Feedback on LH in Testosterone-Clamped Men Depends on the Mode of Testosterone Administration and Body Composition

CONTEXT: Quantitative studies of the short-term feedback of testosterone (T) on luteinizing hormone (LH) secretion in healthy men are relatively rare. Such studies require the shutting down of endogenous T secretion and the imposition of experimentally controlled IV T addback. OBJECTIVE: To evaluate whether pulsatile and continuous T delivery confers equivalent negative feedback on LH secretion. DESIGN: This was a placebo-controlled, blinded, and prospectively randomized crossover study comprising 16 healthy men [age range 23 to 54 years and a body mass index (BMI) between 22.3 and 34.2 kg/m(2)]. Subjects received ketoconazole to block endogenous T secretion and received continuous or 90-minute pulses of IV T addback. SETTING: The study was performed in a Clinical Translational Research Unit. INTERVENTIONS: Subjects underwent 14 hours of blood sampling at 10-minute intervals, with a bolus IV injection of 33 ng/kg gonadotropin-releasing hormone (GnRH). MAIN OUTCOME MEASURES: Log-transformed LH and T concentration ratios before and after GnRH administration. RESULTS: Despite higher T concentrations during pulsatile T feedback, LH concentrations and secretion rates, whether driven by endogenous or exogenous GnRH, were similar to those during continuous T infusion, indicating diminished pulsatile T feedback. Feedback correlated negatively with BMI. Under controlled T feedback, basal but not pulsatile LH secretion correlated negatively with CT-estimated visceral fat mass. CONCLUSION: Feedback by pulsatile T delivery has diminished inhibitory strength compared with continuous infusion. Feedback is negatively correlated with BMI.