Suppression of TRH-Stimulated TSH Secretion by Glucose-Induced Hypothalamic Somatostatin Release

Abstract

To determine whether the combined glucose-thyrotropin-releasing hormone (TRH) test can be a useful method for the evaluation of the hypothalamic somatostatinergic activity, we investigated whether TRH-induced thyroid stimulating hormone (TSH) secretion can be suppressed by the oral glucose administration that stimulates the hypothalamic somatostatin (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 were administered as above. Test 5: Pyridostigmine (PST), 120 mg, was given orally at -90 min followed by the administration of GH and TRH as above. Basal TSH levels were suppressed slightly, but significantly. In Test 3 compared to those observed in Test 1. The oral glucose administration also significantly suppressed TRH-stimulated TSH response by 27-35% between 40 min and 80 min in Test 4. In contrast, the pretreatment 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 administration can suppress TRH-stimulated TSH response in normal men, and the combined glucose-TRH test can be a useful method to evaluate the hypothalamic somatostatinergic activity.

Acute Hyperglycemia and Activation of the .BETA.-Adrenergic System do not Exhibit Synergistic Inhibitory Actions on Thyrotropin-releasing Hormone (TRH)-induced Thyroid Stimulating Hormone (TSH) Secretion

“…As previously reported by our laboratory [20,21], oral glucose significantly suppressed TSH response to TRH stimulation. Also, isoproterenol alone reduced TSH release in response to TRH.…”

“…We have reported that acute hyperglycemia suppresses TRH-induced TSH release, and that pretreatment with pyridostigmine, a cholinesterase inhibitor, causes a reversal of the glucose-induced suppression of TRHstimulated TSH secretion [20]. These findings exclude the possibility that glucose may act by inhibiting endogenous TRH release.…”

“…This is supported by the ability of oral glucose to inhibit TRH-stimulated TSH release [20,21] and by the observation that isoproterenol suppresses basal TSH release [22][23][24]. However, the combined effect of isoproterenol and glucose on the TRHinduced TSH response has not been reported.…”

Somatostatin (SRIF) Receptor Subtype 2 and 5 Gene Expression in Growth Hormone-secreting Pituitary Adenomas: The Relationship with Endogenous SRIF Activity and Response to Octreotide

“…Since the SRIF levels cannot be measured directly in the patient, we estimated HSA by the percent suppression of TSH during the combined glucose-TRH test as we reported previously [13]. Previous studies demonstrated that the increased secretion of SRIF induced by oral glucose loading suppressed the basal and TRH-stimulated TSH release [19,21]. Therefore, measurement of TSH secretion after modulation of SRIF neuronal activity by glucose can be a useful method to evaluate HSA activity.…”

“…On day 3, blood was obtained every hour from 0900-1600 hr for determination of the basal daytime GH secretion pattern. On day 4, the combined glucose-TRH test was undertaken by administering 75 g glucose, orally, 30 min before the injection of TRH as reported previously [19]. HSA was assessed by glucose-induced suppression of TRH-stimulated TSH secretion as described previously [13].…”