The most common international treatment for most well-differentiated thyroid cancers and their metastases is the “total thyroidectomy + 131 iodine therapy + oral thyroid hormone” protocol. Why is this the most common treatment option? Undoubtedly, as with most malignancies, 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 due to the high recurrence rate (median 35%) after surgery. The actual practice varies greatly due to different views. 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 and contralateral lobe; 4) subtotal excision of one lobe, isthmus and 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 micro-carcinoma (TMC): thyroid carcinoma with a maximum diameter of ≤1 cm, regardless of the presence of regional lymph nodes or distant lymph node metastasis, is called TMC. TMC is more common in well-differentiated papillary carcinoma. In the literature, the incidence of papillary thyroid microfocal carcinoma in autopsy thyroid specimens is 5.6%, accounting for 4.2% of thyroid surgeries in the same period and 47.9% of differentiated thyroid carcinomas, and 3.0% in those with clinically inaccessible thyroid masses. TMC is difficult to be detected early and more difficult to be diagnosed preoperatively because of its small diameter, very few spontaneous symptoms and slow clinical progression. 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 the naked eye, clinical research has shifted the focus to the exploration of new treatment methods. It has been shown that 131I therapy given after surgical treatment of thyroid cancer can effectively remove TMC from residual thyroid tissue and at the cellular level and prevent tumor recurrence. It has been reported in the literature that the recurrence rate of thyroid cancer is 35% after surgical resection, and it can be reduced to 1%-2.5% if postoperative nuclear hormone therapy is combined with a larger dose of thyroid hormone replacement therapy. It has also been reported that the recurrence rate of nail cancer is as high as 32.0% with surgery alone, 11% with surgery + oral thyroid hormone, and only 2.7% with surgery + 131I therapy + oral thyroid hormone. Foreign data reported that those who added 131I treatment after surgery had 3.8 ~ 5.2 times lower mortality rate and 4 times lower recurrence rate than patients with surgery alone. Currently, many scholars at home and abroad have developed a more comprehensive plan for the treatment and follow-up of differentiated thyroid cancer, and the detailed process of clinical implementation of this treatment plan is shown in the figure above. Most physicians advocate near-total glandular resection, removing as much of the thyroid as possible, but only if the parathyroid glands and the laryngeal recurrent 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 hypothyroidism 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 inhibit the secretion of thyrotropic hormone from the pituitary gland, because thyrotropic hormone may cause tumor recurrence, and recurrence can be prevented or reduced with thyroid hormone. 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 due to 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 recurrent laryngeal 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. 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 and contralateral lobe; 4) subtotal excision of one lobe, isthmus and contralateral lobe. Regardless of the type, it is impossible to see whether there are metastatic cancer cells in the left thyroid tissue with the naked eye! At present, most of the surgical treatments for thyroid cancer are based on the plan ③④. The thyroid hormone produced by the left behind thyroid tissue cannot possibly meet the metabolic needs of the body! From the physiological point of view, it is already meaningless! But it leaves a hidden danger of possible recurrence! Therefore, complete removal of the thyroid gland is the best treatment method!