Thyroid disease and breast cancer: Is there a link?

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Breast cancer is the most commonly diagnosed cancer among women in the United States, and thyroid disorders affect millions of American women. Many breast cancers are sensitive to hormones like estrogen, and according to researchers, thyroid hormone has estrogen-like effects at high levels. So, for years, scientists have wondered whether having too much thyroid hormone might promote the development of breast cancer. A new study suggests the answer could be “yes.”

It’s all about the thyroid

The thyroid is a butterfly-shaped gland at the base of the neck that produces thyroid hormone. Thyroid hormone affects almost every cell in the body and has many crucial functions, like controlling metabolism, heart rate, and body temperature.

Some people have hyperthyroidism, or an overactive thyroid, in which the thyroid gland produces too much thyroid hormone. This can cause weight loss, thinning hair, sweating, anxiety, and a rapid heartbeat. Women are five to 10 times more likely than men to develop an overactive thyroid.

Hypothyroidism, or an underactive thyroid develops when the thyroid gland does not produce enough thyroid hormone. Symptoms of an underactive thyroid include weight gain, fatigue, constipation, depression, dry hair, and a slow heart rate. Like hyperthyroidism, it’s also more common in women than in men.

In an effort to determine whether having an overactive or underactive thyroid affects a woman’s risk of breast cancer, researchers looked at a large group of women in Denmark diagnosed with thyroid disease between 1978 and 2013. More than 60,000 of the study participants had an underactive thyroid, and more than 80,000 had an overactive thyroid. They followed the patients for five to seven years and found that those with an overactive thyroid had a slightly increased risk of breast cancer — an 11% higher risk, to be specific. However, women with an underactive thyroid had a 6% drop in their breast cancer risk.

Putting the study results into perspective

While this study might lead some to conclude that having excess thyroid hormone promotes the development of breast cancer, experts caution that this study does not prove cause and effect. While there may be an association, it is not clear that an overactive thyroid actually causes breast cancer to develop.

Some critics wonder whether women at risk for hyperthyroidism may also be at risk for breast cancer, or whether the treatment for hyperthyroidism may be to blame for increased cancer risk. Another theory is that women with an overactive thyroid see their doctors more often and therefore are more likely to get screened for other problems like breast cancer.

The effect of thyroid hormone on breast cancer risk clearly requires further study. In the meantime, women with an overactive thyroid should stay in close communication with their doctors and follow routine breast cancer screening recommendations.