In order to find effective treatment method for eliminating radioactive iodine, various thyroid inhibitors were examined. Male rats of Donryu strain were injected with 10-20 μCi of Na
131I intraperitoneally. Thyroidal uptake of
131I was significantly decreased by single intraperitoneal injection of 3mg of 2-thiouracil, which was given 4 hours after
131I. This effect, however, was transient and several hours later the
131I uptake reached a higher level than that of the control group. Single intraperitoneal injection of a thyroid inhibitor, 3mg of 1-methyl-2-mercaptoimidazole, which was given 24 hours after the injection of
131I was ineffective for eliminating radioactive iodine from the thyroid gland. On the other hand, multiple injections of thyroid inhibitors, 2-thiouracil, 6-propyl-2-thiouracil, 1-methyl-2-mercaptoimidazole, or potassium perchlorate, 5 times at intervals of 6 hours resulted in significant decrease of thyroidal deposition of radioiodine, even if the first injection was given 24 hours after
131I. Moreover, no elevation in
131I uptake after the treatment took place. These results suggest that the decrease in the thyroidal deposition of radioiodine by multiple injections of thyroid inhibitors resulted from the blocking of reutilization of free iodine liberated from the thyroid into the blood. In parallel with the decrease in thyroidal deposition of
131I, significant increase of
131I level in blood was found. Possibility of the stimulation on liberation of radioiodine from the thyroid has been discussed. There was no synergistic effect of KClO
4 promoting radioactive iodine removal from the thyroid by 1-methyl-2-mercaptoimidazole. Multiple administration of KSCN, T
3, CoCl
2, and NaI was ineffective for the elimination of radioiodine from the thyroid.
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