Iodine is the main raw material for thyroid hormone synthesis by the thyroid gland, and the thyroid gland has the same highly selective absorption and concentration ability for radioactive iodine. As the rate and amount of thyroid hormone synthesis increase in hyperthyroidism patients, the thyroid gland’s ability to concentrate radioactive iodine is also significantly enhanced, and can be as high as 80-90% of the dose taken. Iodine-131 stays in the thyroid for a long time, with an effective half-life of up to 3,5-4,5 days. Iodine-131 releases beta rays during decay for a longer period of time for concentrated irradiation of the thyroid, but does not damage the surrounding organs and tissues. A few hours after iodine-131 treatment, swelling of some thyroid cells occurs after concentrated irradiation with beta rays, and vacuolated changes occur in the follicular cells. The thyroid gland also shows acute inflammatory changes, such as edema, lymphocyte and phagocyte infiltration, and later becomes non-functional connective tissue, thus reducing the secretory function of the thyroid gland and achieving the purpose of treating hyperthyroidism. Some people call iodine-131 treatment of hyperthyroidism “liquid thyroidectomy”.