Thyrotropin-releasing hormone and the pituitary. New insights into the mechanism of stimulated secretion and clinical usage
Thyrotropin-releasing hormone, a hypothalamic tripeptide, has become a useful pharmacologic tool in clinical medicine. Evidence supporting a role for thyrotropin-releasing hormone as a physiologic regulator of thyroid-stimulating hormone (thyrotropin) but not prolactin secretion is reviewed. Data from animal studies employing thyrotropin- and prolactin-secreting cells that demonstrate that thyrotropin-releasing hormone elevates the concentration of calcium ion free in the cell cytoplasm are presented. These observations are consistent with the hypothesis that calcium ion couples stimulation by thyrotropin-releasing hormone to secretion of thyrotropin and prolactin. A molecular mechanism for thyrotropin-releasing hormone-induced elevation of cytoplasmic free calcium concentration and hormone secretion is proposed. The clinical utility of the thyrotropin-releasing hormone stimulation test in endocrine disorders is discussed. It is recommended that the thyrotropin-releasing hormone stimulation test be used to aid in the diagnosis of hyperthyroidism when other tests show equivocal results, to determine the adequacy of thyroid hormone suppression therapy, to distinguish the two forms of thyrotropin-induced hyperthyroidism, and to assess pituitary reserve of thyrotropin and prolactin.