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 that control our metabolism. 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.
Over 4 million women (4,177,429) living in Denmark were analysed in the study, and those with hyperthyroidism 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.
Dr Mette Sogaard, lead author of the study published in the European Journal of Endocrinology, stated: “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."
Next steps for the research team are to investigate whether using thyroid hormones to treat hypothyroidism may also be associated with an incrased breast cancer risk.
"Our findings emphasise the importance of raising awareness of breast cancer in women with hyperthyroidism, and further our understanding of this potential risk," said Dr Sogaard.