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A randomized study was done in 319 consective patients with Graves thyrotoxicosis who under went radioiodine ablation therapy. They were given 5 millicurie of 131 I. This group was compared to 106 patients given 6 millicurie 131 I. In the former group, 245 (79.62%) remained euthyroid after a mean of one year, 54 (16.92%) became hypothyroid and 11 (3.44%) needed a second dose of radioiodine. In the second group, 72% remained euthyroid, 25% became hypothroid and 1.9% required a repeat dose of radioiodine. In summary, fewer patients with 5 millicurie of radioiodine became hypothyroid after a follow up period of one year, and more remained euthyroid compared to those who received 6 millicurie radioiodine.