Thyroid Hormone Synthesis Process
The Source Components of Thyroid Hormone
Thyroglobulin (Tg), plays an important role in the synthesis and storage of thyroid hormone. Tg is a glycoprotein containing multiple tyrosine residues. It is synthesized in the thyroid follicular epithelial cells and secreted through the apical membrane into the follicular lumen, where it is stored in the colloid. A small amount of noniodinated Tg is also secreted through the basolateral membrane into the circulation. Although circulating levels of Tg can be detected under normal conditions, levels are elevated in diseases such as thyroiditis and Graves disease.
Tg can be considered a scaffold upon which thyroid hormone synthesis takes place. Once Tg is secreted into the follicular lumen, it undergoes major posttranslational modification during the production of thyroid hormones. At the apical surface of the thyroid follicular epithelial cells, multiple tyrosine residues of Tg are iodinated, followed by coupling of some of the iodotyrosine residues to form T3 and T4.
The iodide required for thyroid hormone synthesis is readily absorbed from dietary sources, primarily from iodized salt, but also from seafood and plants grown in soil that is rich in iodine. Following its absorption, iodide is confined to the extracellular fluid, from which it is removed primarily by the thyroid (20%) and the kidney (80%). The total excretion of iodide by the kidneys is approximately equal to daily intake. The balance between dietary intake and renal excretion preserves the total extracellular pool of iodide.
The Uptake and Iodination of Iodine
Iodide is concentrated in thyroid epithelial cells by an active, saturable, energy-dependent process mediated by a Na+/I– symporter located in the basolateral plasma membrane of the follicular cell. Additional tissues that express the Na+/I– symporter include the salivary glands, the gastric mucosa, the placenta, and the mammary glands. However, transport of iodine in these tissues is not under TSH regulation.
Iodine efflux (after the transformation from anion cation?, see below)
The iodination of Tg residues is a process that occurs at the apical membrane. Thus, once inside the cell, iodine must leave the follicular cell through apical efflux by an iodide-permeating mechanism consisting of a chloride-iodide transporting protein (iodide channel) located in the apical membrane of the thyroid follicular cell. The uptake, concentration, and efflux of iodide through the iodide channel are all a function of TSH-stimulated transepithelial transport of iodide.
Organification and coupling
In the follicular lumen, tyrosine residues of Tg are iodinated by iodine (I+; formed by oxidation of I– by TPO). This reaction requires hydrogen peroxide, which is generated by a flavoprotein Ca++-dependent reduced nicotinamide adenine dinucleotide phosphate oxidase at the apical cell surface and serves as an electron acceptor in the reaction process. Iodine bonds to carbon 3 or to carbon 5 of the tyrosine residues on Tg in a process referred to as the organification of iodine. This iodination of specific tyrosines located on Tg yields monoiodinated tyrosine (MIT) and diiodinated tyrosine (DIT) residues that are enzymatically coupled to form triiodothyronine (T3) or tetraiodothyronine (T4). The coupling of iodinated tyrosine residues, either of 2 DIT residues or of 1 MIT and 1 DIT residues, is catalyzed by the enzyme thyroid peroxidase. Because not all of the iodinated tyrosine residues undergo coupling, Tg stored in the follicular lumen contains MIT and DIT residues as well as formed T3 and T4.
Release of Thyroid Hormone
The synthesis of thyroid hormone takes place in the colloid space. As mentioned previously, the apical surface of the follicular epithelial cell faces the colloid and not the interstitial space, and thus has no access to the bloodstream. Therefore, thyroid hormone release involves endocytosis of vesicles containing Tg from the apical surface of the follicular cell. The vesicles fuse with follicular epithelial phagolysosomes, leading to proteolytic digestion and cleavage of Tg. In addition to the thyroid hormones T4 and T3, the products of this reaction include iodinated tyrosine residues (MIT and DIT). MIT and DIT are deiodinated intracellularly, and iodide is transported by apical efflux into the follicular colloid space, where it is reused in thyroid hormone synthesis. T4 and T3 are released from the basolateral membrane into the circulation. The thyroid gland releases greater amounts of T4 than T3, so plasma concentrations of T4 are 40-fold higher than those of T3 (90 vs 2 nM). Most of the circulating T3 is formed peripherally by deiodination of T4, a process that involves the removal of iodine from carbon 5 on the outer ring of T4. Thus, T4 acts as a prohormone for T3. Although this deiodination occurs predominantly in the liver, some occurs in the thyroid follicular epithelial cell itself. This intrathyroidal deiodination of T4 is the result of TSH stimulation of the type I deiodinase.
