The hypothalamic-pituitary-thyroid axis and its feedback control.
The thyroid gland is stimulated by TSH secreted by the anterior pituitary. Secretion of TSH is stimulated in turn by hypothalamic TRH and is inhibited by the hypothalamic factor somatostatin. TSH secretion is also inhibited by the principal secretions of the thyroid ΓÇô T4 and T3, thus forming a negative feedback loop. In addition to exerting direct inhibitory actions at the pituitary level, the thyroid hormones inhibit the secretion of TRH and stimulate the secretion of somatostatin, thus adding a hypothalamic level of negative feedback control of TSH secretion. The regulatory system is made even more complex by the intrahypothalamic and intrapituitary conversion of T4 to the more-potent T3, a process that is regulated by thyroid hormones and by the hypothalamic factors. Modified from Reichlin (1985); by courtesy of WB Saunders Company.
