How is the level of production of thyroid hormones governed? Include the mechanisms involved in hypothalamus, anterior pituitary, and within the thyroid gland itself.
The thyroid gland is responsible for production and secretion of thyroid hormones into the bloodstream. There are two principle forms of thyroid hormones and these are tri-iodothyronine (T3) and thyroxine (T4). These hormones are formed in the follicular cells of thyroid gland following absorption of iodide ions from the bloodstream into these cells where they are converted to iodine. Increase in iodine levels lowers production of thyroid hormones by lowered iodination through an effect that is known as Wolff-Chaikoff (Raff, 2003). Production of these hormones is also regulated by the pituitary and hypothalamus glands.
The hypothalamus synthesizes a hormone known as thyrotropin- releasing hormone (TRH) which is then transported to the anterior part of pituitary gland where it stimulates production and release of thyroid stimulating hormone (TSH) (Greenstein & Wood, 2006). This TSH in turn stimulates the production and secretion of thyroid hormones. The released T3 has the capacity to inhibit the TSH and TRH directly thus controlling its own production and secretion (Greenstein & Wood, 2006). High levels of T4 and T3 in circulation serves to inhibit TSH production and release and conversely if their level falls, there is stimulation of production and release of TSH thus they regulate their own production through feedback mechanism. The pituitary and hypothalamus glands also release enzymes known as deiodinases which regulates the levels of thyroid hormones relative to each other by removing iodine from T4 leading to formation of T3 (Greenstein & Wood, 2006). When the levels of thyroid hormones are high in circulation, the activity of these deiodinases is down-regulated in order to reduce the feedback effects produced by T3. The thyroid cells themselves also have the capacity to auto regulate production of thyroid hormones where in hypothyroidism they preferentially produce T3.
Greenstein, B. G. & Wood, D. F. (2006). The endocrine system at a glance (2nd ed.). USA:
Blackwell Publishing Ltd.
Raff, H. (2003). Physiology secrets. Philadelphia, PA: Hanley & Belfus, Inc.