131I is used to treat hyperthyroidism and thyroid cancer and to measure thyroid function in small doses. It has been used for 40 years. Since iodine is a natural substance used by the thyroid gland to produce thyroid hormones, the thyroid gland takes up radioactive iodine in the same way as non-radioactive iodine. Because the thyroid is the only organ of the body that uses iodine, radioactive iodine is not taken up by other parts of the body, and radioactive iodine that is not taken up by the thyroid cells is excreted by the body (mainly in the urine). Therefore, 131I is a safe and effective way to examine and treat thyroid disease. Further studies have shown that patients treated with 131I do not have an increased incidence of thyroid cancer or other tumors, and this is also true for children and adolescents. If the thyroid cancer is invasive, the entire thyroid gland must be removed and then treated with 131I (a special liquid containing 131I). 131I treatment targets thyroid cells, one of the few types of cells in the body that can take up 131I. 131I can reach any part of the body where thyroid cancer cells are present. One to two 131I treatments are usually needed to destroy the remaining thyroid cells. Since 131I treatment destroys all the thyroid cells in the body, thyroid hormone must be taken to replenish the deficiency in the body. A slightly higher dose of thyroid hormone helps to suppress the hormone produced by the pituitary gland (known as TSH), which is what stimulates the growth of thyroid cells. Thyroid hormone must be taken for life and can be taken once a day in a single dose without any side effects. Patients with inoperable thyroid cancer can be treated with external irradiation to destroy the tumor cells. Chemotherapy is generally ineffective for thyroid cancer and is rarely used. It is important to note that pregnant and breastfeeding women should not receive 131I treatment for any reason, as it may harm the developing fetus’ thyroid gland. 6 to 12 months after 131I treatment should be considered before pregnancy. Women who wish to become pregnant should seek medical advice regarding their current health status. Although 131I threatens the thyroid gland of the fetus, 131I itself does not cause infertility or other birth defects.