Site of negative feedback action of estrogen on gonadotropin secretion in normal women.

We have reported that iv administration of conjugated estrogens results in no significant change in the plasma LH-RH level during the negative feedback phase of LH, suggesting that estrogen does not suppress LH by decreasing hypothalamic LH-RH. To determine the site of estrogen action during the negative feedback phase, we studied the pituitary response to a small amount of LH-RH after estrogen administration in normal cyclic women in the midfollicular phase. The pituitary responses to an iv bolus of 2.5μg of synthetic LH-RH were evaluated by measuring serum LH and FSH 2 h before and 8 h after administration of 20mg of conjugated estrogens (Premarin). The mean levels of serum LH and FSH were significantly (p<0.05) decreased 8 h after the injection. The peak responses of LH and FSH to LH-RH were also significantly (p<0.05) reduced after Premarin administration. These findings suggest that the negative feedback effect of estrogen on gonadotropin secretion is caused by its direct suppression on the pituitary response to LH-RH.