The Mechanism of Iodine Concentration by the Thyroid Gland: Its Non-organic Iodine- Binding Capacity in the Normal and Propylthiouracil-treated Rat*

In normal rats maintained on an adequate iodine intake, the iodine concentration in the thyroid gland is about 10,000 times as great as that in plasma, a finding that illustrates the remarkable iodine-concentrating capacity of this tissue (1). The fact that almost all of the iodine in the normal gland is organically bound (2, 3) would suggest that this concentrating capacity is dependent upon the gland’s ability to convert inorganic iodine to diiodotyrosine and thyroxine. Although this view is supported by the rapid rate at which an injected tracer dose of inorganic radioactive iodide is organically bound by the gland (3), it does not account for the iodide concentration by thyroid tissue when large amounts of iodide are introduced into the animal. Thus, Leblond (4) observed that in guinea pigs that received 500 y of labeled iodide per 100 gm. of body weight the largest part of the iodine fixed by the gland remained in the form of inorganic iodide for some time. Also, Lein (5) reported that most of the radioiodine taken up by the thyroids of rabbits during the first 10 minutes aft,er the intravenous injection of 35 y of labeled iodide was still in the form of iodide. The first demonstration that the mechanism of iodine concentration by thyroid tissue can be completely separated from synthesis of diiodotyrosine and thyroxine was made in this laboratory with the aid of surviving slices of thyroid tissue. Franklin et al. observed that in the presence of 1O-3 fi,r thiouracil surviving thyroid slices failed to convert inorganic iodide to thyroxine and diiodotyrosine but retained their capacity for concentrating iodine (6). p-Aminobenzoic acid, thiourea, and sulfanilamide were shown by Franklin et al. (6) and by Schachner et al. (7) to behave in a similar manner. Subsequently, in intact animals, McGinty (8) and Astwood (9) found that thiouracil treatment does not abolish the iodine-concentrating capacity of the gland, even though it completely blocks the production of the thyroid hormone. The iodine taken up under these conditions, as shown by