131I has become one of the main means of postoperative treatment for DTC.131 I treatment for DTC is to use 131I to remove postoperative residual thyroid tissue, referred to as nail clearing; secondly, to use 131I to remove surgically unresectable metastases of DTC, referred to as focal clearing. The significance of 131I nail clearing after DTC: (1) To facilitate postoperative follow-up monitoring. 131I can clear the normal thyroid tissue that remains after surgery or cannot be removed (e.g. for the protection of parathyroid glands, laryngeal nerve, etc.) to facilitate the monitoring of serum Tg in DTC patients and to improve the sensitivity of 131I whole body imaging (WBS) for the diagnosis of iodine-intake DTC metastases. (2) Nail clearance is the basis of focal clearance treatment and facilitates postoperative 131I focal clearance treatment. The residual normal thyroid tissue has higher 131I uptake than DTC lesions, and the completion of nail clearing helps DTC metastases to uptake iodine more effectively. (3) It facilitates re-staging after DTC surgery. 131I WBS and SPECT/CT fusion imaging after nail clearing can detect partial 131 I uptake of cervical lymph node metastases or even distant metastases, and thus can change the staging and risk stratification of DTC, guiding the subsequent 131I clearing treatment and the development of follow-up plan. (4) Adjuvant treatment of potential DTC lesions. dtc is often characterized by bilateral, micro multifocal, long local latency and development period, and high recurrence rate. Nail clearing treatment has a clearing effect on the cancer cells that may remain after surgery, including microscopic cancer foci hidden in the residual thyroid tissue after surgery, hidden metastatic foci that have invaded beyond the thyroid gland, or potential DTC foci that cannot be removed because the condition does not allow or surgery. Postoperative treatment of DTC with 131I can achieve excellent results and improve the prognosis, including delaying the time to recurrence, reducing the recurrence rate and decreasing distant metastasis. Compared with the surgery + TSH suppression treatment mode, the surgery + 131I clear nail + TSH suppression treatment mode led to a significant reduction in the recurrence rate and morbidity and mortality rate of DTC. The 10-year overall survival rate of DTC patients after 131 I treatment was 92.38%, among which, the 10-year survival rate was 98.09% in the cervical lymph node metastasis group, 87.50% in the lung metastasis group and 80.41% in the bone metastasis group, thus 131I treatment significantly improved the recurrence-free survival, progression-free survival and disease-free survival of patients. Some patients with low-risk DTC did not benefit from clear nail therapy. 131I therapy has its limitations. The age of DTC patients at onset, the uptake and retention time of 131I in the lesion, radiation sensitivity, and patients’ adverse reactions to multiple 131I treatments can affect the therapeutic effect. 131I has a limited therapeutic effect on some high-risk DTC because most of the DTC cells with distant DTC metastasis or in progressive stage have developed a dedifferentiated state and have poor uptake and poor ability to retain 131I. During treatment, about 1/3 of recurrent and metastatic lesions become dedifferentiated, and the expression of sodium/iodine co-transporter (NIS), Tg, and thyroid stimulating hormone receptor (TSHR) genes in DTC cells decreases, and iodine uptake can be reduced or even lost. Note: 131I: iodine 131, DTC: differentiated thyroid cancer (including papillary thyroid cancer and follicular thyroid cancer, the most common type of thyroid cancer).