Thyroid cancer is a malignant tumor of the thyroid gland, which is characterized by easy metastasis and recurrence. The main clinical manifestation is a long lump in the thyroid gland of the neck, and sometimes there may be difficulty in breathing and hoarseness due to the pressure of the lump. Many patients may also have distant metastasis throughout the body such as lymph nodes or lung and bone. Usually, as soon as thyroid cancer is detected, surgery is thought to remove it, and surgery is the first choice of treatment for thyroid cancer. Because of the low malignancy of papillary and follicular thyroid cancer, 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 differentiated thyroid cancer has reached a consensus in the medical field. The “three-step” treatment for differentiated 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 may be left behind. The radiation produced by 131I is used to remove the residual thyroid gland and thyroid cancer tissue, thus improving the patient’s quality of life and prolonging life expectancy. 131I can remove the residual thyroid tissue, and when removing the residual thyroid tissue, it also kills the microscopic cancer lesions, thus reducing the 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” of papillary and follicular thyroid cancer because it can both examine and treat the disease.