Idiopathic Hypogonadotropic Hypogonadism in Men: Dependence of the Hormone Responses to Gonadotropin-Releasing Hormone (GnRH) on the Magnitude of the Endogenous GnRH Secretory Defect*

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Abstract
Idiopathic isolated gonadotropin deficiency (IGD) is associated with a spectrum of clinical findings as well as variable gonadotropin responses to GnRH. In this study we investigated whether patterns of gonadotropin and testosterone responses to pulsatile GnRH therapy (25 ng/kg, iv, every 2 h for 4 days) were related to the magnitude of the GnRH secretory defect in patients with IGD. Eight men with IGD were studied. Patients with partial IGD (p-IGD) and those who had evidence of GnRH secretion (n-IGD) were differentiated by the presence or absence of spontaneous LH secretory pulses during 24 h of every 20-min blood sampling. In response to the first GnRH injection, no LH rise occurred in the n-IGD patients, while LH increases in the p-IGD patients were similar to those in normal men. Continuation of GnRH therapy in patients with n-IGD resulted in predominant FSH secretion and absent or minimal augmentation of LH and T secretion. In contrast, predominant LH secretion occurred in the p-IGD patients and resulted in a significant increase in serum testosterone. A bolus dose of GnRH 2 days after the termination of GnRH therapy caused significant augmentation of gonadotropin responses in the n-IGD, while in the p-IGD group, both LH and FSH responses were unchanged compared to those after the first GnRH pulse. These results indicate that IGD is characterized by variable degrees of endogenous GnRH deficiency. Moreover, the hormone responses to GnRH in IGD patients depend on the magnitude of the underlying GnRH secretory defect.

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