Effect of finasteride, a 5 alpha-reductase inhibitor, on serum gonadotropins in normal men.

Testosterone exerts negative feedback control on gonadotropin secretion either directly, after aromatization to estradiol, or after 5 alpha-reduction to dihydrotestosterone (DHT). Conflicting data exist as to the role of DHT in the modulation of this negative feedback. To determine whether suppression of endogenous DHT alters gonadotropin secretion, we gave the selective 5 alpha-reductase inhibitor finasteride (5 mg daily), or placebo, to 20 healthy men for 28 days. Basal and GnRH-stimulated LH, bioactive LH, FSH, testosterone, and DHT levels were measured before and after 14 and 28 days of treatment. Basal DHT fell from 1.1 +/- 0.2 to 0.15 +/- 0.04 nmol/L after 28 days of finasteride treatment. A significant rise in baseline testosterone from 17.6 +/- 2.0 to 18.3 +/- 2.3 nmol/L was seen at 14 days (P = 0.046), but not at 28 days. No significant changes were seen in either basal or GnRH-stimulated gonadotropin levels on any day. We conclude that suppression of serum DHT levels with 5 mg finasteride daily in healthy young men has no discernible effect on serum gonadotropin levels.