[Seasonal characteristics of the effect of melatonin on thyroid function].

Changes in pituitary-thyroid axis sensitivity to bioactive component of pineal gland, melatonin, have been detected. In winter (short days) melatonin (100 micrograms/100 g of rat body weight) decreased T3 T4, TSH content and 131I uptake by thyroid tissue. However, it was noted that in summer (long photoperiod) the same injection caused suppression of T3 plasma level and 131I uptake ability of the thyroid gland, with T4 and TSH blood levels remaining significantly increased.