Whether or not it is necessary for a patient to have two iodine 131 treatments is mainly determined by the patient’s condition. Generally, it is enough to do it once for hyperthyroidism patients, and it may be necessary to do it a second time for recurrence of hyperthyroidism. For recurrent or metastatic foci of differentiated thyroid cancer, the application of Iodine 131 to clear the foci can also be considered, it is recommended to consult a doctor. The radionuclide iodine 131 is generally used to treat patients with hyperthyroidism or thyroid cancer. For patients with hyperthyroidism, iodine 131 treatment can basically achieve a cure after treatment. Nuclide destroys the thyroid cells and acts precisely with the characteristics of targeted therapy. Generally, the effect is obvious in about 1 month of treatment, and the thyroid function of more than 60% of the patients returns to normal in 3 to 4 months of treatment. For patients whose hyperthyroidism is not relieved after 3 to 6 months of iodine 131 treatment, another iodine 131 treatment may be recommended. In conclusion, whether or not it is necessary for patients to undergo two iodine 131 treatments requires systematic examination and evaluation of the patient’s condition and treatment under the guidance of a doctor.