The
thyroid hormones,
triiodothyronine (
T3) and its
prohormone,
thyroxine (
T4), are
tyrosine-based
hormones produced by the
thyroid gland that are primarily responsible for regulation of metabolism. T
3 and T
4 are partially composed of
iodine (see molecular model). A deficiency of iodine leads to decreased production of T
3 and T
4, 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
4), which has a longer half-life than T
3. In humans, the ratio of T
4 to T
3 released into the blood is roughly 20 to 1. T
4 is converted to the active T
3 (three to four times more potent than T
4) within
cells by
deiodinases (5'-iodinase). These are further processed by
decarboxylation and deiodination to produce
iodothyronamine (
T1a) and
thyronamine (
T0a). All three isoforms of the deiodinases are
selenium-containing enzymes, thus dietary selenium is essential for T
3 production.
