Over the past decade, researchers have found inconsistent evidence about whether radioactive iodine (RAI) treatment for hyperthyroidism can cause cancer in the long term.1,2

The inconsistency of trial data has led researchers and clinicians to continue to use RAI for patients with Graves disease, the leading cause of hyperthyroidism, although in recent years antithyroid therapy has become the primary mode of care. The Cooperative Thyrotoxicosis Therapy Follow-Up Study, which held data from over 35,000 patients, the majority of whom were treated in RAI for hyperthyroidism, found no connection between the treatment and cancer-related death. Researchers enrolled patients over 20 years through 1964 and had follow-up mortality data through 1990.1

A study recently published in JAMA Internal Medicine, however, found that among 18,805 patients treated for hyperthyroidism over the past 24 years, larger doses absorbed by organs were associated with a greater likelihood of cancer-related death.3

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This study, which extended the monitoring from the Cooperative Thyrotoxicosis Therapy Follow-Up Study by another 24 years, used an improved method of determining the degree to which organs absorbed treatment doses; this method was described in a 2015 publication in Radiation Research.4

The authors found a dose-response relationship between RAI and cancer-related mortality, though the findings were significant only for breast cancer in women: each 100 milligray increase to the breast was associated with a 12% increased risk of breast cancer-related death.

Cari Meinhold Kitahara, PhD, an investigator at the National Cancer Institute and lead coauthor of the recent study, noted that it can be difficult to quantify the cancer and cancer-related mortality risks associated with RAI use for hyperthyroidism.