Role of ovarian sex steroids in the regulation of thyrotropin (TSH) secretion of hypogonadal women

Abstract

The exact role of ovarian sex steroids in the neuroendocrine regulation of thyrotropin (TSH) release in women can only be accurately assessed in the absence of any considerable ovarian sex steroid feedback upon the hypothalamic-pituitary unit. Consequently, the unstimulated episodic and thyrotropin-releasing hormone (TRH) stimulated TSH secretion was evaluated in postmenopausal women before and during sequential ovarian sex steroid replacements. Seven euthyroid women (mean age: 59.4 years) were studied initially without any sex steroid replacement (control studies), then on the last day of a 21-day course of oral estradiol-valeriate (E2) administration (2 mg daily) and finally, on the last day of a 21-day course of oral estradiol-progesterone (E2/P4) replacement (2 mg E2 and 200mg micronized P4 daily). During all study occasions, blood was sampled at 10 min intervals for 10 h, while TRH (200 μg iv) was administered 8 h after initiation of blood collections. Compared to the control conditions, serum E2 and P4 concentrations markedly increased (p< 0.001) following oral E2 or E2/P4 treatments. Total triiodothyronine (T3) and thyroxine (T4) concentrations and free T3 and T4 equivalents remained unchanged during E2 and E2/P4 regimens. In the unstimulated secretory profiles, TSH was found to be episodically released, with little interindividual variability for each study condition. Since the TSH pulse attributes (pulse amplitudes, frequencies, interpulse intervals, mean TSH concentrations, by Cluster pulse algorithm) did not significantly change during E2 and E2/P4 replacements, the episodic character of TSH secretion virtually remained unchanged by sex steroid replacements. In addition, the TRH-stimulated TSH releases during E2 and E2/P4 replacement therapies closely resembled those during control conditions. These observations demonstrate that both the unstimulated episodic and TRH-stimulated TSH secretion are unaffected by ovarian sex steroid replacement in hypogonadal women. Collectively, our findings suggest that ovarian sex steroids may not be critically involved in the neuroendocrine regulation of TSH secretion in women.