The thyroid hormones triiodothyronine and thyroxine, usually abbreviated as triiodothyronine and T₄, respectively, are synthesized in the thyroid gland by sequential metabolism of tyrosine residues in the glycosylated homodimeric protein thyroglobulin (TG, P01266) under the influence of the haem-containing protein iodide peroxidase. Iodide peroxidase/TPO is a haem-containing enzyme, from the same structural family as eosinophil peroxidase (EPX, P11678), lactoperoxidase (LPO, P22079) and myeloperoxidase (MPO, P05164). Circulating thyroid hormone is bound to thyroxine-binding globulin (SERPINA7, P05543).

Tissue deiodinases

These are 1 TM selenoproteins that remove an iodine from T₄ (3,3’,5,5’-tetraiodothyronine) to generate triiodothyronine (3,3’,5-triiodothyronine, a more potent agonist at thyroid hormone receptors) or rT₃ (rT3, 3,3’,5’-triiodothyronine, a relatively inactive analogue). DIO1 is also able to deiodinate RT3 to form 3,3’-diidothyronine (T₂). Iodotyrosine deiodinase is a 1TM homodimeric enzyme.

* Key recommended reading is highlighted with an asterisk

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