Studies on the Role of Sex Steroids in the Feedback Control of FSH Concentrations in Men1

Abstract

The effect of sex steroids on plasma concentrations of FSH and LH was studied in 18 men. Steroids were administered by constant infusion for 96 hr at twice the estimated daily production rate of normal men to obtain stable levels in peripheral blood. Testosterone, 15 mg daily, increased plasma testosterone and estradiol levels 2-fold and suppressed FSH and LH approximately 40% during the 3rd and 4th days of infusion. Infusion of estradiol, 90 μg daily, caused similar suppression of FSH and LH (approximately 30%), and addition of 17α-hydroxyprogesterone, 4.5 mg daily, to the estradiol, 90 μg daily, had no additive effect. Dihydrotesterone, in pharmacologic dosage (7.5 mg daily) produced no detectible changes in plasma FSH or LH. There was no evidence for a selective effect of any of the steroids on FSH secretion. This study supports our previous concept that although there is a specific seminiferous tubular factor regulating FSH secretion, testicular steroids also modulate FSH secretion.