To ensure the safety of radioactive iodine 131I treatment, it is generally important not to use too large a dose of 131I for each treatment, and it is advisable to use a fractionated dosing method for each treatment to reduce radiation side effects. If the patient has a large residual thyroid gland, or a large number of metastatic lesions, multiple treatments with 131I will be required. The interval between each 131I treatment is usually 3 to 4 months, which ensures that the patient has enough time to get some recovery while avoiding the accumulation of radiation hazards. However, the interval should not be too long, otherwise the potential growth of thyroid cancer lesions in the interval of treatment will exceed the efficacy of treatment.