1. Iodine is one of the raw materials for synthesizing thyroid hormone. After iodine 131 is taken up by the thyroid gland, beta ray is released, which destroys the thyroid follicular cells and reduces the synthesis and secretion of thyroid hormone and the size of the thyroid gland, thus achieving the purpose of treating hyperthyroidism. 2. The average range of the beta ray is 0.8mm, and its energy is almost completely absorbed by the thyroid gland. 3. The distribution of iodine 131 in extra-thyroidal tissues is small and the retention time is short, so the amount of radiation received by bone marrow, gonads, liver, spleen and gastrointestinal tract is very low. 4. Iodine 131 can enter the fetus through the placenta and can also be secreted into breast milk, so it is prohibited for women during pregnancy and lactation.