Two additional facts regarding thyroid hormone activity and storage should be noted. First, at physiologic levels, T4, is relatively inactive because it possesses 100-fold lower affinity than T3 for binding to the thyroid receptor and does not enter the cell nucleus at high enough concentrations to occupy the ligand-binding site of the thyroid hormone receptor. Second, in contrast to most endocrine glands, which do not have storage capacity for their product, the thyroid gland is able to store 2-3 months' supply of thyroid hormones in the Tg pool.
Transport and Tissue Delivery of Thyroid Hormones
Once thyroid hormones are released into the circulation, most of them circulate bound to protein. Approximately 70% of T4 and T3 is bound to thyroid-binding globulin. Other protein involved in thyroid binding include transthyretin, which binds 10% of T4, and albumin, which binds 15% of T4 and 25% of T3. A small fraction of each hormone (0.03% of T4 and 0.3% of T3) circulates in its free form. This fraction of the circulating hormone pool is bioavailable and can enter the cell to bind to the thyroid receptor. Of the 2 thyroid hormones, T4 binds more tightly to binding proteins than T3 and thus has a lower metabolic clearance rate and a longer half-life (7 days) than T3 (1 day). The kidneys readily excrete free T4 and T3. Binding of thyroid hormones to plasma proteins ensures a circulating reserve and delays their clearance.
The release of hormone from its protein-bound form is in a dynamic equilibrium. Although the role of binding proteins in delivery of hormone to specific tissues remains to be fully understood, it is known that drugs such as salicylate may affect thyroid hormone binding to plasma proteins. The binding-hormone capacity of the individual can also be altered by disease or hormonal changes. The changes in total amount of plasma proteins available to bind thyroid hormone will impact the total amout of circulating thyroid hormone because of a constant homeostatic adjustment to changes in free hormone levels. A decrease in free thyroid hormone because of an increase in plasma-binding proteins will stimulate the release of TSH from the anterior pituitary, which will in turn stimulate the synthesis and release of thyroid hormone from the thyroid gland. In contrast, a decrease in binding-protein levels, with a resulting rise in free thyroid hormone levels, will suppress TSH release and decrease thyroid hormone synthesis and release. These dynamic changes occur throughout the life of the individual, whether in health or disease. Disruption in these feedback mechanisms will result in manifestations of excess or deficient thyroid hormone function.
Thyroid Hormone Metabolism
As already mentioned, the thyroid releases mostly T4 and very small amounts of T3, yet T3 has greater thyroid activity than T4. The main source of circulating T3 is peripheral deiodination of T4 by deiodinases (I, II and III). Approximately 80% of T4 produced by the thyroid undergoes deiodination in the periphery. Approximately 40% of T4 is deiodinated at carbon 5 in the outer ring to yield the more active T3, principally in liver and kidney. In approximately 33% of T4, iodine is removed from carbon 5 in the inner ring to yield reverse T3 (rT3). Reverse T3 has little or no biologic activity, has a higher metabolic clearance rate than T3, and has a lower serum concentration than T3. Following conversion of T4 to T3 or rT3, these are converted to T2, a biologically inactive hormone. Therefore, thyroid hormone peripheral metabolism is a sequential deiodination process, leading first to a more active form of thyroid hormone (T3) and finally to complete inactivation of the hormone. Thus, loss of a single iodine from the outer ring of T4 produces the active hormone T3, which may either exit the cell, enter the nucleus directly, or possibly even both. Thyroid hormones can be excreted following hepatic sulfo- and glucuronide conjugation and biliary excretion.
Type I deiodinase catalyzes outer- and inner-ring deiodination of T4 and rT3. It is found predominantly in the liver, kidney, and thyroid. It is considered the primary deiodinase responsible for T4 to T3 conversion in hyperthyroid patients in the periphery. This enzyme also converts T3 to T2. The activity of type I deiodinase expressed in the thyroid gland is increased by TSH-stimulated cAMP production and has a significant influence on the amount of T3 released by the thyroid. Propylthiouracil and iodinated x-ray contrast agents such as iopanoic acid inhibit the activity of this enzyme and consequently the thyroidal production of T3.
Type II deiodinase is expressed in the brain, pituitary gland, brown adipose tissue, thyroid, placenta, and skeletal and cardiac muscle. Type II deiodinase has only outer-ring activity and converts T4 to T3. This enzyme is thought to be the major source of T3 in the euthyroid state. This enzyme plays an important role in tissues that produce a relatively high proportion of the receptor-bound T3 themseleves, rather than deriving T3 from plasma. In these tissues, type II deiodinases are an important source of intracellular T3 and provide more than 50% of the nuclear receptor-bound T3. The critical role of type II deiodinases is underscored by the fact that T3 formed in the anterior pituitary is necessary for negative feedback inhibition (long loop) of TSH secretion.
