Thyroid disease and breast cancer: Is there a link?

Mallika Marshall, MD

Contributing Editor

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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.


  1. Tonya Dennis

    I believe this study. I have hypothyroid and just finished with breast cancer May 2015. But I also had dense breast, which is another factor. I also had no family history of breast cancer.

  2. will

    so why is the thyroid on the blink… you don’t know

  3. carol van linda

    wow do you ever have the hypo and hyper symptoms messed up!! you can have all the hyper symptoms and be Hypo duh and un treated thyroid cancer is already proven to go to breast cancer you are behind it has NOTHING to do with your thyroid hormones it is poor care, poor testing, and docs who have NEVER talked AND listened to thyroid patients and base treatment off the TSH which is a Pituitary hormone NOT a thyroid hormone a true Thyroid hormone is T4 T3 and reverse T3 please educate yourself before posting junk like this it just shows your ignorance to those of us that have Studied our thyroid condition since 99% of docs are severely under educated in this area( and most Endos and other doc’s are to arrogant to admit this) also try learning from a non pharma sponsored class you might just learn something

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