Does differentiated thyroid cancer (DTC, including papillary thyroid cancer and follicular thyroid cancer) require iodine-131 therapy after surgery? This is one of the key questions that is of high interest to all patients with DTC. Inevitably, there is still some controversy in the academic community regarding individual details because of insufficient research data or conflicts in some aspects. Fortunately, in an effort to provide a comprehensive and authoritative guide for patients and clinicians involved in this disease, in January 2016 the American Thyroid Association (ATA) published a revised version of the 2015 American Thyroid Association Guidelines for the Management of Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer in its official journal, THYROID. The guidelines provide an indication-based recommendation on whether or not to treat patients with iodine-131 after DTC surgery after stratifying the risk of recurrence. I. Risk of recurrence stratification for post-DTC patients 1. Low risk: All of the following conditions are met: complete resection of the tumor confined to the thyroid gland, no extrathyroidal breakthrough, invasion (if there is vascular invasion, it should not exceed 4 places), metastasis (if there is metastasis in the cervical lymph nodes, it should be limited to clinically negative or no more than 5 and the maximum diameter does not exceed 0.2 cm pathologically positive), no invasive histological manifestations (such as high cell, columnar cell, peg cell carcinoma, etc.). 2.Intermediate risk: Any one of the following: microscopic invasion of soft tissue outside the thyroid gland, clinically positive or more than 5 lymph node metastases in the neck with a maximum diameter between 0.2-3 cm, abnormal iodine uptake foci outside the thyroid bed, invasion of blood vessels, invasive histological manifestations. 3.High risk: Any one of the following conditions: tumor invading soft tissue outside the thyroid gland (T4), incomplete surgical resection, distant metastasis, high TGemia, unlimited number of pathologically positive cervical lymph node metastases with maximum diameter greater than or equal to 3 cm, follicular thyroid cancer with extensive vascular invasion (vascular invasion more than 4 places). Iodine-131 is recommended for DTC treatment. 1. Known distant metastasis (M1) to lung, bone and other organs, high risk, iodine-131 treatment is highly recommended (to improve disease-specific survival and disease-free survival). 2. Intraoperative visualization of tumor breaking through the thyroid envelope and invading subcutaneous soft tissue, larynx, trachea, esophagus, laryngeal nerve, prevertebral fascia or encircling carotid artery and mediastinal vessels ( Regardless of tumor size, T4), incomplete surgical resection, distant metastasis, hyper-TGemia, an unlimited number of metastatic lymph nodes in the neck with a maximum diameter greater than or equal to 3 cm, and follicular thyroid cancer with extensive vascular invasion (vascular invasion in more than 4 places). Those with one of the above are considered high risk and are highly recommended for iodine-131 treatment (to improve disease-specific survival and disease-free survival); 3. Primary tumor diameter over 4 cm or microscopic invasion (T3), cervical lymph node metastasis (N1), intermediate risk, should be treated with iodine-131 selectively according to age, extent of tumor invasion, number and size of lymph node metastases; 4. Although the tumor does not break through the thyroid envelope and have a diameter between 1-4 cm (T1b-2), low-risk, iodine-131 therapy is usually not recommended, but may be considered if surgical pathology suggests aggressive histological manifestations (e.g., high-cell, columnar cell, peg cell carcinoma, etc.). 5. Microfocal carcinoma without external invasion and metastasis (less than 1 cm in diameter), whether single or multiple foci, should be considered as low risk and iodine-131 therapy is not routinely recommended unless there are considerations of recurrence risk adjustment, disease follow-up and patient’s wishes. It is worth noting that, strictly speaking, postoperative iodine-131 therapy for thyroid cancer in the broad sense can be subdivided into three specific situations in terms of practical methods and purposes, namely, residual nail ablation, adjuvant iodine-131 therapy, and iodine-131 therapy for thyroid cancer. Ablation of residual nail, commonly known as “iodine-131 nail cleansing”, means that the normal (postoperative) thyroid tissue is necrosed by targeted ionizing radiation through oral administration of iodine-131 to fully deactivate the thyroid tissue. The effect is to reduce the risk of recurrence and death of postoperative thyroid cancer and to facilitate disease staging and follow-up (monitoring of serum thyroglobulin TG). The dose of iodine-131 should be decided depending on the size of the residual thyroid gland and its iodine uptake capacity, usually in the range of 30-150 millicurie. Iodine-131 treatment for thyroid cancer in a narrow sense means that the residual, recurrent or metastatic thyroid cancer will be necrosed by targeted ionizing radiation through oral administration of iodine-131, which will suppress or even cure the thyroid cancer. The dose (activity) of iodine-131 usually used ranges from 150-250 millicurie. Of course, during specific clinical practice, especially during the first admission, some patients have the possibility of both residual nail and potential metastases (or recurrence or residual). In order to maximize the efficacy, reduce the number of treatment sessions and reduce radiation damage and medical expenses, iodine-131 treatment can be used, if conditions permit, to simultaneously perform the dual functions of ablation of residual nail and adjuvant treatment of thyroid cancer lesions, and it is difficult to strictly distinguish between “ablation” and “treatment”. It is difficult to strictly distinguish between “ablation” and “treatment”, or the two can be performed simultaneously, which is called Iodine-131 adjuvant therapy.