As the incidence of thyroid cancer is increasing year by year, 131 iodine treatment after thyroid cancer surgery is getting more and more attention from doctors and patients. However, because some patients do not pay enough attention to the seriousness of thyroid cancer or still hold the stereotype of “talking about nuclear”, many thyroid cancer patients miss the best time for 131 iodine treatment and the great opportunity to eradicate tumor cells. In clinical practice, we often encounter patients with thyroid cancer who have recurrence and metastasis several years after surgery, and some patients even wait until they have extensive metastasis before they remember to receive 131 iodine treatment, which is really saddening. In fact, although the occurrence and progression of most thyroid cancers are slower than other tumors, it does not mean that surgical removal of the primary site can solve all the problems, and subsequent 131 iodine treatment is not only necessary, but also necessary as early as possible. Cancer cells are good at “lurking” A characteristic of thyroid cancer is that it is especially good at “lurking”. It has a long progression cycle in the thyroid tissue and is mostly bilateral, microscopic and multifocal, and is prone to local organ invasion and local lymph node metastasis. Therefore, postoperative 131 iodine clearance of the residual thyroid gland is essential. If metastases (e.g. lymph node metastases, lung metastases, bone metastases) are already found at the time of surgery, it is more important to remove the metastatic lesions as soon as possible. In addition, after 131 iodine treatment, it is better to monitor thyroid cancer by whole body 131 iodine imaging and measuring serum thyroglobulin level. Studies have shown that the recurrence rate of thyroid cancer after surgical resection alone is 3-5 times higher than the recurrence rate of surgical resection plus postoperative radioactive 131 iodine nail clearance. 131 iodine clearance of residual thyroid tissue after thyroid cancer surgery can significantly reduce the chance of tumor recurrence and metastasis, as well as decrease the rate of death. 131 iodine treatment is not as scary as imagined. Many patients are willing to receive 131 iodine treatment in order to eradicate the lesion, but they are hesitant to accept the legendary “atomic bomb” of radiation, whether it will destroy cancer cells as well as healthy cells in the body. Can their bodies withstand this seemingly powerful treatment, and what are the side effects of 131 iodine treatment? Will the treatment endanger family members? Will the patient’s reproductive function be affected after treatment? In fact, 131 iodine treatment is a very safe and reliable treatment method, and the side effects are not as terrible as they are said to be. Although 131 iodine is a radioactive treatment drug, the side effects of this treatment are far less than those of radiotherapy and chemotherapy, and do not cause serious vomiting, hair loss, hematuria and other toxic side effects. Studies have shown that 131 iodine itself releases a small amount of gamma rays, and because of its small amount of radiation, it has no adverse effects on the patient’s future marriage or fertility, and does not enhance the probability of tumors in other areas. During treatment, patients generally have only mild gastrointestinal reactions, neck swelling and parotid swelling, most of which will resolve on their own. Since many foods and drugs can affect the effect of 131 iodine treatment, the following must be done during the preparation phase before hospitalization: 1. stop thyroid hormone replacement therapy; 2. strictly avoid iodine-rich foods (such as kelp, seaweed, sea fish, sea cucumber, shark fin, abalone, etc.); 3. take non-iodized salt if possible; 4. refrain from performing Enhanced CT (because the contrast agent will affect the 131 iodine treatment).