The Effect of Acute Iodide Depletion on Thyroid Function in Man*

Abstract

Acute depletion of extracellular iodide has been achieved in 8 patients, including 2 with diffuse toxic goiter, by institution of a low-iodine diet, followed 24 hr. later by an infusion of mannitol to increase renal clearance of iodide. Measurements of plasma iodide (I127) concentration, thyroid I131 transfer and clearance rates, renal I131 clearance rate, absolute iodine (I127) accumulation rate, and serum protein-bound iodine were made before, during (4 patients), and 24 hr. after mannitol infusion. This iodide depletion procedure resulted in decreased plasma iodide concentration, increased thyroid transfer and clearance rates, increased uptake of I131, and decreased absolute iodine accumulation. These changes, which were usually evident during the mannitol infusion, were uniformly present by the following day. Serum protein-bound iodines varied only slightly from control values. Decreases in plasma iodide concentration were prevented by inclusion of only 800 ug of sodium iodide in the mannitol infusion, and, under these conditions, the increase in I131 clearance and uptake did not occur. It is concluded that there exists a mechanism for acutely increasing thyroid function in response to a relatively abrupt decrease in plasma iodide concentration. This mechanism apparently is not mediated by the pituitary, but resides within the thyroid gland itself.