[Epiphysis cerebri-thyroid interrelations. Distorting effect of partial thyroidectomy on the antithyroid action of...

The effect of pineal methoxyindoles (MI) on hypothalamus-hypophysis-thyroid gland system was studied in intact and partially thyroidectomized male Wistar rats in conditions of a short light day (winter). Melatonin administration for 10 days suppressed 131I uptake by the thyroid gland and decreased the levels of T3, T4 and thyrotropic hormone (TTH) in the blood serum of intact animals, with TTH reaction to thyroid hormone (TH) retained. 5-methoxytryptamine administration was less effective. Partial thyroidectomy distorted the direction of MI effect: melatonin and to a lesser extent 5-methoxytryptamine caused a marked normalization of a decreased TH content and an increased TTH level in the blood serum of partially thyroidectomized rats. TTH-TH reaction also corresponded to the control. A predominantly modulating character or pineal MI effect on thyroid system is suggested.