Radioactive iodine 131I generally works only on thyroid cancer cells that are able to take up iodine. The vast majority of thyroid cancers have well differentiated cells and can be treated with this method. There are two situations where 131I treatment is necessary: one is when a patient with thyroid cancer has a recurrence of the cancer lesion after surgery; the other is when metastasis of thyroid cancer (including lymphatic metastasis in the neck and metastasis in other areas) has been identified during surgery or after other diagnoses. There are some patients with thyroid cancer who can also be temporarily excluded from 131I treatment. If the primary thyroid cancer is small (e.g. less than 1 cm) and the lesion is only solitary and does not invade the envelope (carcinoma in situ), these patients have a low probability of recurrence and metastasis after surgery to remove the cancer. However, patients must insist on taking oral thyroxine tablets daily after surgery to prevent recurrence. If there is any recurrence later, surgery should be performed as soon as possible followed by 131I treatment. If the patient has a large primary thyroid cancer, or multiple thyroid cancer lesions in the thyroid gland, or the cancer lesions have invaded the envelope, it is better to use 131I treatment even if no obvious metastatic lesions are detected by surgical treatment, so that the surgical residual thyroid gland and hidden metastatic lesions can be effectively removed to prevent recurrence (of course, you must also insist on taking oral thyroxine tablets all the time afterwards). Only some rare thyroid cancers (such as undifferentiated thyroid cancer, medullary thyroid cancer, etc.) cannot be treated with radioactive iodine 131I and other treatments are needed. In addition, for patients with poor liver and kidney function, severely low blood count or some advanced and severe thyroid cancer, although the cancer lesion can take up and collect a certain amount of radioactive iodine 131I, 131I treatment also has side effects, and when our doctors consider that the disadvantages of the treatment outweigh the benefits, they will advise the patient not to take 131I treatment. It is important to note that if the patient is able to undergo surgery, all thyroid cancer lesions (including the primary lesion, normal thyroid tissue in the neck and metastases) should be prioritized for surgery to maximize the removal of the lesion before 131I treatment. In this way, 131I treatment is fast-acting and efficient, and the side effects of treatment are less.