Synthesis of thyroid hormone begins when iodide molecules enter the follicular cell of the thyroid gland via the sodium–iodide transporter and are then transported into the colloid. Once in the thyroid gland, iodide is oxidized and is quickly associated with thyroglobulin. Organification of the iodide results in the inactive iodotyrosines, monoiodotyrosine and diiodotyrosine. Subsequently, these tyrosine residues are coupled to form the 2 active thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Organification and coupling occur in the matrix of thyroglobulin. This complex thyroglobulin, which resides in the colloid of the thyroid gland, serves as the reservoir for the production of T3 and T4. Thyroid hormones are then moved via endocytosis into the follicular cell. This vesicle fuses with lysosomes, after which T4 and T3 are disassociated from thyroglobulin and eventually released into the bloodstream.
