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Radioactive Iodine for Hyperthyroidism Increases Cancer Risk

Would you believe that I-131 has been administered for over 70 years? Only now do we learn that radioactive iodine for hyperthyroidism might cause cancer.

A new study in JAMA Internal Medicine (July 1, 2019) has linked exposure to radioactive iodine for hyperthyroidism to a 6 percent increased risk of death for all solid cancers and a 12 percent increased risk of death for breast cancer.  Before you panic, however, please recognize that the absolute risk is relatively small. The authors note: “For every 1000 patients currently treated at age 50 years, we estimated an excess of 12 radiation-associated solid cancer deaths, including 3 breast cancer deaths.” Most of these deaths occurred more than 20 years after the original treatment.

What is Hyperthyroidism?

Mention thyroid problems and most people assume you are talking about HYPOthyroidism or underactive thyroid function. That’s because it is extremely common. But a substantial number of people have the opposite condition, ie, HYPERthyroidism. It is also called Graves’ disease.

In this autoimmune condition, the thyroid gland goes into overdrive. Symptoms include hand tremor, weight loss, insomnia, anxiety, restlessness, sensitivity to heat, frequent trips to the bathroom, rapid pulse or irregular heart rhythms.

Why Would Anyone Take Radioactive Iodine for Hyperthyroidism?

To “cure” hyperthyroidism, doctors have been prescribing radioactive iodine (I-131) since the 1940s. Abbreviated RAI, this treatment for Graves’ disease is the most common hyperthyroid treatment in the United States. The radioactive iodine is taken up by the thyroid gland and basically zaps the tissue, leaving the gland nonfunctional. Find out more about this process and unexpected consequences at this link.

Health professionals generally prefer this approach because radioactive iodine for hyperthyroidism is permanent. They often refer to RAI as “a safe and effective definitive treatment for Graves’ disease” (BMC Endocrine Disorders, Dec. 12, 2018). 

The alternatives for treating Graves’ disease are surgery or thyroid-suppressing drugs. Medications like methimazole or propylthiouracil are not considered as reliable as RAI. Thyroid surgery (total thyroidectomy) is usually reserved for therapeutic failures. It is generally the last choice for treating hyperthyroidism (Journal of Community Hospital Internal Medicine Perspectives, online, Sept 7, 2016). 

Problems with Radioactive Iodine for Hyperthyroidism:

When I learned about I-131 in graduate school at the Univeristy of Michigan, the professor was somewhat cautious. He implied that this radioactive isotope of iodine might have long-term consequences that had not yet been fully discovered.

The latest research seems to have proved him right. The authors tracked nearly 19,000 patients who received radioactive iodine for hyperthyroidism. In some cases this was over nearly seven decades. Without doubt, this is the largest and most comprehensive study of RAI ever undertaken.

We suspect that most health professionals assume that radioactive iodine goes straight to the thyroid gland without impacting any other organs. Although the thyroid gets by far the biggest dose of radiation, other organs are exposed. They include the esophagus, stomach, lungs, female breast, pancreas, kidney, uterus, brain, bladder, ovary, prostate, colon or rectum.

The researchers point out that higher radioactive doses are “now currently recommended for the treatment of patients with Graves disease…” The authors project that:

“At these dose levels, we would expect between 19 and 32 excess solid cancer deaths per 1000 patients treated at age 40 years and between 18 and 31 excess solid cancer deaths per 1000 patients treated at age 50 years.”

What to Make of the New Research?

Radioactive iodine for hyperthyroidism has been prescribed for at least 70 years. Only now do we learn that there could be an increased risk of cancer associated with RAI. Please keep in mind that this research in JAMA Internal Medicine (July 1, 2019) is reporting on cancer deaths, not total cancer cases. We suspect that the number of people diagnosed with cancer due to I-131 exposure is substantially higher.

That said, the authors emphasize that this is a relatively small risk. They offer these thoughts on the significance of their research.


“This study’s findings suggest a modest positive association between greater organ-absorbed doses of radioactive iodine and risk of solid cancer death; additional studies are needed to fully weigh the risks and advantages of radioactive iodine and other treatment options for patients with hyperthyroidism.”

So, 70 years after radioactive iodine for hyperthyroidism was introduced into “modern medicine,” we are told that “additional studies are needed.” Seems to us that the horses have been out of the barn for a very long time.

If you are interested in HYPO- and HYPERthyroidism, you may wish to listen to our interview with Dr. David Cooper, Director of the Thyroid Clinic at Johns Hopkins University. Here is a link to the streaming audio or the free mp3 podcast.

Hyperthyroidism in Cats!

You may also want to learn about the epidemic of hyperthyroidism in cats. Don’t let the title about dental floss fool you. It isn’t just cats, and it’s not just dental floss. This article discusses endocrine-disrupting chemicals that might be contributing to hyperthyroidism.

Does Your Dental Floss Contain PFAS Chemicals That Disrupt Hormones?

Share your own experience with radioactive iodine for hyperthyroidism in the comment section.

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About the Author
Joe Graedon is a pharmacologist who has dedicated his career to making drug information understandable to consumers. His best-selling book, The People’s Pharmacy, was published in 1976 and led to a syndicated newspaper column, syndicated public radio show and web site. In 2006, Long Island University awarded him an honorary doctorate as “one of the country's leading drug experts for the consumer.”.
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  • Kitahara, C. M., et al, "Association of Radioactive Iodine Treatment With Cancer Mortality in Patients With Hyperthyroidism," JAMA Internal Medicine, July 1, 2019,doi:10.1001/jamainternmed.2019.0981
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