Iodine 131 is a radioactive isotope of elemental iodine that is used clinically as a treatment for the destruction of thyroid follicles and thus for hyperthyroidism and thyroid cancer. Usually we would call this biochemical therapy, or chemotherapy. The radioactive iodine 131 emits beta rays that, after precise localization of the thyroid gland, will act on the thyroid gland within 8 days, effectively reducing the entry of thyroid hormones into the bloodstream to relieve the exacerbation of hyperthyroidism. It is especially good for patients who have thyroid crisis, and has a very good healing effect. This is due to the relatively high ionizing radiation of iodine 131, which is more self-healing. However, for the treatment of patients who have developed metastatic lesions from thyroid cancer, intense reactions in the upper gastrointestinal tract, such as nausea and vomiting, may occur with high doses of the drug. There may be pain or swelling in the neck and chest, dry vomiting, dry and painful throat, and hair loss in two to three months after treatment. In conclusion, iodine 131 is often used clinically as an important tool for chemotherapy, not only to kill cancer cells, but also to play an important role in thyroid cancer. As the cancer cells are killed by eating the iodine containing radioactive elements, the chemotherapy effect, but its side effects are also obvious, especially for people with low immunity may be fatal.