Women with an overactive thyroid have an increased risk of breast cancer, a new study suggests.
Hyperthyroidism is a relatively common condition which occurs when there is too much thyroid hormone in the body.
Women with an underactive thyroid gland, also known as hypothyroidism, carry a lower risk than the general population, the study concluded.
Experts in Denmark used nationwide registries to identify women with over and under-active thyroids between 1978 and 2013.
They then estimated their risk of breast cancer compared with the expected risk in the general population.
Of the data pertaining to more than four million women, they identified 61,873 women diagnosed with hypothyroidism and 80,343 women diagnosed with hyperthyroidism.
Writing in the European Journal of Endocrinology, the experts said that hyperthyroid women were 11 per cent more likely to develop breast cancer, whereas those with hypothyroidism were 6 per cent less likely to develop the condition.
"Our findings emphasise the importance of raising awareness of breast cancer in women with hyperthyroidism, and further our understanding of this potential risk," said lead author Dr Mette Sogaard from Aarhus University Hospital in Denmark.
"In vitro experiments show that sex hormones such as oestrogen play an important role in the proliferation of breast-cancer cells.
"High levels of thyroid hormone levels can have oestrogen-like effects, which may explain why hyperthyroidism is associated with higher risk of breast cancer."
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