Figure 5.

The hypothalamic-pituitary-thyroid axis. The hypothalamus produces and secretes thyrotropin-releasing hormone (TRH), stimulating cells of the anterior pituitary to produce thyroid-stimulating hormone (TSH). TSH then stimulates secretion of thyroxine (T₄) and triiodothyronine (T₃) by the thyroid gland, the primary effect of which is to regulate cell metabolism. T₄ and T₃ both feed back to the hypothalamus and the pituitary to reduce TRH and TSH secretion.

SOURCE: Guyton, A.C. Human Physiology and Mechanisms of Disease. 5th ed. Philadephia: W.B. Saunders, 1992. pp. 566–568.