For metastatic differentiated thyroid cancer, radioactive iodine (131I) therapy can eradicate tumor cells and is highly effective. However, even with adequate thyroid stimulation and avoidance of excessive iodine intake, only 2/3 of patients with metastatic differentiated thyroid cancer can consume large amounts of iodine, with a cure rate of only 42%. Patients with ineffective radioiodine therapy have an expected survival of 3-5 years and lack of appropriate medication. Treatment was once hampered by the lack of consensus on the diagnosis and treatment of radioiodine-resistant differentiated thyroid cancer. Patients with radioiodine-resistant differentiated thyroid cancer are broadly divided into the following four categories: 1. Patients with metastatic lesions without iodine uptake at the time of initial treatment The following patients cannot benefit from radioiodine therapy: clear lesions with no iodine uptake at the time of radioiodine whole-body scan; lesions with iodine uptake but cannot benefit after radioiodine therapy scan. 2.Patients with loss of iodine uptake before metastases This group of patients has multiple metastases, and the loss of iodine uptake may be due to the eradication of better differentiated cells with iodine uptake and the metastasis of poorly differentiated cancer cells with loss of iodine uptake. 3.Patients with some lesions with iodine uptake function and some without iodine uptake function Most commonly seen in patients with multiple metastases, which are usually clarified by PET-CT scan or diagnostic whole-body CT scan with radioiodine as tracer by 124I, 18-FDG, whose metastases will further lose their iodine uptake function (especially able to uptake 18-FDG) and cannot benefit from radioiodine therapy. 4. Patients whose lesions have iodine uptake but whose metastases are further progressing The consensus clearly states that if the lesions are still deteriorating after adequate radioiodine treatment, continued treatment is ineffective. During radioiodine therapy, the tumor response is mainly observed by imaging (CT or MRI) and functional (lesion iodine uptake and serum thyroglobulin measurement). The assessment methods are diverse, but some differences may exist, such as when imaging shows a decrease in iodine uptake but an increase in serum thyroglobulin concentration, which requires a thorough evaluation of the condition. Some conditions are still unexplained, such as some patients whose metastases are all iodine uptake, but who are not cured after several radioiodine treatments (these patients also have no progression of the lesions according to RECIST criteria). This group of patients has a low probability of cure with continued radioiodine therapy and a progressive risk of side effects, such as secondary tumors and leukemia. Whether this group of patients (especially those who received 600 mCi) can be classified as radioiodine resistant and whether to discontinue radioiodine therapy is still controversial. Support for continuation of radioiodine therapy is mainly based on the response to the previous course of treatment: continued significant iodine uptake, reduced 18F-FDG uptake and few adverse effects. If PET-CT shows high 18F-FDG uptake in the lesion, the likelihood that radioiodine therapy will be fully effective is reduced; therefore, radioiodine therapy should be avoided if 18F-FDG uptake or enhanced function is found in the lesion. Finally, radioiodine therapy is usually not recommended for patients who cannot undergo thyroidectomy because the presence of the thyroid gland makes iodine uptake unassessable and radioiodine therapy ineffective, and such patients should be managed as radioiodine resistant.