Melatonin and the thyroid.

Chronic melatonin administration (100 microgram daily for 21 days) to male weanling rats increased basal TSH without altering peripheral hormone levels. There was an increase in radioactive thyroidal iodine uptake in the melatonin treated animals. Melatonin increased the thyroid gland size relative to body weight and increased the total T4 content and T4:T3 ratio in the thyroid gland. There was no effect of melatonin on TSH response to TRH or hypothalamic TRH content. Optic nerve section increases serum TSH and T3 levels. The increase in T3, but not TSH, is abolished by pinealectomy. The alterations in circulating thyroid hormone levels after optic nerve section were not altered by chronic melatonin administration.