The main complication after 131I treatment of hyperthyroidism is hypothyroidism. Overseas: early (within 1 year) hypothyroidism is about 20%, and then it increases by 2% to 3% every year, and after 10 years it reaches about 50% to 70%. Domestic: comprehensive 1977-1991 11 follow-up report, 131I treatment of hyperthyroidism 5 to 24 years 4,167 cases, early hypothyroidism 2.5 ± 1.7% (0% to 10%), late hypothyroidism 12.7 ± 17.3% (2.2% to 59.8%). In order to reduce the incidence of hypothyroidism, there have been studies on the treatment of hyperthyroidism with small doses of 131I or the addition of ATD before and after 131I treatment or the treatment of hyperthyroidism with 125I and other methods, but none of them have achieved the expected results. Therefore, some foreign scholars believe that hypothyroidism is an unavoidable result of 131I treatment of hyperthyroidism, and is not a complication of 131I treatment of hyperthyroidism.The ATA published “Guidelines for the Treatment of Hyperthyroidism and Hypothyroidism” in 1995, pointing out that: 131I is the most commonly used treatment of hyperthyroidism in the United States, and the treatment is safe, and the main side effects are early or late hypothyroidism. The main side effect of the treatment is the development of early or late hypothyroidism, which requires lifelong thyroid hormone replacement therapy [1]. European experts believe that the goal of 131I treatment of hyperthyroidism is to eradicate hyperthyroidism to hypothyroidism with 131I followed by thyroid hormone replacement therapy [2]. This requires informed and signed consent from the patient. The doctor should also inform the patient of the precautions related to radiation protection after 131I treatment [1]: take 131I orally on an empty stomach, eat only 2h after taking 131I, pay attention to rest, prevent infection and avoid mental stimulation, do not squeeze the thyroid gland, avoid iodine for 1 month, avoid close contact with infants and young children for 1 week, and do not get pregnant for 0.5 years. The cause of hypothyroidism is related to the therapeutic dose of 131I. Foreign countries mostly use 80 to 200µCi/g thyroid, some uniformly give 10mCi or 15mCi at 1 time, that hypothyroidism is the purpose of 131I treatment of hyperthyroidism, not a complication of treatment. China’s nuclear medicine experts advocate that the therapeutic dose of 131I should be individualized, generally only 70-120µCi/g thyroid, and advocate to minimize the occurrence of hypothyroidism. This may be the main reason for the different incidence of hypothyroidism at home and abroad. The treatment of hypothyroidism with L-T4 after 131I treatment of hyperthyroidism can make the patient’s thyroid function remain normal, the general state is good, can live, work and study normally, for women of childbearing age, can be normal pregnancy and childbirth . In addition, about half of the patients with early hypothyroidism treated with L-T4 for a period of time, the thyroid function can return to normal and discontinue the use of L-T4[1] . Some patients are afraid of hypothyroidism, unwilling to use 131I treatment in time, long-term disease can not work normally, low quality of life, combined with atrial fibrillation patients, some cerebral infarction hemiplegia, some patients with hyperthyroidism cardiac disease sudden death, is really a misunderstanding caused by the bitter fruit.