The most rational approach to treat most well-differentiated thyroid cancers and their metastases is the “three-in-one” approach of total thyroidectomy + 131 iodine therapy + oral thyroid hormone. Why is the “three-in-one” treatment plan the most reasonable approach? As with most malignant tumors, the first choice of treatment for thyroid cancer should be surgery. However, the extent of thyroidectomy for differentiated thyroid cancer has long been the subject of surgical debate. The overall treatment options are subtotal thyroidectomy and total thyroidectomy, but there are at least four options for subtotal thyroidectomy: 1) partial excision of one lobe; 2) excision of one lobe and isthmus; 3) partial excision of one lobe, isthmus + contralateral lobe; 4) subtotal excision of one lobe, isthmus + contralateral lobe. Therefore, it is difficult to determine the best plan for complex lesions, and the specific implementation is also very difficult, and one wrong move will have a great impact on clinical treatment. In 1988, WHO proposed the definition of thyroid microfocal carcinoma (TMC): any thyroid carcinoma with a maximum diameter of less than 1 cm, regardless of the presence of regional lymph nodes or distant lymph node metastasis, is called TMC. TMC is more commonly seen in well-differentiated papillary carcinoma. Because TMC has small diameter, very few spontaneous symptoms and slow clinical progression, it is difficult to be detected early and preoperative diagnosis is also difficult. There are even metastatic lesions at the cellular level that are inaccessible to the naked eye (studies have reported that microscopic detection of metastases in the contralateral gland of differentiated thyroid cancer can reach 38% to 87%), so it is also difficult to diagnose intraoperatively. It is speculated that TMC is likely the main reason for the high recurrence rate after conventional surgery for this disease. Since it is difficult to determine the early diagnosis of TMC and the presence of TMC in the thyroid gland seen intraoperatively with bare eyes, clinical studies have shifted the focus to the exploration of new treatment methods. It has been shown that 131I treatment after surgical treatment of thyroid cancer can effectively remove residual thyroid tissue and TMC at the cellular level to prevent tumor recurrence. Although the “three-in-one” treatment plan for thyroid cancer has been increasingly accepted by the industry, different scholars still have different opinions on the extent of resection for different lesions. Most physicians advocate near-total thyroidectomy, removing as much of the thyroid as possible, but only if the parathyroid glands and the laryngeal nerve are protected. In fact, total thyroidectomy is associated with high complications and is extremely unnecessary, as high doses of radioactive iodine are effective in removing residual functional thyroid tissue from the neck after surgery. Another consideration in favor of subtotal thyroidectomy is that 131I is more effective in removing postoperative residual thyroid because the number of residual thyroid glands is small and the dose of 131I required is small. In addition, proximal total gland excision causes low thyroid and increased TSH, allowing for a more sensitive determination of early functional metastases. The traditional approach recognizes the significance of thyroid hormone therapy ① to maintain the normal function of the thyroid gland; ② to suppress the secretion of thyrotropic hormone from the pituitary gland, because thyrotropic hormone may cause tumor recurrence, and using thyroid hormone can prevent or reduce recurrence. Therefore, thyroid hormone replacement therapy is applied whether the thyroid gland is completely resected or partially resected; it may be that 131I is rarely used in postoperative treatment because of the lack of understanding of the significance of 131I in removing residual thyroid tissue. Since thyroid hormone does not completely inhibit the growth of possible TMC and microscopically accessible metastases, there has long been a high recurrence rate after conventional surgery. It is now recognized that the principle of surgery is to remove as much cancerous tissue as possible and to remove the lymph nodes in the neck where metastases may have occurred. In order not to damage the parathyroid glands and the laryngeal recurrent nerve, it is difficult to completely remove the thyroid gland by surgery (cancer cells are found in the residual thyroid gland under microscope). Therefore, after surgical removal of the thyroid gland, the residual thyroid tissue should be removed promptly using 131I and then thyroid hormone replacement therapy should be given to reduce the recurrence rate.