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

Abstract

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 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. © 1985 by The Endocrine Society.