Why can radioactive iodine-131 treat hyperthyroidism? The reason why Iodine-131 can treat hyperthyroidism is that after entering the body, most of the iodine-131 accumulates in the thyroid gland. During the decay process, iodine-131 releases beta rays, which have a range of only 2 mm in the thyroid gland. Since the iodine enters the body and accumulates mainly in the thyroid gland, what is not absorbed by the thyroid gland is quickly excreted in the urine, thus causing no damage to other organs. Long-term follow-up of a large number of patients shows that radioactive iodine-131 treatment does not cause infertility or cancer; iodine-131 treatment for hyperthyroidism is an extremely effective, safe, simple and economical treatment method. Are all patients with hyperthyroidism suitable for Iodine-131 treatment? Toxic diffuse goiter, especially those with the following conditions, can be treated with iodine-131: abnormal liver function, hypocellularity unsuitable for antithyroid drug (ATD) treatment; allergy to ATD drugs; relapse after ATD treatment; relapse after surgical treatment or unwillingness to undergo surgery. Toxic nodular goiter with hyperthyroidism; chronic lymphocytic thyroiditis with hyperthyroidism; non-toxic goiter may also be treated with iodine-131 from a cosmetic point of view. Iodine-131 therapy should not be used in the following cases: 1. Patients who are pregnant or breastfeeding and patients in the acute phase of acute myocardial infarction should not be treated with iodine-131. 2. Iodine-131 therapy cannot be used in patients with severe renal impairment, because all iodine-131 is excreted in the urine except for the uptake of iodine-131 by the thyroid gland. If the kidney function is bad, iodine-131 cannot be eliminated in the body in time, and iodine-131 accumulates in the body, especially in the kidney for a long time making the patient unnecessarily irradiated. 3. Iodine-131 treatment is prohibited or prudent for extremely enlarged thyroid gland or post-thoracic goiter with compression symptoms, because the oversized thyroid gland will aggravate the compression symptoms due to mild inflammatory reaction during iodine-131 treatment, resulting in breathing difficulties for the patient. Can hyperthyroidism treated with iodine-131 cause hypothyroidism? All three current treatments for hyperthyroidism (anti-thyroid medication, iodine-131, and surgery) can cause hypothyroidism. In people who are in remission from hyperthyroidism, a certain percentage of hypothyroidism occurs every year due to autoimmune effects in the patient’s body, so the occurrence of hypothyroidism is not due to iodine-131 treatment. The incidence of hypothyroidism due to iodine-131 therapy varies from hospital to hospital in China and abroad and tends to increase year by year. Hypothyroidism is easy to diagnose and treat.