INTRATHYROIDAL IODINE METABOLISM IN THE RAT. THE INFLUENCE OF DIET AND THE ADMINISTRATION OF THYROID-STIMULATING HORMONE.

1. Ratios of mono[(131)I]iodotyrosine and di[(131)I]iodotyrosine (R values) and the incorporation of (131)I into iodothyronines have been estimated in rat thyroid glands from 30min. to 38hr. after the administration of [(131)I]iodide. 2. In rats receiving a powdered low-iodine diet the R values were close to unity and did not change with time after the administration of [(131)I]iodide. In rats receiving a commercial pellet diet the R values fell from a mean of 0.8 at 30min. after [(131)I]iodide administration to 0.49 at 38hr. 3. Administration of 0.5-2.0i.u. of thyroid-stimulating hormone before giving the injection of [(131)I]iodide caused a small diminution in the R value when the time between injecting [(131)I]iodide and killing the animal was 16hr. or more. 4. Iodothyronines represented a greater percentage of the total thyroid-gland radioactivity in the iodine-deficient animals than in animals fed on the pellet diet. Thyroid-stimulating hormone had little effect, if any, on the iodothyronine contents.