Low blood levels of the thyroid hormones — triiodothyronine (T3) and thyroxine (T4) — signal the hypothalamus to release the thyrotropin-releasing hormone (TRH). TRH then reaches the pituitary gland and stimulates the release of thyroid-stimulating hormone(TSH) into the bloodstream.
Upon reaching the thyroid gland, TSH stimulates the follicular cells' active uptake of iodide ions from the blood. The ions diffuse to the apical surface of the cells and are oxidized to iodine. The iodine is then added to tyrosine residues of the thyroglobulin protein stored in the lumen colloid, forming monoiodotyrosine (T1) and diiodotyrosine (T2). The coupling of one molecule of T1 with T2 forms a T3 molecule, whereas two T2 molecules combine together to create T4.
Finally, endocytosis and enzymatic cleavage release the T3 and T4 in the follicular cells. These hormones then cross into the blood vessels for transport to target tissues. An increase in blood levels of T3 and T4 temporarily inhibits the production of TRH and TSH, regulating the level of thyroid hormones in the body.
