Sodium iodide [131I] – the “nemesis” of papillary and follicular thyroid cancer Usually, whenever thyroid cancer is detected, surgery is the first treatment option for thyroid cancer. Because of the low malignancy of papillary and follicular thyroid cancer, the comprehensive treatment of thyroid cancer has not been given enough attention for a long time. In some places, surgery alone is used; in better cases, thyroid hormone therapy is given after surgery. With the strengthening of international academic exchanges and a large amount of evidence-based medical data, the “surgery + 131I + thyroid hormone” comprehensive treatment plan is now being promoted in China, and the “three-step” treatment for thyroid cancer has reached a consensus in the medical community and is gradually becoming popular. The “three-step” treatment for thyroid cancer has been gradually popularized. It has been found that thyroid cancer is multifocal in nature, and it is impossible to completely remove the thyroid tissue under the microscope during thyroid cancer surgery, and some of the thyroid tissues will be left behind. 131I radiation is used to remove the residual thyroid gland and thyroid cancer tissues, thus improving the quality of life of patients and prolonging their lives. 131 I can remove residual thyroid tissue, and when removing residual thyroid tissue, it also kills microscopic cancer lesions, thus reducing recurrence and metastasis. The results of many studies show that the recurrence and metastasis rate of thyroid cancer is about 30% with surgery alone, but the recurrence and metastasis rate is reduced to about 2.7% with the combined treatment of “surgery + 131I + thyroid hormone”. It is also the best treatment for papillary and follicular thyroid cancer recurrence and metastasis, and is an excellent representative of molecular targeted therapy with low toxicity and good efficacy. In addition, whole-body 131I scan is also a specific test for early diagnosis of recurrent metastasis of papillary and follicular thyroid cancer, observation of 131I absorption in lesions during treatment, and determination of efficacy and prognosis. In summary, 131I is a true “nemesis” for papillary and follicular thyroid cancer because it can both examine and treat the disease.