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The biphasic pattern in the feedback action of ethinyl estradiol on the release of pituitary FSH and LH.

ABSTRACT Assessment of sequential changes of circulating LH and FSH in response to ethinyl estradiol (EE) treatment was made in 7 postmenopausal women. During a relatively short period of treatment (5–8 days) EE (400 μg/day) initially exerts a negative feedback action which was greater for FSH (36% of control) than for LH (49% of control). This was followed by the occurrence on the 3rd and 4th day of an augmented phase of gonadotropin release which consisted of either acute surges or oscillations. This positive feedback action of EE is quantitatively much greater for LH than for FSH. These results indicate that EE exerts a biphasic action (suppression followed by augmentation) on the gonadotropin-releasing mechanism and that a differential estrogen feedback control between FSH and LH exists.