<Li> The endocytosed vesicles fuse with the lysosomes of the follicular cell . The lysosomal enzymes cleave the T from the iodinated thyroglobulin . </Li> <Li> The thyroid hormones cross the follicular cell membrane towards the blood vessels by an unknown mechanism . Text books have stated that diffusion is the main means of transport, but recent studies indicate that monocarboxylate transporter (MCT) 8 and 10 play major roles in the efflux of the thyroid hormones from the thyroid cells . </Li> <P> Thyroglobulin (Tg) is a 660 kDa, dimeric protein produced by the follicular cells of the thyroid and used entirely within the thyroid gland . Thyroxine is produced by attaching iodine atoms to the ring structures of this protein's tyrosine residues; thyroxine (T) contains four iodine atoms, while triiodothyronine (T), otherwise identical to T, has one less iodine atom per molecule . The thyroglobulin protein accounts for approximately half of the protein content of the thyroid gland . Each thyroglobulin molecule contains approximately 100 - 120 tyrosine residues, a small number of which (<20) are subject to iodination catalysed by thyroperoxidase . The same enzyme then catalyses "coupling" of one modified tyrosine with another, via a free radical - mediated reaction, and when these iodinated bicyclic molecules are released by hydrolysis of the protein, T3 and T4 are the result . hence, each Tg protein molecule ultimately yields very small amounts of thyroid hormone (experimentally observed to be on the order of 5 - 6 molecules of either T4 or T3 per original molecule of Tg). </P> <P> More specifically, the monoatomic, anionic form of iodine, iodide, I (\ displaystyle I), is actively absorbed from the bloodstream by a process called iodide trapping . In this process, sodium is cotransported with iodide from the basolateral side of the membrane into the cell, and then concentrated in the thyroid follicles to about thirty times its concentration in the blood . Then, in the first reaction catalysed by the enzyme thyroperoxidase, tyrosine residues in the protein thyroglobulin are iodinated on their phenol rings, at one or both of the positions ortho to the phenolic hydroxyl group, yielding monoiodotyrosine (MIT) and diiodotyrosine (DIT), respectively . This introduces 1 - 2 atoms of the element iodine, covalently bound, per tyrosine residue . The further coupling together of two fully iodinated tyrosine residues, also catalysed by thyroperoxidase, yields the peptidic (still peptide - bound) precursor of thyroxine, and coupling one molecule of MIT and one molecule of DIT yields the comparable precursor of triiodothyronine: </P>

Protein that serves as the precursor molecular for thyroid hormones