The two major active hormones produced by the thyroid are T3 and T4. These are synthesised from tyrosine residues in thyroglobulin in a series of discrete steps (Fig. 15.8). They are subsequently stored in the colloid bound in thyroglobulin, and later released from the follicular cells into the circulation. The steps involved in thyroid hormone synthesis are iodine concentration and transport; oxidation; coupling and secretion; colloid resorption; proteolysis; and deiodination.
