THE ROLE OF MELATONIN IN THE REGULATION OF THYROTROPHIN SECRETION

Abstract

The fact that melatonin, a pineal substance, causes a depression in thyroid function of prepuberal rats has been reported. A possible site of this action on the thyroid gland has been studied by estimating the TSH content of blood and hypophysis in rats from 35 days to 65 days of age. Hemagglutination-inhibition technique has been used to assay very small amounts of TSH in plasma. Rats receiving 100 [mu]g of melatonin daily for 10 days were sacrificed at 35, 45, 55 and 65 days of age. The mean thyroid weight of each group was markedly higher than that of the corresponding controls. The plasma TSH/ml level was higher in experimental groups, with a marked decrease (0.10 > P> 0.05) in TSH/mg of hypophysis (wet), especially at 45 and 55 days of age. Rats similarly treated with 400 [mu]g/100 g body weight of tapazole dally for 10 days and sacrificed at 45 and 55 days of age showed higher plasma and lower hypophyseal TSH levels than the controls and the results were comparable to those of the melatonin treated groups. The dry-fat-free tissue of the thyroid glands of the melatonin treated groups were higher than the corresponding controls and their DNA content was significantly higher (0.050 > P > 0.025) also, indicating hyperplasia and hypertrophy of the thyroid glands resulting from the action of melatonin. The histological picture of the melatonin treated animals showed goitrogenic effect. It may be concluded from these data that melatonin exerts its regulatory effect on TSH secretion directly acting on the thyroid gland and in some way inhibiting thyroid hormone synthesis or release.