Effects of the pineal and melatonin on thyroid activity of male golden hamsters

The effects of blinding and pinealectomy in normal and gonadectomized hamsters were studied. Blinding was associated with depressed thyroxin levels and free thyroxin index in normal but not in gonadectomized animals. Pinealectomy, by itself, had no effect on thyroxin levels but reversed the effects of blinding. Pinealectomy of gonadectomized animals had no effect on thyroxin levels but was associated with increased thyroxin levels and free thyroxin index in gonadectomized hamsters that were blinded. Different effects of blinding and thiourea treatment suggested different sites of action. Melatonin injections, like blinding, were associated with a depression of thyroxin levels and free thyroxin index. Similar effects of the pineal and melatonin on gonad weights and on thyroxin levels suggest a common site of action of the pineal gland in regulation of pituitary-gonadal and pituitary-thyroid axes. Another interpretation of these data is that the effects of the pineal and melatonin on thyroid function are secondary to their effects on the reproductive system.