Thyroid hormone


The thyroid hormones, thyroxine (T<sub>4</sub>) and triiodothyronine (T<sub>3</sub>), are tyrosine-based hormones produced by the thyroid gland. An important component in the synthesis is iodine. The major form of thyroid hormone in the blood is thyroxine (T<sub>4</sub>). The ratio of T<sub>4</sub> to T<sub>3</sub> released in the blood is roughly 20 to 1. Thyroxine is converted to the active T<sub>3</sub> (three to four times more potent than T<sub>4</sub>) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T<sub>1</sub>a) and thyronamine (T<sub>0</sub>a).