Prepubertal girls and gonadotropin-releasing hormone (GnRH)-deficient females secrete follicle-stimulating hormone (FSH) preferentially in response to intravenous GnRH. With continued pulsatile GnRH stimulation, FSH secretion is reduced when plasma estradiol (E2) is increasing. To delineate the mechanisms involved in these changing gonadotrpin responses, we studied the effect of low dose (0.025 μg/kg) pulsatile injections of GnRH in females with varying degrees and/or duration of endogenous GnRH deficiency (idiopathic panhypopituitarism, PHP; isolated growth hormone deficiency, IGHD; isolated gonadotropin deficiency, IGD; and anorexia nervosa, AN; both at low body weight and after weight regain). In patients presumed to have the most severe GnRH deficiency (PHP), responses of both FSH and luteinizing hormone (LH) were small and delayed, and no increase in plasma estradiol occurred during the 5 d of GnRH injections. In patients previously exposed to prepubertal or adult levels of endogenous GnRH secretion (IGHD, IGD, AN at low body weight), a rapid initial FSH response occurred that subsequently declined when plasma estradiol rose to concentrations > 40-50 pg/ml. Prior therapy with estrogen (micronized estradiol, Estrace) abolished FSH responses but LH responses were only slightly impaired. The degree of FSH response was dependent upon the time of initiation of estrogen relative to the onset of GnRH injections. Administration of estrogen after the first GnRH injection inhibited gonadotropin responses, whereas later estrogen therapy (after 1 d of GnRH pulses) blunted the GnRH induced FSH secretion without significantly impairing the LH response. In weight-regained anorexic patients who had spontaneous pulsatile LH secretion and a mean basal plasma estradiol concentration of 53 ± 15 pg/ml, administration of GnRH pulses did not change plasma LH and a minimal FSH response was seen. The data indicate that the pattern of gonadotropin responses to low dose GnRH injections depends upon the degree of previous exposure of the pituitary to endogenous GnRH. Furthermore, estradiol selectively inhibits FSH secretion by a direct action on the pituitary gland. This action of estradiol provides an explanation for the selective reduction in FSH responses to GnRH seen during pubertal maturation in girls and during the mid-follicular stage of the menstrual cycle.
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