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.
Hormone
Many hormones and their structural and functional analogs are used as medication. The most commonly prescribed hormones are estrogens and progestogens (as methods of hormonal contraception and as HRT), thyroxine (as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulin is used by many diabetics. Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline, while steroid and vitamin D creams are used extensively in dermatological practice.