Effects of melatonin on thyroid physiology of female hamsters

The effects of melatonin administration of thyroid physiology of female hamsters was investigated. A protocol of 25 μg given daily as subcutaneous injections late in the light period was found to inhibit blood levels of thyroxin (T₄), triiodothyronine (T₃) and thyrotropin (TSH). The free T₄ index (FT₄I) and the free T₃ index (FT₃I) were also significantly inhibited by melatonin injections. Decreasing the photoperiod under which the hamsters were kept, from 14 h light/10 h dark (14L/10D) to 10L/14D also resulted in decreased blood levels of these hormones. A protocol of melatonin injections using 2.5 mg daily, on the other hand, did not significantly inhibit blood levels of thyroid hormones or TSH; injection of this dose every afternoon into hamsters in long photoperiod significantly augmented the blood levels of T₄. Continuously available melatonin in the form of subcutaneous implants of 1 mg melatonin in beeswax did not inhibit blood levels of thyroid hormones; furthermore, such implants prevented the inhibitory effects of injections of 25 μg melatonin. The results are consistent with the hypothesis that melatonin interferes with neurotransmitters which influence the synthesis or release of hypothalamic thyrotropin-releasing hormone.