The thyroid gland regulates the body’s metabolism and growth and development by synthesizing and releasing thyroid hormones. Elemental iodine is an essential raw material for this, and the thyroid gland has a high capacity to polymerize iodine. Since radioactive iodine and stable iodine have the same physicochemical properties, the thyroid gland has the same high absorption and concentration capacity for it. Under normal circumstances, the concentration of iodine in the thyroid gland can reach 25 times the plasma concentration, hyperthyroidism patients due to the synthesis of thyroid hormone speed and amount of increase, at this time to the radioactive iodine concentration ability is higher, up to 80 ~ 90%. The effective half-life of iodine in the thyroid gland is 3.5 to 4.5 days on average. A large amount of concentrated radioactive iodine exposes the thyroid gland to radiation, and part of the thyroid tissue is destroyed, resulting in a decrease in thyroid hormone production and the alleviation or cure of hyperthyroidism. Iodine-131 in the decay process emits β-rays, the range is shorter, the average 1 mm, the longest 2.2 mm, so both the destruction of thyroid tissue, but the impact on the surrounding tissues and organs of the thyroid gland is very small. This shows that radioactive iodine treatment of hyperthyroidism is a safe and simple method.