Female Syrian hamsters maintained on a 14 h light, 10 h dark photoperiod were injected once daily (1–2 h before lights out) with melatonin (25 μg), alone or in combination with thiourea, or with thiourea plus thyroxine. Serum levels of the somatomedin, Insulin‐like growth factor‐I (IGF‐I), were significantly reduced by thiourea as well as by melatonin administration. These data suggest that in the female hamster melatonin‐induced reduction of circulating IGF‐I depends largely on a reduction in circulating levels of thyroid hormones. However, melatonin‐induced changes in secretion of thyroid hormones, gonadal hormones, and hypothalamic hormones could contribute to decreased growth hormone (GH)‐stimulated somat
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