The thyroid hormones, thyroxine (T₄) and triiodothyronine (T₃), 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 (T₄). This is converted to the active T₃ within cells by deiodinases. These are further processed by decarboxylation and deiodination to produce iodothyronamine (T₁a) and thyronamine (T₀a).

Circulation

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Most of the thyroid hormone circulating in the blood is bound to transport proteins:

• Thyroxine-binding globulin (TBG, 70%)
• Thyroxine-binding prealbumin (TBPA, 10-15%): this protein is also responsible for the transport of retinol, and so now has the preferred name of transthyretin (TTR)
• Albumin (15-20%).

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