SUPPRESSION OF TRH-STIMULATED TSH SECRETION BY GLUCOSE-INDUCED HYPOTHALAMIC SOMATOSTATIN RELEASE

To determine whether the combined glucose-thyrotropin-releasing hormon e (TRH) test can be a useful method for the evaluation of the hypothal amic somatostatinergic activity, we investigated whether TRH-induced t hyroid stimulating hormone (TSH) secretion can be suppressed by the or al glucose administration that stimulates the hypothalamic somatostati n (SRIH) secretion. Six tests were performed in ten healthy young men. Test 1: 1 ml of normal saline was intravenously administered at 0 min . Test 2: TRH was administered intravenously at 0 min. Test 3: Glucose , 75 g, was administered orally at -60 min. Test 4: Glucose and TRH we re administered as above. Test 5: Pyridostigmine (PST), 120 mg, was gi ven orally at -90 min followed by the administration of GH and TRH as above. Basal TSH levels were suppressed slightly, but significantly, i n Test 3 compared to those observed in Test 1. The oral glucose admini stration also significantly suppressed TRH-stimulated TSH response by 27 - 35 % between 40 min and 80 min in Test 4. In contrast, the pretre atment with PST completely reverted the suppressive effect of glucose on TRH-stimulated TSH response in Test 5. These data suggest that the increased hypothalamic SRIH secretion induced by oral glucose administ ration can suppress TRH-stimulated TSH response in normal men, and the combined glucose-TRH test can be a useful method to evaluate the hypo thalamic somatostatinergic activity.