Thyroxine is 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 (T_1a) and thyronamine (T_0a).