July 03, 2019
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Radioactive iodine treatment increases cancer risk in patients with hyperthyroidism

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Photo of Cari M. Kitahara
Cari M. Kitahara

In patients with hyperthyroidism treated with radioactive iodine, organ-absorbed doses were associated with increased risk for death from all solid cancers, including breast cancer, according to study results published in JAMA Internal Medicine.

“In the United States, the prevalence of hyperthyroidism is 1.2% (0.5% overt and 0.7% subclinical), and most cases are due to Graves disease,” Cari M. Kitahara, PhD, of the radiation epidemiology branch in the division of cancer epidemiology and genetics at the NIH’s National Cancer Institute, and colleagues wrote. “Radioactive iodine (RAI; sodium iodide I 131, or Na131I) has been extensively used to treat hyperthyroidism since the 1940s and has been the preferred first line treatment by U.S. physicians for uncomplicated Graves disease.”

Researchers created a 24-year extension of the Cooperative Thyrotoxicosis Therapy Follow-Up Study to evaluate site-specific and solid cancer mortality among the study cohort. The study included patients from the U.S. and U.K. with hyperthyroidism who were enrolled between 1946 and 1964 and initially followed up through 1968. After 1968, participants were asked to return to clinics every 2 years. Causes of death were identified through Dec. 31, 2014.

After exclusions, 18,805 patients with a mean age of 49 years and no history of cancer were included in the study. Of those, 14,671 (78%) were women and 17,615 (93.7%) had Graves disease.

Researchers found that the risk for death from all solid cancers increased by 6% with each 100-mGy dose of radioactive iodine to the stomach (RR = 1.06; 95% CI, 1.02-1.1). The risk for death from breast cancer also increased with each radioactive iodine dose, with a 12% increased risk for death for every 100-mGy dose to the breast (RR = 1.12; 95% CI, 1.003-1.3). Excluding breast cancer, all other solid cancers combined had a 5% increased risk for death per 100-mGy dose to the stomach (RR = 1.05; 95% CI, 1.01-1.1).

Researchers found that for every 1,000 patients with hyperthyroidism treated at 40 years of age who received typical radioactive iodine doses (150 mGy to 250 mGy) to the stomach, an additional 19 (95% CI, 3-40) to 32 (95% CI, 5-66) solid cancer cases could occur.

“The results of our study suggest a long-term, modest radiation dose-dependent relationship for radioactive iodine treatment and risk of solid cancer-related death, including breast cancer death, among patients with hyperthyroidism,” Kitahara told Healio Primary Care. “These findings might be considered by physicians and patients when discussing and weighing the risks and benefits of different treatment options.”

Kitahara noted that aside from radioactive iodine, anti-thyroid drugs and thyroidectomy are available treatment options for hyperthyroidism. – by Erin Michael

Disclosures: Kitahara reports no relevant financial disclosures. The research was funded by the Intramural Research Program of the National Cancer Institute, NIH. Please see study for all other authors’ relevant financial disclosures.