Demonstration of a Second Thyroidal Iodide Pool in Rat Thyroid Glands by Double Isotope Labeling1

Abstract

Rats were brought to specific activity equilibrium by repeated daily injections of a mixture of I¹²⁷ and I¹³¹. The animals were then given a blocking dose of propylthiouracil, followed by inorganic I¹³², and were killed 1 hr later. From the plasma concentration of inorganic I¹³¹ and thyroid/plasma iodide concentration ratios for I¹³², the expected thyroidal content of inorganic I¹³¹ was calculated. Invariably, direct measurement revealed far more inorganic I¹³¹ in the thyroid than could be accounted for in the “external” iodide pool, i.e., that pool of iodide in rapid equilibrium with iodide in the plasma, as measured by I¹³². Thus, a second, or “internal,” iodide pool is postulated to exist. Evidence is presented which indicates that iodide in the internal pool arises from the deiodination of organic moieties by physiologic, rather than artifactual, means, and that it is not in rapid equilibrium with iodide in the external pool, since it is not discharged by thiocyanate, perchlorate or large doses of iodide. Triiodothyronine decreases and acute doses of TSH increase the concentration of iodide in the internal iodide pool.