Abstract：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.