EFFECTS OF OESTROGEN ADMINISTRATION ON TESTOSTERONE METABOLISM IN NORMAL MEN

SUMMARY The effects of oestrogen administration on various parameters of testosterone metabolism (metabolic clearance rate, plasma concentration, production rate, level of binding to plasma proteins) were studied in normal men. Oral oestrogen administration (ethinyl oestradiol, 0.05 mg b.i.d. for 4 days) decreased the metabolic clearance rate (32%), plasma concentration (49%) and production rate (65%) and increased the level of binding of testosterone to plasma proteins (4.6%); the testosterone‘free index’ (plasma concentration X% unbound) decreased by 75%. An intravenous infusion of oestrogen (i.v. injection of 5 mg stilboestrol diphosphate followed by constant infusion of 47.5 μg/min for 30 min prior to and during the experiment) had no significant effects on the above parameters. The net effect of oral oestrogen administration is to decrease the amount of testosterone available for entry into tissues. The testosterone ‘free index’ showed a significant correlation with the metabolic clearance rate.