It is not easy to determine how long it takes for iodine 131 to completely disappear from the body, but it takes about 35~40 days for it to be basically metabolized or to decline. Iodine 131 is a radioactive isotope of elemental iodine, with a half-life of 8.02 days, and an effective half-life of 7.6 days for the human body. Therefore, after 35-40 days, the iodine 131 in the body has already gone through 5 half-lives, and is close to being metabolized, and therefore it is safer at this time. Usually patients who have received iodine 131 treatment try not to come into contact with pregnant women or children after they are discharged from the hospital. The body’s thyroid tissue is highly selective for iodine, so hyperthyroidism can be treated with iodine 131. After iodine 131 is taken orally, most of it is absorbed by the thyroid tissue, so the treatment has a high cure rate, low recurrence rate and good safety. The side effects of iodine 131 treatment are hypothyroidism and radiation thyroiditis. If you need to use iodine 131 for treatment, please follow your doctor’s instructions.