Type III Deiodinase is expressed in the brain, placenta, and skin. Type III deiodinase has inner-ring activity and converts T4 to rT3, and T3 to T2, thus inactivating T4 and T3. This process is an important feature in placental protection of the fetus. The placental conversion of T4 to rT3, and of T3 to T2 reduces the flow of T3 from mother to fetus. Small amounts of maternal T4 are transferred to the fetus and converted to T3, which increases the T3 concentration in the fetal brain, preventing hypothyroidism. In the adult brain, the expression of type III deiodinases is enhanced by thyroid hormone excess, serving as a protective mechanism against high thyroid hormone concentrations.
The Hypothalamic-pituitary-thyroid Axis
Hypothalamic Regulation of Thyroid-Stimulating Hormone Release (releasing factor)
Thyroid hormone synthesis and release are under negative feedback regualtion by the hypothalamic-pituitary-thyroid axis. TRH is a tripeptide synthesized in the hypothalamus and released from nerve terminals in the median eminence from where it is transported through the portal capillary plexus to the anterior pituitary. TRH binds to cell membrane Gq/11 receptors on thyrotrophs of the anterior pituitary gland, where it activates phospholipase C, resulting in the hydrolysis of phosphatidylinositol bisphosphate and the generation of inositol triphosphate and diacylylycerol. This process leads to an increase in the intracellular Ca2+ concentration, resulting in stimulation of exocytosis and release of TSH into the systemic circulation.
Thyroid-Stimulating Hormone Regulation of Thyroid Hormone Release (tropic effect)
TSH is transported in the bloodstream to the thyroid gland, where it binds to the TSH receptor located on the basolateral membrane of thyroid follicular epithelial cells. The TSH receptor is a cell membrane G protein-coupled receptor. Binding of TSH to its receptor initiates signaling through cyclic 3', 5'-adenosine monophosphate (cAMP), phospholipase C, and the protein kinase A signal transduction systems. Activation of adenylate cyclase, formation of cAMP, and activation of protein kinase A regulate iodide uptake and transcription of Tg, thyroid peroxidase (TPO), and the activity of the sodium-iodide (Na+/I–) symporter. Signaling through phospholipase C and intracellular Ca2+ regulate iodide efflux, H2O2 production, and Tg iodination. The TSH receptor is an important antigenic site involved in thyroid autoimmune disease. Autoantibodies to the receptor may act as agonists mimicking the actions of TSH, or antagonists in the case of autoimmune hypothyroidism.
TSH receptor activation results in stimulation of all of the steps involved in thyroid hormone synthesis, including 1) iodine uptake and organification, 2) production and release of iodothyronines from the gland, and 3) promotion of thyroid growth. Specifically, the biologic effects of TSH include stimulation of gene transcription of the following: 1) Na+/I– symporter, the protein involved in transporting and concentrating iodide in the thyroid epithelial cell; 2) Tg, the glycoprotein that serves as a scaffold for tyrosine iodination and thyroid hormone synthesis, as well as storage of thyroid hormone; 3) TPO, the enzyme involved in catalyzing the oxidation of iodide and its incorporation into thyrosine residues of Tg; and 4) thyroid hormones T4 and T3 (triiodothyronine).
TSH control the energy-dependent uptake and concentration of iodide by the thyroid gland and its transcellular transport through the follicular epithelial cell. However, iodine metabolism within the thyroid can also be reglated independently of TSH. This mechanism is important when plasma iodide levels are elevated (15-20-fold above normal) because this elevation inhibits the organic binding of iodine within the thyroid. This autoregulatory phenomenon consisting of inhibition of the organification of iodine by elevated circulating levels of iodide is known as the Wolff-Chaikoff effect. This effect lasts for a few days and is followed by the so-called escape phenomenon, at which point the organification of intra-thyroidal iodine resumes and the normal synthesis of T4 and T3 returns. The escape phenomenon results from a decrease in the inorganic iodine concentration inside the thyroid gland from downregulation of the Na+/I– symporter. This relative decrease in intrathyroidal inorganic iodine allows the TPO-H2O2 system to resume normal activity. The mechanisms responsible for the acute Wolff-Chaikoff effect have not been elucidated but may be caused by the formation of organic iodocompounds within the thyroid.
Thyroid Hormone Regulation of Thyroid-Stimulating Hormone Release (long loop)
The production and release of thyroid hormones are under negative feedback regulation by the hypothalamic-pituitary-thyroid axis. The release of TSH is inhibited mainly by T3, produced by conversion of T4 to T3 in the hypothalamus, and in the anterior pituitary by type II deiodinase. The contribution of this intracellularly derived T3 in producing the negative feedback inhibition of TSH release is greater than that of T3 derived from the circualtion. Other neuroendocrine mediators that inhibit TSH release include dopamine, somatostatin, and glucocorticoids at high levels, which produce partial suppression of TSH release.