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Radioactive iodine was first introduced as a tool for treatment and diagnosis of thyroid disease in the 1940s by Dr. Saul Hertz of the Massachusetts General Hospital in Boston. He realized that radioactive forms of iodine could be used to check the function of the thyroid gland, and – in larger doses – to treat an overactive thyroid (hyperthyroidism). Radioactive iodine quickly became the number one treatment for Graves’ Disease, an autoimmune disorder that causes hyperthyroidism, in the United States. According to the American Thyroid Association, about 60% of patients who are treated for an overactive thyroid in the U.S. receive radioactive iodine therapy.  Radioactive iodine is also commonly used as part of the treatment of the most common types of thyroid cancer.

Now, a new study from the National Institutes of Health has confirmed a link between the use of radioactive iodine and an increased risk of other cancers down the line, and found a “modest” link between the amount of radioactive iodine absorbed by different organs and the risk of dying from a cancer arising in those organs.

Dr. Bryan McIver, endocrinologist and Moffitt Cancer Center Deputy Physician-in-Chief.

“We have known about the link between radioactive iodine and cancer for many years,” said endocrinologist and Moffitt Cancer Center Deputy Physician-in-Chief Dr. Bryan McIver. “This effect isn’t unique to radioactive iodine, but results from all forms of radiation. If we treat with radiation, sometimes we are actually increasing the risk downstream for other cancers.”

The study estimated for every 1,000 patients treated with radioactive iodine, about 20 to 30 cancer deaths, including breast cancer, would occur. “The surprise in this study was the relatively high number of cancers that seem to be triggered by doses of radioactive iodine that have often been considered to be ‘safe’ in the past,” said McIver.

Radioactive iodine is also used to treat thyroid cancer, where it is given in higher doses than are used for Graves’ Disease. It became a common standard of care in the late 1990s and early 2000s because it appeared to work so well, but the link between the treatment and risk for other cancers has caused endocrine oncologists to use it more sparingly, especially in patients with low-grade, early stage thyroid cancer.

“Here at Moffitt, we use radioactive iodine in only a minority of our patients with thyroid cancer,” said McIver. “We use it in selected patients who can really benefit from it and where the risk is worth it.” In contrast, McIver estimates that more than half of all thyroid cancer patients receive the treatment in the U.S., often unnecessarily.

Increased cancer risk isn’t the only side effect of radioactive iodine. Patients can also experience damage to saliva glands and tear ducts, dental and gum decay and changes in taste buds. “In the past, well-meaning physicians have tended to overstate the safety of radioactive iodine and have often dismissed concerns about radiation side-effects and risks,” said McIver. “We need to be much more honest with our patients that these treatments can have consequences.”

McIver says you should always discuss with an expert before deciding which treatment option is right for you. “There are a range of options to treat Graves’ Disease and thyroid cancer, and with every treatment decision it’s important to carefully balance risk and benefit. You have to have a conversation with a true expert who understands those risks and benefits and accepts that a cookie-cutter decision doesn’t suit everyone. Treatment with radioactive iodine needs to be a personalized, carefully thought out decision.”