Radioactive iodine-131 has essentially the same properties as iodine in food, except that iodine-131 releases rays that destroy normal and abnormal thyroid cells. Iodine is the raw material for the synthesis of thyroid hormones, and most of the iodine that enters the body is taken up by the thyroid gland. When iodine-131 is given orally to a hyperthyroid patient, this radioactive iodine also enters the patient’s thyroid gland. Iodine-131 can remain in the thyroid gland of a hyperthyroid patient for a considerable period of time, releasing rays that cause degeneration and reduction of part of the thyroid gland, a decrease in the synthesis and secretion of thyroid hormones, and a reduction in the size of the thyroid gland, thus achieving the goal of treating hyperthyroidism. The iodine-131 that enters the body is mainly concentrated in the thyroid gland. The amount of iodine-131 radiation used in the treatment of hyperthyroidism is very small, and the amount of radiation produced to the bone marrow, gonads, liver and gastrointestinal tract of the body is very low. Therefore, Iodine-131 treatment for hyperthyroidism usually does not require strict radiation isolation, and outpatient treatment is usually sufficient for general patients (a few patients need to be hospitalized for observation and adjuvant treatment because of their serious conditions). The advantages of iodine-131 treatment for hyperthyroidism include: (1) Iodine-131 treatment is relatively simple, usually requiring only one oral dose, with only a small number of patients requiring another oral dose after 2 to 4 months of treatment. (2) Compared with anti-thyroid medication, iodine-131 treatment has a higher remission rate and a lower recurrence rate. (3) The treatment can significantly reduce the size of the enlarged thyroid gland, so that the patient’s neck swelling is eliminated. (4) This treatment is especially suitable for hyperthyroidism with toxic side effects after antithyroid drug therapy (such as decreased peripheral blood count, drug allergy, liver function impairment, etc.), hyperthyroidism with serious complications, and hyperthyroidism in which the patient is unable or unwilling to undergo surgical treatment. (5) The treatment has no significant toxic side effects on other tissues in the body (especially the hematopoietic system and liver system); it does not induce tumors, etc.; and it does not affect future generations. (6) The overall cost of treatment is low. Despite the advantages of iodine-131 treatment, there are some limitations of this method. These include: (1) Some patients may develop hypothyroidism soon after iodine-131 treatment, and some patients may become hypothyroid for life, requiring daily thyroid supplementation (eugenol or other thyroid preparations). (2) Compared with and anti-thyroid drug treatment, some patients have a slower onset of action after iodine-131 treatment. Also, since iodine-131 treatment early on mainly destroys some of the thyroid cells, if many thyroid cells release large amounts of thyroid hormones in a short period of time it can lead to the possibility of exacerbation in a short period of time (within 1 to 2 weeks after treatment). Occasionally, some patients with severe hyperthyroidism may experience a critical phase of hyperthyroidism, increased heart failure, increased proptosis, or more pronounced eye swelling after iodine-131 treatment. Of course, effective adjuvant therapy can prevent and alleviate these conditions. (3) Very few patients are insensitive to iodine-131 treatment and require multiple iodine-131 treatments or can only use other treatments.