BACKGROUND
Graves’ disease is the most common cause of hyperthyroidism. Radioactive iodine therapy is sometimes used for Graves’ disease that has not responded to medication. There is a concern that radioactive iodine treatments might increase risk of thyroid cancer developing in the thyroid cells that are not destroyed. A group of patients were studied in the 1950s, and no increased risk of thyroid cancer with radioactive iodine treatments was found. However, when this same group was studied for an additional 24 years, an increase in thyroid cancers and other solid cancers was found. The criticism of the longer study is that the radioactive iodine doses were not accurate and that the hyperthyroidism itself can increase thyroid cancer risk. Nonetheless, after this study, fewer physicians have been ordering radioactive iodine treatments for their patients.

The goal of this study is to clarify if there is a link between radioactive iodine treatments and thyroid cancer and identify various characteristics of the thyroid cancers that occur in these patients treated with radioactive iodine.

THE FULL ARTICLE TITLE
Hiruma S, et al. Occurrence of newly diagnosed thyroid cancer is not increased after radioactive iodine therapy for Graves’ disease. J Clin Endocrinol Metab. Epub 2025 Apr 11:dgaf231; doi: 10.1210/clinem/dgaf231. PMID: 40215268

SUMMARY OF THE STUDY
The medical charts of patients with Graves’ disease from a specialized Japanese thyroid institute were analyzed from 2007 to 2016 to see if there was a link between receiving radioactive iodine for Graves’ disease and thyroid cancer. After removing patients with pre-existing cancer, a total of 13,874 patients were studied. The patients were on average 30-50 years old and were followed for 5-10 years after the treatments. Separately, a second group of 23,179 patients who underwent radioactive iodine therapy for Graves’ disease from April 1997 to December 2022 were studied.

A total of 2,284 Graves’ patients were treated with radioactive iodine. A total of 12,418 patients were treated with antithyroid medications, and 287 patients had their thyroid glands surgically removed. New thyroid cancer was found in 8 patients who received the radioactive iodine treatment. In the group whose Graves’ disease was only treated with medication, 39 patients developed cancer, with cancer found 0.43 times in a span of 1000 years of patients’ lives. Over a total of 107,218 years of patient’s lives studied, there was no significant difference in the incidence rates of thyroid cancer between the radioactive iodine group and the medication-only group. The sizes of the cancers were also not different between the two groups. Out of the 23,179 patients in the second group who underwent radioactive iodine for Graves’ disease, 17 patients developed thyroid cancer. A total of 16 patients had the most common type of thyroid cancer, papillary thyroid carcinoma, and 15 were less than 1 cm in size. Only one patient who had received radioactive iodine developed anaplastic thyroid carcinoma, the most aggressive thyroid cancer.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
For patients who received radioactive iodine, thyroid cancer occurred 0.53 times in a span of 1,000 years of patients’ lives. This was not significantly different that the thyroid cancer incidence rates in patients who were treated with medication for their Graves’ disease. The thyroid cancers found in patients who had received radioactive iodine were mostly in the category of American Thyroid Association low risk of recurrence, less than 1 cm, and with no concerning features of the cancer cells or genetic mutations. Patients and physicians can feel safe about using radioactive iodine treatment for Graves’ disease that is not responding to medical therapy, as radioactive iodine does not seem to increase the risk of thyroid cancer significantly compared to medical therapies.

— Pinar Smith, MD

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