Iodine-131 is a radiopharmaceutical, an isotope of iodine, which has a physical half-life of 8.04 days; it emits γ-rays for imaging and β-rays for treatment to play a diagnostic and therapeutic role. Since the synthesis of thyroxine by the thyroid tissue in the body requires the participation of iodine, iodine-131 can gather in the thyroid tissue after entering the body, and the beta ray has a range of only 2 mm in the thyroid gland. Therefore, iodine-131 is mainly used for the treatment of hyperthyroidism such as Graves’ disease and Plummer’s disease. It is also used for the determination of thyroid function, thyroid imaging, and the treatment and follow-up of well-differentiated thyroid cancer and its metastases.