Lack of Iodide Effect on Serum and Pituitary Thyrotropin in Vivo

The secretion of thyrotropin is controlled by the serum concentration of thyroid hormones, and it has been postulated, but neither established nor excluded, that iodide also may play a direct role. Attempts were made to confirm observations of a direct effect of iodide, by measuring the concentration of thyrotropin in the blood and pituitary gland of rats injected with iodide after surgical thyroidectomy or administrationadministration of antithyroid drugs. Iodide failed to decrease thyrotropin concentrations. When rats given propylthiouracil and low-iodine diet were injected with iodide, there was significantly less increase in thyroid weight, despite unchanged serum and pituitary thyrotropin concentrations. It is probable that, in these experiments, iodide exerted a direct effect on the thyroid gland. (Endocrinology 81: 871, 1967)