Thyroid hormones, thyroxine and triiodothyronine, are synthesised from the tyrosine residues of the protein thyroglobulin and activated iodine. Iodination of tyrosine ring occurs to produce mono-iodination and diiodotyrosine, from which triiodothyronine (T₃) and thyroxine undergo proteolytic breakdown to release the free hormones, T₃ and T₄.