THE CLINICAL IMPACT OF THE THYROTROPIN-RELEASING-HORMONE TEST

Because of its ability to cause the release of thyrotropin (TSH), prol actin (PRL), and, under particular circumstances, also of other adenoh ypopyseal hormones, from the pituitary, thyrotropin-releasing hormone (TRH) has been widely used as a diagnostic tool for about 30 years. Th e recent introduction of an ultrasensitive TSH assay, able to clearly distinguish suppressed from unsuppressed TSH levels, has tendered the use of the TRH test obsolete in the diagnosis of classic hyperthyroidi sm. On the contrary, the TRH test is still extremely useful in hyperth yroid patients with inappropriate secretion of thyrotropin, allowing t he distinction between TSH-secreting pituitary tumors (usually unrespo nsive) and the pituitary variant of resistance to thyroid hormone (PRT H) syndrome (always responsive). In hypothyroidism, the TRH test is st ill of value in patients with preclinical primary hypothyroidism, as t hey show exaggerated TSH response, and in those with central hypothyro idism, allowing the differentiation between pituitary (secondary) and hypothalamic (tertiary) hypothyroidism. The availability of high-resol ution imaging techniques such as magnetic resonance has rendered the u se of the TRH test obsolete, to distinguish microprolactionomas from f unctional hyperprolactinemia. The TRH test still has great clinical va lue in the follow-up of patients with pituitary tumors (in particular somatotropinomas and clinically nonfunctioning pituitary adenomas) sho wing abnormal responses of anterior pituitary hormones other than TSH.