131 iodine has been used for over 50 years for the treatment of differentiated thyroid cancer. 131 iodine is a radioactive form of iodine that is ingested into the body mainly in the thyroid gland and other tissues that take up iodine. When the normal thyroid tissue is removed, the differentiated thyroid cancer tissue can take in a certain amount of 131 iodine and use the beta radiation emitted by the decay of 131 iodine to destroy the tumor cells for the purpose of treatment. Therefore, it is also called “internal radiotherapy”.