Does Hyperthyroid Diagnosis Equal Higher Breast Cancer Risk?

With commentary by Minisha Sood, MD, endocrinologist and director of inpatient diabetes, Lenox Hill Hospital, New York.

If you've been diagnosed with an overactive thyroid (hyperthyroidism), it is important to get the proper treatment. New research suggests it also may be especially important to pay close attention to breast cancer screenings.


While thyroid issues and breast cancer may seem unrelated, research has found a link between hyperthyroidism and a slightly higher breast cancer risk.

"From our preliminary findings, there is a statistically significant increased risk of having breast cancer in people with hyperthyroidism," says Chien-Hsiang Weng, MD, MPH, resident physician at NH Dartmouth Family Medicine Residence at Concord Hospital in New Hampshire. He presented the results of his study, which included 100,000 people, at the American Thyroid Association's 86th annual meeting in Denver Sept. 22.1

Earlier this year, two other studies also found a link between overactive thyroid and a higher risk of breast cancer. 2,3 It's too soon to suggest a change in cancer screening guidelines, according to Dr. Weng, but he believes women with hyperthyroidism should at least be aware of the recent findings.

More About Hyperthyroidism and the Link

If you've been diagnosed with hyperthyroidism, your thyroid gland (a butterfly-shaped organ in the front of your neck) is making too much thyroid hormone. These hormones control metabolism, weight, breathing, heart rate, the nervous system and many other functions.

If you have an overactive thyroid, you may have noticed symptoms such as excessive sweating, weight loss, sleep problems and nervousness, among other issues, before receiving treatment.  (Treatment options include antithyroid medications that interfere with thyroid hormone production, radioactive iodine therapy or, rarely, surgery.)

Exactly why an overactive thyroid may boost breast cancer risk is unknown, Dr. Weng says, but he suspects one possibility is that thyroid hormones may have estrogen-like effects, promoting growth of breast cancer cells. Another possible explanation? If you are under treatment for an overactive thyroid, you may go to the doctor more often--and other conditions, including breast abnormalities, may be found during those visits.

More About the Studies

Here, in brief, is what the researchers studied and found in three different studies looking at thyroid problems and breast cancer risk.

  • Dr. Weng's team evaluated more than 100,000 women in the Taiwanese National Health Insurance Research Data Base and found more than 51,000 women with a breast cancer diagnosis from 2006 through 2011. The researchers compared them to another 51,000 women without breast cancer. In each group, some women had a diagnosis of overactive or underactive thyroid. Those who had hyperthyroidism had a 9% increased risk of getting breast cancer, while having an underactive thyroid did not affect risk. A surprise finding: the five-year survival rate was better among those who had a thyroid disorder. 1
  • In a Dutch study, reported in September in the Journal of Clinical Endocrinology & Metabolism, researchers found that higher levels of one thyroid hormone, T4, was linked with a higher risk of breast cancer, as well as other cancers, including lung cancer.  The researchers studied more than 10,000 people who had baseline measurements of thyroid hormones. 2
  •  In another study, published in the European Journal of Endocrinology, researchers tracked nearly 62,000 women with underactive thyroid and more than 80,000 with overactive thyroid in Denmark for nearly five years. Those with an underactive thyroid had an 11% increased risk of breast cancer compared to the general population; those with an underactive had a very slightly lower risk.3  

Perspective and Action Plan

Endocrinologist Minisha Sood, MD, is the director of inpatient diabetes at Lenox Hill Hospital, New York, and reviewed the research for Endocrine Web. "The link between thyroid hormone levels and breast cancer is, in fact, plausible," she says. Thyroid hormones play an important role in growth and cell differentiation, she says, and could increase breast cancer cell growth, she says.

Other research, however, has suggested a protective effect of thyroid hormones on some cancers, she says. The studies are preliminary, she says, and ''there is not conclusive evidence for either of these hypotheses."

However, the new findings do highlight the fact that women with overactive thyroid should get prompt treatment, Dr. Sood says, even if they are wary of side effects. "Many women fear that in treating their hyperthyroidism, they may gain weight or suffer other untoward consequences, but this may be a reason not to ignore thyroid disease," she says.

Awareness of the research is warranted, Dr. Weng agrees. "Based on our preliminary finding," Dr. Weng says, ''patients with hyperthyroidism should be more aware of the [possible] increased risk of breast cancer and have regular follow up with their primary care physicians (or ob-gyn doctors) for breast cancer screening."

Several organizations issue guidelines about breast cancer screening, and they differ. Experts do agree that a woman and her doctor should discuss the best screening timetable for her, based on factors such as personal and family history. The American Cancer Society recommends women with an average risk of breast cancer start regular screening mammography at age 45, but also have the option of beginning at age 40. Women 45 to 54 should undergo annual screening, the ACS says, and those 55 and older can transition to every other year or continue screening annually. 4


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