Regulation of thyroid hormone synthesis. Left. Thyroid hormones T₄ and T₃ feed back to inhibit hypothalamic production of thyrotropin-releasing hormone (TRH) and pituitary production of thyroid-stimulating hormone (TSH). TSH stimulates thyroid gland production of T₄ and T₃. Right. Thyroid follicles are formed by thyroid epithelial cells surrounding proteinaceous colloid, which contains thyroglobulin. Follicular cells, which are polarized, synthesize thyroglobulin and carry out thyroid hormone biosynthesis (see text for details). TSH-R, thyroid-stimulating hormone receptor; Tg, thyroglobulin; NIS, sodium-iodide symporter; TPO, thyroid peroxidase; DIT, di-iodotyrosine; MIT, monoiodotyrosine