• testosterone [ Time Frame: daily for 7 days ] [ Designated as safety issue: No ]
  
• LH [ Time Frame: frequent sampling for 2 hours ] [ Designated as safety issue: No ]
  
• FSH [ Time Frame: frequent sampling for 2 hours ] [ Designated as safety issue: No ]
  
• Inhibin B [ Time Frame: daily for 7 days ] [ Designated as safety issue: No ]
  
• free alpha subunit [ Time Frame: daily for 7 days ] [ Designated as safety issue: No ]
  

Drug: gonadotropin releasing hormone (GnRH)
Pulsatile GnRH (25 ng/kg per bolus every two hours via microinfusion pump titrated to reach normal serum testosterone levels)

Despite variability in the triggers, timing, and pace of sexual maturity between species, all species utilize the final pathway of hypothalamic secretion of gonadotropin releasing hormone (GnRH) to initiate and maintain the reproductive axis. Thus, GnRH is required for reproductive competence in the human. The classic studies from the 1970s clearly demonstrate that pulsatile release of GnRH from the hypothalamus is a prerequisite for physiologic gonadotrope function. Absence, decreased frequency or decreased amplitude of pulsatile GnRH release results in the clinical syndrome of hypogonadotropic hypogonadism (HH). The phenotypic expression of GnRH deficiency in the human demonstrates considerable heterogeneity. Defining the physiology of GnRH is critical to understanding the clinical heterogeneity of isolated GnRH deficiency and its comparison to other conditions resulting in hypogonadotropic hypogonadism (HH). The overall goal of this protocol is to investigate the neuroendocrine control of reproduction and specifically the physiology and pathophysiology of GnRH secretion and action in the human male.

Subjects will be selected from a group of adult men (18-65 years)based on the demonstration of a low testosterone level (<100 ng/dL) in association with low or inappropriately normal gonadotropin levels. All patients will undergo an initial assessment that includes an overnight 12-hour frequent blood sampling study to determine their degree of endogenous GnRH secretion. Following the overnight evaluation, subjects will have daily outpatient visits for 7 consecutive days when they will receive a GnRH bolus followed by 2hrs of blood sampling.