Common Treatment for Hyperthyroidism Increases Cancer Risk

Findings from a study of patients who received radioactive iodine (RAI) treatment for hyperthyroidism show an association between the dose of treatment and long-term risk of death from solid cancers, including breast cancer. The study, led by researchers at NCI, was published July 1 in JAMA Internal Medicine.

“We identified a clear dose-response relationship between this widely used treatment and long-term risk of death from solid cancer, including breast cancer, in the largest cohort study to date of patients treated for hyperthyroidism,” said lead author of the study Dr. Cari Kitahara of NCI’s Division of Cancer Epidemiology and Genetics. “We estimated that for every 1,000 patients treated currently using a standard range of doses, about 20 to 30 additional solid cancer deaths would occur as a result of the radiation exposure.”

RAI, which has been used widely in the United States for the treatment of hyperthyroidism since the 1940s, is one of three commonly used treatments for hyperthyroidism. The other two are anti-thyroid drugs, which have been rising in popularity, and surgical treatment, which is used least often.

The new findings are from a long-term follow-up study of a large cohort of people with hyperthyroidism (mainly Graves’ disease) who were treated with radiation between 1946 and 1964, the Cooperative Thyrotoxicosis Therapy Follow-up Study.

In the new analysis—which included nearly 19,000 people from the original cohort, all of whom had received RAI and none of whom had had cancer at study entry—the researchers used a novel, comprehensive method of estimating radiation doses to each organ or tissue.

Most of the radiation is absorbed by the thyroid gland, but other organs like the breast and stomach are also exposed during treatment.