Hypothalamic–pituitary–gonadal axis in two men with aromatase deficiency: evidence that circulating estrogens are required at the hypothalamic level for the integrity of gonadotropin negative feedback

Background: In men, the feedback of gonadotropins is regulated by estrogens that come from the aromatization of testosterone, but the relative contribution to the inhibition of LH and FSH secretion by the amount of locally produced estrogens within the hypothalamus and/or the pituitary, and the amount of circulating estrogens still remains unknown. Objective: In order to evaluate the effect of regulation induced by estradiol on the hypothalamic–pituitary–gonadal (HPG) axis, we studied the pulsatility of LH and FSH in two aromatase-deficient men (called subject 1 and subject 2), in which the production rate of estrogen (both local and circulating) is completely, or at least severely, impaired. Design: FSH and LH were evaluated in terms of their pulsated secretion and as GnRH-stimulated secretion in two phases: phase 1, before estrogen treatment; and phase 2, during estrogen treatment with 25 μg transdermal estradiol twice weekly. Methods: Blood samples were taken during phase 1 and phase 2 at 0800 h for basal measurements of LH, FSH, inhibin B, testosterone, and estradiol. The analysis of the pulsatility of LH and FSH was performed by sampling every 10 min for 8 h in the two phases. Gonadotropin response to GnRH-stimulation test was studied by serial standard sampling after 100 μg GnRH i.v. bolus in phases 1 and 2. Results: Estrogen treatment led to a significant reduction in both LH-pulsated frequency (7.5 ± 0.7 in phase 1, 4.5 ± 0.7 in phase 2) and amplitudes (3.5 ± 0.006 in phase 1, 1.9 ± 0.4 in phase 2) of peaks, whereas FSH showed only a conspicuous reduction in serum levels and a trend towards the reduction of the amplitudes of its peaks without modification of the frequency of the pulses. Both testosterone and gonadotropins decreased during phase 2, whereas estradiol reached the normal range in both subjects. Transdermal estradiol treatment significantly lowered the peaks of both serum LH and FSH after GnRH as well as the incremental area under the curve after GnRH administration in both subjects. Basal serum inhibin B levels were slightly higher before transdermal estradiol treatment (phase 1) than during estrogen treatment (phase 2) in both subjects. Conclusions: The administration of estrogen to aromatase-deficient men discloses the effects of circulating estrogens on LH secretion, exerted both at pituitary level, as shown by the decrease of basal and GnRH-stimulated secretion of LH and the LH pulsed amplitude, and at hypothalamic level as shown b