Effects of melatonin and thyroxine treatment on reproductive organs and thyroid hormone levels in male hamsters.

The effects of exogenous thyroid hormone administration on melatonin-induced gonadal atrophy were studied in young adult male hamsters. Animals were given daily afternoon injections of 25 micrograms melatonin and/or thrice-weekly injections of 7 micrograms thyroxine. A control group received injections of saline vehicle only. No significant effect of either treatment was seen on body weight. Melatonin-treated animals had significantly reduced testicular and seminal vesicle weights, compared with vehicle-treated control animals: serum T4 levels and the FT4I were reduced by melatonin treatment, but serum T3 and the FT3I were increased. No effect of melatonin was seen on in vitro T3 uptake. Thyroxine treatment alone, while leading to elevated serum T4 levels and FT4I, had no effect on T3, the FT3I, or T3 uptake. Thyroxine treatment normalized circulating T4 levels and the FT4I in melatonin-treated animals, without preventing the melatonin-induced testicular regression. The results are taken as evidence that the inhibitory effects of melatonin on gonadal and thyroid function are independently mediated.