There is a growing trend of treating hyperthyroidism with 131I, but can 131I also be used to treat thyroid cancer? In fact, 131I has been used to treat thyroid cancer for more than half a century, but its application is mainly for post-thyroidectomy treatment to provide better adjuvant clearance. Among thyroid cancers, follicular carcinoma and papillary carcinoma account for about 90% of well-differentiated thyroid cancers. With proper treatment, patients with this type of well-differentiated thyroid cancer can have a 10-year survival rate of 90%, but once metastasis occurs, the 10-year survival rate will only be about 20% to 30%. In order to reduce the chance of recurrence and metastasis of this type of cancer, complete removal of thyroid tissue should be the standard of treatment. However, due to the special structure of the thyroid gland, it is not easy to completely remove all thyroid tissue with thyroidectomy. Studies have also shown that the addition of post-operative atomic iodine clearance significantly reduces mortality compared to thyroidectomy alone. Once a patient is diagnosed with well-differentiated thyroid cancer, a total thyroidectomy should be performed by a surgeon, followed by discontinuation of thyroxine and a low iodine diet for four weeks, followed by a 131I whole-body scan, and if the scan shows residual thyroid tissue or metastatic lesions, the patient should be treated with 131I clearance and have another whole-body scan one week after treatment. Patients should be followed up with 131I whole body scan and serum thyroglobulin concentration every six months to one year after treatment to evaluate the effect of treatment and whether there is any recurrence. It is important to note that patients with thyroid cancer who are pregnant, breastfeeding, or have a severely low blood count (e.g., receiving chemotherapy or systemic radiation therapy) should not receive 131I thyroid cancer clearance therapy for the time being. 131I treatment can be given orally in the easiest way possible. 131I can be taken up by functioning thyroid tissue due to the highly selective uptake of iodine by thyroid tissue. If the thyroid gland is not removed, most of the 131I will be taken up by normal thyroid tissue, but if most of the thyroid tissue is removed, the residual tissue and cancerous tissue will also take up 131I. Therefore, it does not affect the surrounding normal tissues much. Another topic of concern is whether there are any side effects of 131I clearance treatment. 131I clearance treatment complications include anorexia, nausea, dizziness, itchy skin, neck swelling, salivary gland swelling, bone marrow suppression, etc. They are usually temporary and patients who are extremely uncomfortable can return to the clinic for symptomatic treatment. Long-term serious complications are uncommon, and there have been reports of leukemia in patients receiving very large doses (1000 mCi) or more, but there have been no reports of increased leukemia rates with long-term follow-up of the usual treatment doses and fractionated doses currently used. Nuclear medicine 131I clearance therapy is convenient and effective as it requires only simple oral fluids or capsules. The combination of total thyroidectomy and 131I clearance treatment significantly reduces the chance of recurrence, metastasis and death in patients with well-differentiated thyroid cancer and is the most complete treatment for well-differentiated thyroid cancer.