Thyroid hormones
Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T) and thyroxine (T). They are tyrosine-based hormones that are primarily responsible for regulation of metabolism. T and T are partially composed of iodine. A deficiency of iodine leads to decreased production of T and T, enlarges the thyroid tissue and will cause the disease known as simple goitre. The major form of thyroid hormone in the blood is thyroxine (T), which has a longer half-life than T. In humans, the ratio of T to T released into the blood is approximately 14:1. T is converted to the active T (three to four times more potent than T) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (Ta) and thyronamine (Ta). All three isoforms of the deiodinases are selenium-containing enzymes, thus dietary selenium is essential for T production. Edward Calvin Kendall was responsible for the isolation of thyroxine in 1915.
Thyroid-stimulating hormone
The TSH receptor is found mainly on thyroid follicular cells. Stimulation of the receptor increases T and T production and secretion. This occurs through stimulation of six steps in thyroid hormone synthesis: (1) Up-regulating the activity of the sodium-iodide symporter (NIS) on the basolateral membrane of follicular cells, thereby increasing intracellular concentrations of iodine (iodine trapping). (2) Stimulating iodination of thyroglobulin in the follicular lumen, a precursor protein of thyroid hormone. (3) Stimulating the conjugation of iodinated tyrosine residues. This leads to the formation of thyroxine (T) and triiodothyronine (T) that remain attached to the thyroglobulin protein. (4) Increased endocytocis of the iodinated thyroglobulin protein across the apical membrane back into the follicular cell. (5) Stimulation of proteolysis of iodinated thyroglobulin to form free thyroxine (T) and triiodothyronine (T). (6) Secretion of thyroxine (T) and triiodothyronine (T) across the basolateral membrane of follicular cells to enter the circulation. This occurs by an unknown mechanism.