Effects of melatonin and thyroxine replacement on thyrotropin, luteinizing hormone, and prolactin in male hypothyroid hamsters.

The effects of daily evening melatonin injections on plasma and pituitary levels of TSH, LH, and PRL in hypothyroid hamsters maintained under a 14-h light, 10-h dark photoperiod were investigated. Circulating levels of thyroid hormones were monitored, and testicular weights were recorded. Thiourea-induced increases in serum and pituitary TSH were significantly reduced by melatonin injections. Control hamsters, not receiving thiourea, responded to daily evening melatonin injections with a decrease in serum T4. Serum T3 levels were decreased by thiourea and increased by T4 replacement. The pituitary PRL content was significantly reduced below control values in hamsters receiving melatonin injections; the sensitivity to melatonin was inhibited by thiourea and restored by T4 replacement. T4 replacement injections were associated with a significant decrease in serum PRL and a significant increase in serum LH. Melatonin-induced testicular involution was attenuated by thiourea administration; this attenuation was reversed by T4 replacement. Similarly, a melatonin-induced decrease in serum LH was prevented by thiourea administration and restored by T4 replacement. The data suggest that melatonin injections increase the sensitivity of the pituitary to T4/T3 feedback inhibition of TSH. Furthermore, these data show that thyroid status may influence melatonin-induced changes in release of PRL and LH from the hamster pituitary. The data showing an interaction of thyroid hormones in the control of testicular size are interpreted as evidence that thyroid hormones play a major role in the mechanism regulating gonadal cycles in the hamster.