<Li> Thyroid - stimulating hormone (TSH) released from the anterior pituitary (also known as the adenohypophysis) binds the TSH receptor (a G protein - coupled receptor) on the basolateral membrane of the cell and stimulates the endocytosis of the colloid . </Li> <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 . Therefore, each thyroglobulin 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 thyroglobulin). </P>

What is the action of t3 and t4