Synthesis & Transport of Thyroid Hormones

The thyroid secretes 2 iodine-containing hormones: thyroxine (T4 ) and triiodothyronine (T3). The iodine necessary for the synthesis of these molecules comes from food or iodide supplements. Iodide ion is actively taken up by and highly concentrated in the thyroid gland, where it is converted to elemental iodine by thyroidal peroxidase ( See the Figure ).
The protein thyroglobulin serves as a scaffold for thyroid hormone synthesis. Tyrosine residues in thyroglobulin are iodinated to form monoiodotyrosine (MIT) or diiodotyrosine (DIT) in a process known as iodineorganification.
Within thyroglobulin, 2 molecules of DIT combine to form T4, while 1 molecule each of MIT and DIT combine to form T3. Proteolysis of thyroglobulin liberates the T4 and T3, which are then released from the thyroid. After release from the gland, T4 and T3 are transported in the blood by thyroxine-binding globulin, a protein synthesized in the liver.

This figure also show Sites of action of some antithyroid drugs. I–, iodide ion; I°, elemental iodine. Not shown: radioactive iodine (131I), which destroys the gland through radiation.