Testosterone doping remains a prevalent and potent form of drug cheating among elite athletes. In men, the urine testosterone (T) to epitestosterone (E) ratio (T/E ratio) can identify administration of exogenous T by its suppression of endogenous T production through strong negative feedback on endogenous T and E production as well as spill over into urine of extra testosterone. However, this mechanism may be partially inoperative in females whose much lower circulating T derives from three sources, none subject to powerful negative T feedback. Hence, additional methods to detect T doping in females are required. In this study we report two cases of elite female athletes who were sanctioned for T doping proven by measurement of serum T using liquid chromatography-mass spectrometry (LC-MS), when serial urine T and T/E ratio in one were not indicative of T doping, and in the other were nullified by incidental genetic inactivation of T glucuronidation through the uridine diphosphoglucuronosyl transferase 2B17 (UGT2B17) deletion genotype-phenotype. These findings indicate the potential for serum T measurement by LC-MS to detect T doping in female athletes, especially if implemented in the Bayesian format of an athlete biological passport.