Acute Effects of Intravenous Infusion of 17β-Estradiol on Gonadotropin Release in Pre- and Post-menopausal Women

In order to quantitate the feedback effect of estradiol on the release of pituitary gonadotropins, the acute responses of circulating LH and FSH to the constant infusion of 17β-estradiol (E2) were studied in pre- and postmenopausal women. The duration of infusion ranged from 3 to 12 hr (mean 7.5 hr) and the dosage from 50 μg to 4 mg. In premenopausal women, during the follicular phase of the cycle, a significant and rapid decline in serum FSH and LH in response to E2 infusion was observed. The quantitative response to the negative feedback effect of E2 infusion is greater for LH (47%) than for FSH (17%), which could be accounted for at least in part by the slow disappearance rate of FSH. This disparity was also noted during the recovery period. A positive teedback effect of E2 to raise the LH concentration to above the preinfusion value was observed at 24 and 48 hr following the infusion. By contrast, the recovery of FSH followed a progressive course and returned to the preinfusion value at 72 hr following the infusion, as did LH. In post-menopausal subjects, the pattern of decline was similar but the magnitude is greater for both LH (55 vs. 47%) and FSH (39 vs. 17%). A similar recovery pattern was noted except that the LH concentration did not exceed the preinfusion level. It is suggested that: 1) E2 infusion in doses greater than 50 μg in premenopausal women and 400 μg in post-menopausal women showed consistently suppressive effect on the serum concentrations of FSH and LH. 2) The positive feedback effect of E2 on LH release following the infusion is probably related to a temporary increase in pituitary store which resulted from a decreased release and/or increased synthesis in response to E2 infusion. 3) The slow recovery and the absence of abrupt increase in FSH concentration following the infusion suggests that estradiol exerts more negative feedback action on FSH than on LH.