Thyroid hormones are constructed by adding iodine residues to the amino acid tyrosine (shown on the right), which is a major component of the protein thyroglobulin. Thyroglobulin is produced in the endoplasmic reticulum of the thyroid follicular cells and excreted into the colloid follicles. Within the follicles, iodination (the addition of iodine) to tyrosine occurs.
Initially, the compound that is formed is monoiodotyrosine (MIT), which is a single iodine added to tyrosine through the action of the enzyme thyroperoxidase (TPO).
A second iodine is added to MIT through the TPO enzyme, forming diiodotyrosine (DIT). The structures of MIT and DIT are provided below to show where the iodine residues attach.
When two DIT molecules combine through the action of TPO, the hormone tetraiodothyronine is formed, which is more commonly called thyroxine or T4. T4 is then catabolized to triiodothyronine, or T3, by removing an iodine molecule, through the action of the enzyme 5' deiodinase (5DEO), which requires the cofactor selenium.
Upon release of the thyroid hormones into circulation, thyroglobulin is largely conserved and recycled within the thyroid follicles.