A Differential Effect of Ethinyl Estradiol upon Plasma FSH and LH Relating to Time of Administration in the Menstrual Cycle

Abstract

We have investigated the hypothesis that orally administered exogenous estrogen might exert differing effects upon circulating levels of plasma FSH and LH depending upon the time of administration during the menstrual cycle. Ethinyl estradiol (EE), 100 μg/day, was administered to 7 normal volunteers from days 9–18, counting the day of onset of menses as day 0. Plasma FSH and LH concentrations were compared to those from 7 untreated controls. FSH showed a transitory elevation, marked decline, and prominent rebound following cessation of EE. LH showed a sustained elevation during EE therapy, with a maximum elevation near midcycle. Maximal levels of FSH and LH, which occurred concomitantly in controls, were dissociated in 6 of 7 women receiving EE. These results were contrasted to those observed when the same dose of EE was administered from cycle days 0–7 (1) when suppression of both FSH and LH was noted without the occurrence of peaks during therapy. These results provide evidence that the effect of orally administered exogenous estrogen upon plasma gonadotropins varies with the time of administration in the menstrual cycle.