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Breast cancer risk higher in women with overactive thyroid

10 February 2016

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A 36-year long study has linked an overactive thyroid with an increased risk of breast cancer in women.

The thyroid is an important gland that releases hormones which control our metabolism; up to 6.7% of Europe’s population have undiagnosed thyroid disease. Hyperthyroidism, in which the gland makes too much thyroid hormone, occurs in 51 per 100 000 people per year, and is six times more common in women than in men.

More than four million women (4,177,429) living in Denmark were analysed, and those with hyperthyroidism (too much thyroid hormone) were 11% more likely to develop breast cancer, whereas those with hypothyroidism (too little thyroid hormone) were 6% less likely to develop the condition, compared to women in the general population.

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“High levels of thyroid hormone levels can have oestrogen-like effects, which may explain why hyperthyroidism is associated with higher risk of breast cancer. In vitro experiments show that sex hormones such as oestrogen play an important role in the proliferation of breast cancer cells”, said Dr Mette Sogaard, lead author of the study, published in the European Journal of Endocrinology today.

The research team’s next step is to investigate whether using thyroid hormones to treat hypothyroidism may also be associated with an increased breast cancer risk.

“Our findings emphasize the importance of raising awareness of breast cancer in women with hyperthyroidism, and further our understanding of this potential risk,” said Dr Sogaard.