THE CHANGES INDUCED IN IODINE METABOLISM OF THE RAT BY INTERNAL RADIATION OF ITS THYROID WITH I1311

Abstract

100 adult male rats of the Long-Evans strain were injected intraperit. with either 24, 30, 300, or 875 microcuries of carrier-free I131, were sacrificed at 1, 2, 3, 6, 8, or 10 days thereafter. A method for estimating the dose, in reps delivered to the center and to a point near the surface of the small rat thyroid gland, is presented. The following criteria were used as indicators of metabolic changes induced in the thyroid glands by the internal radiation of the injected I131: (1) the concn. of protein-bound iodine in plasma, (2) the chemical iodine content of the gland, (3) retention of the initial injn. of I131 by the gland, and (4) the capacity of the irradiated gland to concentrate a 2d injn. of I131. In addition, all glands were examined histologically. The earliest metabolic indicator of thyroid dysfunction was the concn. of protein-bound iodine in plasma. Following the injn. of 875 micro-curies of I131, a triphasic response in plasma protein-bound iodine was observed. An initial decrease occurred 24 hrs. after the injn., at which time 150,000 reps had been delivered to the center of the gland. This initial response was followed by a sharp rise in the level of plasma protein-bound I. The highest values were found on the 3d day after the injn. of 875 microcuries. The 3d, or final, response consisted of a reduction in plasma I to thyroidectomy levels. The injn. of 300 microcuries of I131 decreased the concn. of protein-bound I in plasma. With this dose, however, no subsequent rise in plasma I was observed; the level fell steadily. A change in the iodine (I127) content of the thyroid gland was not observed until 2 days after the injn. of either 300 or 875 microcuries of I131. By this time, the center of the gland had received 110,000 and 280,000 reps, respectively. By the 8th day, the gland''s iodine was practically depleted. The percentages of the initially injected I131 present in the gland one day after the injn. of 300 or 875 microcuries of I131 were the same as those observed in rats injected with 24 microcuries. At the end of 2 days, however, the thyroid glands of rats which had received 300 and 875 microcuries, respectively, failed to retain the trapped I to the same extent as did the glands of rats that had received 24 microcuries. This 48-hr. interval corre-sponds to the time when the chemical I was being rapidly lost from the strongly irradiated glands. Rats were treated with 300 microcuries of I131. Three days later the capacity of their thyroid glands to concentrate a 2d injn. of radioactive I was detd. The irradiated glands collected-about 1/10 the amt. of I131 found in the glands of the control (untreated) group. As judges by the metabolic criteria outlined above, thyroid function is not disturbed in the rat up to 10 days after the injn. of 24 or 30 microcuries of I131.