Differentiation between endogenous and exogenous testosterone in human plasma and urine after oral administration of deuterium-labeled testosterone by mass fragmentography.

A mass fragmentographic method employing stable isotopically labeled testosterone was employed to simultaneously determine the concentrations of endogenous and exogenous testosterone in the urine and plasma of two healthy male volunteers. After oral administration of 20 mg testosterone-19,19,19-d3, a slight but significant amount of the deuterated testosterone appeared in the plasma and was rapidly cleared. The results show that the exogenous testosterone did not influence the plasma levels or urinary excretion of endogenous testosterone.