The Iodide Pools of the Rat Thyroid

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Abstract
Radioactive iodide in the rat thyroid was separated from iodine-containing organic compounds by means of dialysis at 3 C, followed by ion exchange resin chromatography. The thyroid: serum radioactive I- concentration ratio (T:S) one hour after the injection of I131 was much lower when organic binding of I131 was permitted than when it was blocked with propylthiouracil (7 vs. 130); it could be further depressed—to 2– with perchlorate. However, the T:S for binding thyroids rose progressively and eventually exceeded the value determined by giving I131 after propylthiouracil administration; 24–48 hours after the injection of I131, the concentration of radioactive I- in the binding thyroid was no longer diminished by ClO4- nor was it affected by propylthiouracil injected 2 hours before the rats were killed. In rats which were given I131 for 3 weeks to achieve uniform specific activity of iodine throughout the body, 0.26 % of thyroidal iodine was estimated to be in the form of I-; the T:S was 500. It is suggested that there are two major I- pools in the thyroid: transported, ClO4--dischargeable I-, which has a fast turnover, and a much greater I- pool, derived presumably from the iodotyrosines of the gland, which is turned over less rapidly.

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