Free plasma testosterone levels during the normal menstrual cycle.

This report describes the free or unbound testosterone levels in ten normal females during the menstrual cycle, using a simplified technique of equilibrium dialysis of undiluted plasma. Total testosterone concentration fell progressively during the menstrual cycle, whereas the percent free testosterone increased from the follicular to the luteal phase. Free testosterone levels also fell but only significantly at the late luteal phase. For comparison two patients with anovulatory cycles were evaluated. Progesterone displacement of endogenous testosterone from its binding protein(s) was suggested by in vitro studies.