Acute suppression of endogenous testosterone levels by exogenous testosterone in normal men

The effect of exogenous testosterone on endogenous plasma testosterone was studied in normal men. Intramuscularly administered testosterone-19,19,19-d3 rapidly appeared in the systemic circulation in large amounts. Endogenous plasma testosterone was suppressed to near-castrate levels. The suppressed level began to rise between 6 and 10 h, and reached a preinjection level at 24 h after the injection. Plasma LH decreased with a concomitant decrease in endogenous testosterone and began to rise as soon as plasma total testosterone returned to physiological levels.