Melatonin concentration before and during testosterone replacement in primary hypogonadic men.

OBJECTIVE: To study circadian levels of melatonin in primary hypogonadic adult men before and after testosterone treatment. DESIGN AND METHODS: Circadian serum melatonin profiles were studied in six men with primary hypogonadism before and during testosterone substitution and compared with an age-matched control group (n = 6). RESULTS: Hypogonadal patients had higher plasma melatonin concentrations than the control group during day time (34.2 +/- 8.8 compared with 5.4 +/- 0.5 ng/l, means +/- SD; P < 0.005) and night-time (74.8 +/- 34.5 compared with 30.8 +/- 3.2 ng/l). A 3 months course of testosterone replacement treatment in the hypogonadal group was followed by a diminution of the amplified melatonin circadian rhythm, with lower mean values both during the day (34.2.8 +/- 8 compared with 12.7 +/- 2.45 ng/l, P < 0.001) and at night (74.8 +/- 34.5 compared with 41.5 +/- 13.5 ng/l, P < 0.01), and a decrease in the total area under the curve (958 +/- 318 compared with 475.5 +/- 222.9, P = 0.046). There was a significant negative correlation between melatonin (r = -0.69) and testosterone concentrations. CONCLUSIONS: These data indicate that diminished testosterone in male primary hypogonadism is associated with enhanced plasma levels of melatonin, and that testosterone substitution treatment induces a deamplification of the circadian rhythm of melatonin values in humans.