Several recent studies found a link between an overactive thyroid, or hyperthyroidism, and a higher risk of breast cancer among women. The new findings prompt doctors to suggest that women with this condition seek treatment quickly, even if they are wary about the side effects of slowing down their metabolisms, Endocrine Web reports.

Hyperthyroidism occurs when the thyroid, a butterfly-shaped gland in the front of the neck, produces too much thyroid hormone. These hormones regulate metabolism, weight, breathing, heart rate and the nervous system’s control of body movements, among other functions. Symptoms of the condition include weight loss, excessive sweating, sleep problems, heart palpitations and anxiety. 

For the latest study, which was presented at the American Thyroid Association’s recent annual meeting, researchers evaluated more than 51,000 women in Taiwan’s National Health Insurance Research database who received a breast cancer diagnosis between 2006 and 2011. After researchers compared them with another 51,000 women without breast cancer, they found that hyperthyroidism was associated with a 9 percent increased risk of the disease. (Cancer risk was not affected by an underactive thyroid, or hypothyroidism.) 

In an earlier Dutch study, reported in the Journal of Clinical Endocrinology & Metabolism, researchers studied more than 10,000 women and found that higher levels of one thyroid hormone, T4, was also linked to a higher risk of breast cancer. Additionally, findings from a third study conducted in Denmark that involved more than 80,000 women with hyperthyroidism similarly showed that women with an overactive thyroid had an 11 percent increased risk of breast cancer.

“Based on our preliminary finding, patients with hyperthyroidism should be more aware of the increased risk of breast cancer and have regular follow up with their primary care physicians [or ob/gyn] for breast cancer screening,” said Chien-Hsiang Weng, MD, MHP, a resident physician at Concord Hospital in New Hampshire, who presented the results of the first study. 

Researchers said that although these findings are preliminary and it’s far too soon to suggest a change in cancer screening guidelines, women with hyperthyroidism should at least be aware of the possible connection of the illness to breast cancer.

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