Women with an overactive thyroid may have an increased risk of developing breast cancer, a new study has found.

The thyroid is a key gland in the body, releasing hormones that are essential for metabolism. According to Danish researchers, up to 6% of people in Europe have undiagnosed thyroid disease.

Hyperthyroidism refers to a condition in which the thyroid makes too much thyroid hormone. It is six times more common in women than men.

The researchers set out to see if this had any effect on the risk of breast cancer. They followed over 4.1 million women living in Denmark between 1978 and 2013.

Over 80,000 of the women were diagnosed with hyperthyroidism, while more than 61,000 were diagnosed with hypothyroidism (too little thyroid hormone).

The study found that women with hyperthyroidism were 11% more likely to develop breast cancer, while those with hypothyroidism were 6% less likely to develop it.

"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 a higher risk of breast cancer," suggested the study's lead author, Dr Mette Søgaard, of Aarhus University Hospital.

The researchers now want to assess whether treating hypothyroidism with thyroid hormones also increases the risk of breast cancer.

"Our findings emphasise the importance of raising awareness of breast cancer in women with hyperthyroidism, and further our understanding of this potential risk," Dr Søgaard added.

Details of these findings are published in the European Journal of Endocrinology.