Thyroid cancer was first documented in the late 18th century, and radioactive iodine therapy was first introduced in 1946, thus ending the century and a half-long history of surgery as the only effective means of tumor treatment. At present, radioactive iodine therapy for thyroid cancer has a history of more than 50 years at home and abroad, but unfortunately, many thyroid cancer patients are still so unfamiliar with radioactive iodine therapy, and even some medical practitioners know little about it. The thyroid gland is an endocrine organ of the human body, divided into two lobes, located on both sides of the trachea, which can produce thyroid hormones to meet the needs of the human body, and iodine is one of the substances necessary for synthesizing thyroid hormones. It has the same chemical properties as iodine, but unlike ordinary iodine, it emits gamma rays for imaging and beta rays for treatment, thus performing diagnostic and therapeutic functions. When thyroid cancer patients take iodine-131 orally, the lesion can take up iodine-131 at a high level, and the beta-rays emitted from the lesion can effectively kill the tumor cells, which is called “radioactive iodine therapy”. However, not all thyroid cancer patients can be treated with radioactive iodine therapy. Generally, thyroid cancer is divided into four types according to pathological types: papillary carcinoma, follicular carcinoma, medullary carcinoma and undifferentiated carcinoma. Radioactive iodine treatment is not suitable for medullary thyroid cancer and undifferentiated cancer because they do not have the function of iodine-131 uptake. Perhaps we may ask, since iodine-131 can treat thyroid cancer, does it mean that surgery is not necessary? The answer is no. The first choice of treatment for thyroid cancer is still surgical resection, while radioactive iodine treatment is only a further treatment after surgery. However, it is essential to perform total thyroidectomy or subtotal thyroidectomy before radioiodine therapy. According to the latest foreign guidelines for thyroid cancer treatment, radioactive iodine therapy should be performed after surgical resection for the vast majority of thyroid cancer patients, especially when there are distant metastases such as lymph nodes, lungs and bones, and postoperative radioactive iodine therapy is preferred. Iodine-131 whole-body imaging after treatment can detect new metastatic foci that cannot be detected by other imaging examinations; third, by measuring thyroglobulin (Tg) in blood, it can monitor whether there is recurrence or metastasis, which is both sensitive and accurate, and easy to follow up; fourth, local or distant metastatic foci (such as lung, bone, brain, etc.) have good results after repeated iodine-131 treatment, which can be seen to be significantly reduced in size and symptoms, and improve the quality of life. Improve the quality of life, and some patients can be cured. Although radioactive iodine therapy has many implications, thyroid cancer should not be over-treated. Some low-risk patients or early stage patients (such as micro papillary carcinoma) can be followed up after surgery without further radioactive iodine therapy, and external radiotherapy or chemotherapy is not recommended, because differentiated thyroid cancer itself is not sensitive to chemotherapy and external radiotherapy, and the efficacy is poor. Radioactive iodine therapy is one of the only treatments with evidence-based medical evidence, which is both an internal radiotherapy and a targeted therapy with convenient treatment, non-invasive and good efficacy. However, certain preparations are needed before the treatment: firstly, the surgical incision should be well healed to avoid infection and healing difficulties after radioactive iodine treatment; secondly, in order to enhance the iodine uptake capacity of the lesion and make the treatment effective, it is necessary to avoid iodine diet (do not eat iodine-rich food and medication) and to stop using “Eugenol” (a drug for thyroxine replacement therapy) for about four weeks. (a type of thyroxine replacement therapy) for about four weeks. After radioactive iodine treatment, it is necessary to take thyroxine (eugenol) replacement therapy for life to replenish the normal physiological needs of the body and to suppress tumor recurrence. If there are metastases in the cervical lymph nodes or distant metastases in the lungs, bones and soft tissues, multiple radioactive iodine treatments may be required. There are two sides to everything, and radioactive iodine therapy is no exception. It has the side of treating diseases, but also the side of affecting others. Because iodine-131 is a kind of radioactive drug, after the patient takes a large dose of iodine-131 orally, the iodine-131 gathered in the lesion will emit a large amount of γ-rays, thus producing a certain amount of ionizing radiation to the surrounding people, so radioactive iodine treatment must be carried out in a special protective ward for inpatient treatment. Only after the decay and excretion of iodine-131 in the body have been reduced to meet the national radiation protection requirements can the patient be discharged. At present, the Department of Nuclear Medicine of Zhejiang Cancer Hospital has a special ward for radionuclide treatment, which is fully equipped, marking Zhejiang Cancer Hospital as one of the only few medical institutions in China and the only one in Zhejiang Province that meets the national environmental protection requirements for regular radioactive iodine treatment. The standard treatment mode of differentiated thyroid cancer is “thyroid surgery + radioactive iodine therapy + thyroid hormone replacement therapy”, which is also the mainstream and most reasonable. Thyroid cancer is not scary, what is scary is that we should not be blinded by the darkness. Radioactive iodine therapy is like the brightest light, shining our way forward.