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Cancer rates elevated after hyperthyroid treatment

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Effective treatments for one medical condition can, unfortunately, increase an individual's risk for another medical condition. One clear-cut example of this is the special risk of former cancer patients later developing a second cancer or another serious medical condition related to their earlier cancer treatment.

Radioactive iodine is a time-honored and effective treatment option for hyperthyroidism, a condition in which the thyroid gland is overactive. The thyroid gland naturally "absorbs" iodine in the body. So if the iodine is made radioactive, the absorbed radioactive iodine delivers radiation directly to the overactive thyroid gland, where it "quiets" the gland and treats the condition. The research reported here found that people who received radioactive iodine to treat hyperthyroidism had an overall 25% increased risk of cancer compared to those who did not. Stomach and breast cancer risk were particularly increased.

The researchers found that women who received radioactive iodine were 53% more likely to develop breast cancer compared to those who did not receive radioactive iodine. The increased cancer risk was seen only after five years from the time of radioactive iodine treatment. Because of this delay in the increased risk, and the fact that radiation takes time to induce cancer, the researchers think that the radiation may be the culprit for the increased cancer risk. It's important to note that while cancer risk was increased in treated patients, the fact is that the vast majority of patients treated with radioactive iodine did NOT develop cancer.

If you are being treated or have been treated with radioactive iodine, the findings of this research do not at all mean that your treatment is, or was, wrong. But you and your doctor should consider the findings of this research in your overall care. In recognizing the possibly increased cancer risk resulting from thyroid treatment, your doctor can plan with you and provide to you the counseling, monitoring, and screening appropriate for that increased risk. This might include a more aggressive approach to breast cancer screening, possibly including mammograms starting at an earlier age, as well as other imaging techniques such as MRI.

This article was made possible by an educational grant from GlaxoSmithKline.

Last Updated: 2007-06-04 10:40:40 -0400 (Reuters Health)

NEW YORK (Reuters Health) - Patients who are treated with radioactive iodine for an over active thyroid, a condition also referred to as hyperthyroidism, have an increased risk of cancer -- especially cancers of the stomach, kidney, and breast -- according to the results of a study published in the medical journal Cancer.

"Radioactive iodine is commonly used as first-line therapy for hyperthyroidism," Dr. Saara Metso, of Tampere University Hospital, Finland, and colleagues write. "Even though it has been used for this purpose since the 1940s, concerns remain about the subsequent risk of malignant disease."

The researchers therefore followed 2,793 hyperthyroid patients treated with radioactive iodine at Tampere University Hospital between January 1966 and June 2002, and 2,793 similar, but untreated subjects, who were used for comparison (controls). After therapy, the subjects were followed for an average of 10 years through the Finnish Cancer Registry.

Overall, 367 patients and 308 controls received a cancer diagnosis during the follow-up period. Twenty-one patients and eight controls were diagnosed with more than two different cancer types.

The researchers determined that the overall cancer rate in the hyperthyroid treated-patients was 25 percent higher compared with the controls. This is the equivalent of 118.9 cases for every 10,000 people compared with 94.9 cases for every 10,000 people, respectively.

Radioactive iodine-treated patients had a 75-percent higher incidence of stomach cancer, a 53-percent higher incidence of breast cancer and the rates of kidney cancer were more than doubled. The risk of cancer also increased with higher doses of radioactive iodine.

Patient age was also a factor. Patients who were between 50 and 59 years old at the beginning of follow-up had a 44-percent increased risk of cancer. The corresponding risk for patients older than 70 years was 38 percent.

The authors observed a 5-year "latent" period after the radioactive iodine treatment before the cancer rates began to differ between the hyperthyroid patients and controls.

Because the risk of cancer increased as the radioactive iodine dose increased and because the cancer risk was not elevated until after 5 years, which is the minimum for radiation-induced cancer to emerge, Metso's team suggests that it was radiation that might explain the excess cancer risk.

However, they add that the "absolute risk of cancer" for all patients was still on the low side.

SOURCE: Cancer, May 15, 2007.

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This page was last modified on: July 26, 2007

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