Effects of Daily Afternoon Melatonin Administration on Body Weight and Thyroid Hormones in Female Hamsters

Female hamsters were treated each afternoon for 8 weeks with subcutaneous injections of 25 μg of either melatonin or vehicle solution. Animals were sacrificed in either the morning or afternoon of diestrus and proestrus, along with their respective melatonin‐induced acyclic pairs. Melatonin‐treated hamsters had significantly greater mean body weights than did the vehicle‐treated hamsters (P < 0.05). Terminal mean serum T4 levels and free thyroxine index (FT41) were significantly reduced (P < 0.05) in melatonin‐treated hamsters compared with values obtained from animals subjected to the vehicle alone. In addition, T4 levels, FT41, T3 levels, and FT31 were significantly influenced by day and time of sacrifice independently of the melatonin treatment. T3 uptake was significantly lower on the morning of proestrus in melatonin‐treated animals. These results demonstrate that chronic afternoon melatonin administration in female hamsters results in the loss of estrous cyclicity, a significant gain in body weight, and the reduction of T4 levels and T3 uptake. Changes in serum T3 levels are a function of the time of sample collection and are not influenced by melatonin treatment. In addition, these data indicate that thyroid function in general changes during the estrous cycle in these animals.