Disparate serum free testosterone concentrations and degrees of hypothalamo-pituitary-luteinizing hormone suppression are achieved by continuous versus pulsatile intravenous androgen replacement in men: a clinical experimental model of ketoconazole-induced reversible hypoandrogenemia with controlled testosterone add-back.

To investigate the neuroendocrine mechanisms underlying the negative feedback actions of testosterone on both the pulsatile mode of LH release and the entropy or disorderliness of the LH release process, we blocked testicular androgen biosynthesis using oral high dose ketoconazole treatment with concomitant low dose glucocorticoid replacement for 48 h in six healthy young men. Volunteers were then infused iv with saline or a total of 8.0 mg testosterone base over the second 24 h via either a continuous or a pulsatile (90-min boluses) delivery pattern. Discrete peak detection (Cluster analysis) was applied to obtain a model-independent estimate of the frequency of serum LH concentration peaks, maximal and incremental LH peak amplitudes, peak area, and interpeak nadir serum LH concentrations. Approximate entropy was used to quantify the relative orderliness/disorderliness of the LH release process over 24 h. Ketoconazole treatment markedly lowered 24-h mean serum total and free testosterone concentrations (by 17- and 9-fold respectively), and significantly increased LH pulse frequency, maximal LH peak height, and interpeak nadir serum LH concentrations. Continuous iv testosterone add-back increased 24-h pooled serum free testosterone concentrations 3-fold more and concomitantly reduced mean (24-h) serum LH concentrations by at least 2-fold more than pulsatile delivery of the same total daily amount of androgen. Both modes of testosterone infusion suppressed pulsatile LH release, but the effects were distinguishable; namely, treatment with continuous vs. intermittent androgen add-back, respectively, decreased LH pulse frequency and incremental LH pulse amplitude. Ketoconazole treatment alone also significantly increased approximate entropy values, indicating greater disorderliness of LH release during androgen removal. Approximate entropy/orderliness was restored to baseline by continuous, but not pulsatile, iv testosterone replacement. In conclusion, the present novel testosterone add-back clinical experimental paradigm indicates that 1) remarkably different 24-h mean serum free testosterone concentrations can result from continuous vs. pulsatile testosterone delivery into the bloodstream; 2) androgen negative feedback can exert frequency- as well as amplitude-dependent suppression of pulsatile LH release; and 3) testosterone is required to maintain an orderly 24-h LH release process in young men.