Biphasic Change in Pituitary Capacity Induced by Estrogen in Hypogonadal Women*

Changes in response to pulses of LRF (10 ¼g at 2-h intervals × 3) and TRF (200 ¼g at 2-h intervals × 3) for LH, FSH, and PRL were measured in 5 post-menopausal women before and during the administration of a large dose of ethinyl estradiol (400 μg/day) for 5 days. Responses to LRF and TRF stimulation on the 2nd and 5th days of ethinyl estradiol treatment were studied in 2 experimental periods in the same individual at an interval of at least 10 weeks between treatments. During 5 days of ethinyl estradiol treatment, LH first declines (50%) and then returns to its original level (U-shaped curve). For FSH, the initial fall in the basal level is not followed by a return to pretreatment level. These dynamic changes in gonadotropin levels are accompanied by an exponential increase of more than 3-fold in the PRL levels. The bidirectional (U-shaped) pattern of LH release is associated with parallel changes in pituitary sensitivity to a 10-fig pulse of LRF. Pituitary reserve, defined as the responseto the 2nd and 3rd pulses of LRF (10 μg), however, exhibits a remarkable undirectional augmentation. The progressive increase in PRL release is unaccompanied by changes in pituitary PRL sensitivity and reserve. The contribution of hypothalamic LRF and dopamine in the participation ofthese functional changes of LH, FSH, and PRL are discussed. (J Clin Endocrinol Metab46: 369, 1978)