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.
Liothyronine
Liothyronine is a synthetic form of triiodothyronine (T3), a thyroid hormone used to treat hypothyroidism and myxedema coma. T3 is the metabolically active thyroid hormone, which causes feedback inhibition, and lowers elevated TSH levels. It increases metabolism in peripheral tissues and is indicated when there is an impaired conversion of T4 to T3 in peripheral tissues. It is also used as an augmentation strategy in treating major depressive disorder when used in combination with antidepressants. It is marketed as the sodium salt under the brand name Cytomel (or Tertroxin in Australia).