rT3. Approximately three fourths of circulating T3arises from the peripheral conversion of T4, which occurs principally in the liver and kidneys.

In the circulation, both T4 and T3 are highly bound to plasma proteins. Thyroid-binding globulin (TBG), albumin, and transthyretin (TTR) account for most of this binding. The affinity of these binding proteins is sufficiently high that, for T4, >99.98% of the hormone circulates tightly bound to protein. T3 is bound only slightly less: ~99.5% is protein bound. Because the free or unbound hormone in the circulation is responsible for the actions of the thyroid hormones on their target tissues, the large amount of bound hormone has considerably confounded our ability to use simple measurements of the total amount of either T4 or T3 in the plasma to provide a reliable index of the adequacy of thyroid hormone secretion. For example, the amount of TBG in the serum can change substantially in different physiological states. Pregnancy, oral estrogen therapy, hepatitis, and chronic heroin abuse can all elevate the amount of TBG and hence the total concentration of T4 and T3. Decreased levels of TBG, associated with diminished concentration of total T4 and T3, can accompany steroid usage and nephrotic syndrome. However, despite the marked increases or decreases in the amounts of circulating TBG, the concentrations of free T4 and T3do not change in the aforementioned cases. Box 49-2 indicates how one can calculate levels of free T4 or T3, knowing the concentration of TBG and the concentration of total T4 or total T3.

Box 49-2

Free versus Bound Thyroxine

Most of the T4 and T3 in the serum is bound to proteins, the most important of which is TBG. For the binding of T4 to TBG, the reaction is

image

image

The binding constant K is ~2 × 1010M−1 for T4. The comparable binding constant for T3 is ~5 × 108M−1. Approximately one third of TBG's binding sites are occupied by T4. Therefore, we have all the information we need to compute the concentration of free T4:

image

A reasonable value for [T4TBG] would be 100 nM, and for [TBG], 250 nM. Thus,

image

Because the bound T4 in this example is 100 nM, and the free T4 is only 20 pM, we can conclude that only ~0.02% of the total T4 in the plasma is free. Because 99.98% of the total T4 in the plasma is bound, moderate fluctuations in the rate of T4 release from the thyroid have only tiny effects on the level of free T4. To simplify, we have not included the minor contribution of albumin and TTR in this sample calculation.

The liver makes each of the thyroid-binding proteins. TBG is a 54-kDa glycoprotein consisting of 450 amino acids. It has the highest affinity for T4 and T3 and is responsible for most of the thyroid-binding capacity in the plasma. The extensive binding of thyroid hormones to plasma proteins serves several functions. It provides a large buffer pool of thyroid hormones in the circulation, so that the active concentrations of hormone in the circulation change very little on a minute-to-minute basis. The binding to plasma proteins markedly prolongs the half-lives of both T4 and T3. T4 has a half-life of 8 days, and T3, of ~24 hours; each is longer than the half-life of steroid or peptide hormones. Finally, because much of the T3 in the circulation is formed by the conversion of T4 to T3 in extrathyroidal tissues, the presence of a large pool of T4 in the plasma provides a reserve of prohormone available for synthesis of T3. This reserve may be of particular importance because T3 is responsible for most of the biological activity of thyroid hormones.

Peripheral tissues deiodinate T4 to produce T3

The thyroid synthesizes and stores much more T4 than T3, and this is reflected in the ~10 : 1 ratio of T4 to T3 secreted by the thyroid. However, certain tissues in the body have the capacity to selectively deiodinate T4, thereby producing either T3 or rT3. T3 and rT3 can each be further deiodinated to various DITs and MITs (Fig. 49-4); both DITs and MITs are biologically inactive. Both iodine atoms on the inner ring, and at least one iodine atom on the outer ring, appear essential for biological activity. Similarly, the loss of the amino group renders T4 or T3 inactive. The importance of the peripheral deiodination of T4 to T3 can be readily appreciated from the observation that persons whose thyroids have been removed have normal circulating concentrations of T3 when they receive oral T4 supplementation.

image

FIGURE 49-4 Peripheral metabolism of T4. The 5′/3′-monodeiodinases (type 1 and type 2; green arrows) remove I from the outer benzyl ring, whereas the 5/3-monodeiodinase (type 3; orange arrows) removes I from the inner benzyl ring. Thus, the action of the 5′/3′-monodeiodinases on T4 yields T3, whereas the action of the 5/3-monodeiodinase yields rT3. Sequential deiodination yields T0 (thyronine).

Inasmuch as T3 is biologically much more active than the far more abundant T4, the regulated conversion of T4 to T3 in peripheral tissues—as well the conversion of T4 and T3 to inactive metabolites—assumes considerable importance. These conversions are under the control of three deiodinases. Two deiodinases are 5′/3′-deiodinases that remove an I from the outer ring and thereby convert T4 to T3 (see Fig. 49-4). The first of these 5′/3′-deiodinases—type 1 deiodinase—is present at high concentrations in the liver, kidneys, skeletal muscle, and thyroid. It appears to be responsible for generating most of the T3 that reaches the circulation. The second 5′/3′-deiodinase—type 2 deiodinase—is found predominantly in the pituitary, central nervous system (CNS), and placenta, and is involved in supplying those tissues with T3 by local generation from plasma-derived T4. As shown below, the type 2 enzyme in the pituitary is of particular importance because the T3 that is generated there is responsible for the feedback inhibition of the release of thyrotropin (or thyroid-stimulating hormone, TSH).

A third 5/3-deiodinase—type 3 deiodinase—removes an I from the inner ring, thereby converting T4 to the inactive rT3. Because the 3′ and 5′ positions in T4 are equivalent stereochemically, removing either of these by type 1 or type 2 deiodinase yields T3. Similarly, removal of the I from either the 3 or the 5 position of the inner ring of T4 by type 3 deiodinase yields rT3. Further deiodination by any of the three enzymes ultimately yields T0 (i.e., thyronine).

The relative activity of the outer-ring deiodinases changes in response to physiological and pathological stimuli. Caloric restriction or severe stress inhibits the type 1 deiodinase; this process decreases the conversion of T4 to T3—and thus reduces the levels of T3. In contrast, levels of rT3 rise by default in these situations, in part because of reduced conversion to DITs. These decreases in T3 levels are accompanied by a decline in metabolic rate. You may think that because plasma levels of T3 fall, there would be a compensatory rise in TSH, the secretion of which is inhibited by T3. However, because type 2 deiodinase mediates the conversion of T4 to T3 within the pituitary and CNS, and because caloric restriction does not affect this enzyme, local T3 levels in the pituitary are normal. Thus, the thyrotrophs in the pituitary continue to have adequate amounts of T3, and no compensatory rise in TSH occurs. Teleologically, the rationale to restrain calorie expenditure in settings of decreased caloric intake is appealing. imageN49-2

N49-2

Effect of Calorie Restriction on Type 1 Deiodinase

Contributed by Eugene Barrett

As noted in the text, calorie restriction inhibits type 1 monodeiodinase, reducing the conversion of T4 to T3 and thereby lowering circulating levels of T3. This effect of caloric restriction makes sense for someone who is starving because it tends to conserve body stores of fuel. On the other hand, this effect makes it more difficult to lose weight intentionally while dieting.