Influence of the pineal body on experimental TSH dominance in rat.

Iodide uptake of the thyroid following the joint administration of 1-methyl-2-mercapto-imidazole (MMI) and TSH was studied by autoradiography at various points of time throughout 125 days in pinealectomized and sham-operated rats. In the pinealectomized group iodide uptake decreased. The enhanced TSH effect also decreased by the end of the experiment, suggesting the existence of a mechanism antagonizing the TSH effect. It is assumed that prolonged TSH dominance stimulates the pineal which, in turn, inhibits iodide uptake by the thyroid. This mechanism would thus prevent the TSH dominant state to influence the thyroid. As this prevention occurred also in pinealectomized animals it is postulated that some other as yet unknown structures exert a similar effect. It is suggested that the pineal body acts upon the follicular cells of the thyroid, exerting thus a peripheral inhibitory effect on iodide-uptake.