Figure 1.

TRH-TSH-T3 feedback loop. The hypothalamic neurons secrete thyrotropin releasing hormone (TRH) which is carried down to the adenohypophysis of the pituitary by the hypothalamic portal vein where it releases thyroid stimulating hormone (TSH). The released TSH reaches thyroid glands via blood stream to bind to TSH receptor (TSHR) to stimulate production and release of thyroxin (T4) and T3. T3 exerts its actions on bone mainly by binding to TRa. TSH can also act directly on bone cells by binding to TSHR. Increased levels of T3 can act by negative feedback loop via TRβ to inhibit release of TRH and TSH, thereby preventing hyperparathyroidism.