One of two tyrosine-based hormones
secreted by the thyroid gland, the
other being triiodothyronine (T3).
Thyroxine (T4) is the major form of thyroid
hormone in the blood. It is converted to the active T3 within
cells by deiodinases. These are further processed by decarboxylation and
deiodination to produce iodothyronamine (T1a) and thyronamine
Picture credit: Wikipedia