There are very few risks associated with the ingestion of radioactive iodine when it is administered in doses that are commonly used to treat Graves' disease and other forms of hyperthyroidism. Some patients may experience slight nausea after ingesting a dose of 131-Iodine due to mild irritation of the lining of the stomach. As 131-Iodine travels through the bloodstream, some of it may concentrate in the large salivary glands located in the cheeks. Some patients may develop mild temporary irritation and swelling of these salivary glands, a condition called parotitis.
In patients who have moderate to severe thyroid eye disease, there is some concern that treatment with radioactive iodine may contribute to progression of inflammation and swelling in the tissues that surround the eyeballs that may increase the risk of developing complications. Clinical trials have demonstrated that treatment with a course of immunosuppressive therapy started before a dose of 131-Iodine is administered may help to prevent the progression of thyroid eye disease.