Hypothyroidism (hypothyroidism) is a systemic hypometabolic syndrome caused by a decrease in the synthesis and secretion of thyroid hormones or insufficient tissue utilization. It affects approximately 1.0% of the general population (more common in women than men) and increases with age. There are many causes of hypothyroidism, which can occur after radioactive iodine treatment. Hypothyroidism after iodine-131 treatment can be classified as early-onset hypothyroidism or late-onset hypothyroidism. Any treatment inevitably produces a certain percentage of hypothyroidism. Now that early diagnosis of hypothyroidism and physiologic replacement can be easily and sensitively performed, clinicians in some parts of the world (represented by North America) use high doses of 131I for the treatment of hyperthyroidism to ensure that a high cure rate can be obtained. Early-onset hypothyroidism: refers to hypothyroidism that occurs within one year after treatment and is reported to be about 10% in China. The cause of early onset hypothyroidism is the direct destruction of thyroid cells by radiation, which is related to the dose of iodine-131 given and the sensitivity of the individual to radiation. The occurrence of early onset hypothyroidism cannot be predicted or avoided, and even with a lower dose of iodine-131 for hyperthyroidism, there is no guarantee that hypothyroidism will not occur. If the incidence of early onset hypothyroidism is reduced by lowering the dose of iodine-131, the cure rate is bound to decrease. The main purpose and main advantage of using iodine-131 therapy is to control hyperthyroidism as soon as possible, not to avoid the occurrence of hypothyroidism. If thyroid hormone replacement therapy is given promptly after the occurrence of hypothyroidism, patients can grow and have children normally and maintain a normal quality of life. Some patients with early onset hypothyroidism can recover their thyroid function on their own. Late onset hypothyroidism: It occurs after one year of iodine-131I treatment and increases at a rate of 2% to 3% per year. The cause of late onset hypothyroidism is not clear, but may be related to autoimmune dysfunction, independent of the dose of iodine-131. Late onset hypothyroidism is not unique to iodine-131 treatment, but can also occur after medical medication and surgical treatment, even without any therapeutic measures GD spontaneous hypothyroidism can occur in up to 16%-20%. Therefore, hypothyroidism may be one of the natural regressions in the course of GD. The principle of management is early detection and timely replacement therapy.