[en] Objective: To explore the impact of interference through different does of Euthyrox (L-T4) on the early hypothyroidism of patients with hyperthyroid after ¹³¹I treatment. Methods: 218 patients with Graves's disease were divided into 3 random groups. Group A contained 69 patients only treated by ¹³¹I and severed as control group; Group B had 78 samples which gave L-T4 25 μg/d for the next successive 20 days after 21 day of ¹³¹I treatment. Group C had 71 samples which gave L-T4 100 μg/d for the next successive 20 days after 21 day of ¹³¹I treatment. The early hypothyroidism rate and the cure rate of hyperthyrosis were observed by measurement of thyroid function. Results: The early hypothyroidism rate and the cure rate of hyperthyrosis of group A after 3, 6, 12 months treated by ¹³¹I were 8.72%, 12.33%, 20.28% and 81.16% respectively. Group B were 4.29%, 5.49%, 7.69% and 76.92%. Group C were 4.37%, 5.02%, 7.04% and 61.97%. Conclusion: Small does of Euthyrox could reduce the early hypothyroidism rate in patients with hyperthyrosis after ¹³¹I treatment I and would not reduce the cure rate of hyperthyrosis. (authors)