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In Men, Peripheral Estradiol Levels Directly Reflect the Action of Estrogens at the Hypothalamo-Pituitary Level to Inhibit Gonadotropin Secretion

Department of Endocrinology (G.R., W.d.R.), Vrije Universiteit Medical Center, 1007 MB Amsterdam, The Netherlands; Department of Internal Medicine (F.H.d.J.), Erasmus Medical Center, 3000 DR Rotterdam, The Netherlands; and Department of Internal Medicine (J.-M.K.), Ghent University Hospital, B-9000 Ghent, Belgium

Address all correspondence and requests for reprints to: W. de Ronde, Department of Endocrinology, VU University Medical Center, de Boelelaan 1117, P.O. Box 7057, 1007 MB Amsterdam, The Netherlands. E-mail: p.deronde{at}vumc.nl.

Context: Estradiol inhibits gonadotropin release in men by an action at the hypothalamus and pituitary. Because of the tissue-specific regulation of aromatase, peripheral estradiol levels may not reflect brain estradiol concentrations.

Objective: We evaluated whether local aromatization of testosterone in the hypothalamus or pituitary is important for gonadotropin release and to what extent circulating estrogens affect gonadotropin levels and peripheral testosterone levels.

Design, Subjects, and Interventions: We suppressed aromatase activity in 10 young healthy men with letrozole 2.5 mg once daily, restored plasma estradiol levels with estradiol patches (100 µg/d for the first week, 50 µg/d the second week, 25 µg/d the third week, and no estradiol patch the fourth week) and measured plasma testosterone, estradiol, LH, FSH, and SHBG levels.

Results: The mean estradiol and testosterone levels during the study ranged between 68.6 ± 38.3 and 12.6 ± 7.21 pg/ml for estradiol and 179 ± 91 and 955 ± 292 ng/dl (mean ± SD) for testosterone. Levels of testosterone, LH, and FSH were inversely related to peripheral estradiol levels. During letrozole use, the mean plasma estradiol level needed to restore testosterone, LH, and FSH levels to baseline levels was not significantly different from the baseline mean estradiol level.

Conclusions: Local aromatization of testosterone in the hypothalamo-pituitary compartment is not a prerequisite for expression of the inhibitory action of estrogens on gonadotropin secretion in men. Peripheral estradiol levels directly reflect the inhibitory tone exerted by estrogens on gonadotropin release and are a major determinant of peripheral testosterone, LH, and FSH levels.

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