Breast cancer risk higher in women with overactive thyroid

Feb 2016

High levels of thyroid hormone are associated with an increased risk of breast cancer in women, according to a 36 yearlong study of more than four million women published today in the European Journal of Endocrinology.

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.

In this study, researchers from Aarhus University Hospital followed 4,177,429 women living in Denmark between 1978 and 2013, accounting for all the women registered and using Denmark’s health service over the entire 36 year period. Only women with a first-time diagnosis of thyroid disease and no history of cancer were included.

They found that 80,343 women were diagnosed with hyperthyroidism (too much thyroid hormone) and 61,873 with hypothyroidism (too little thyroid hormone). Hyperthyroid women were 11% more likely to develop breast cancer, whereas those with hypothyroidism were 6% less likely to develop the condition, compared to women in the general population.

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

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 Søgaard.



Notes for editors:



  1. For further information about the study please contact the authors.


  1. The study Hypothyroidism and hyperthyroidism and breast cancer risk: a nationwide cohort study will be published in the European Journal of Endocrinology the day the embargo lifts. For copies of the study, please contact the press office using the details below.


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  1. The European Journal of Endocrinology is published by Bioscientifica, an innovative and agile publisher. Bioscientifica collaborates with learned societies worldwide to develop new and existingquality products that meet the ever-changing needs of the biomedical community. Our publishing portfolio includes journals and online resources, including Journal of Endocrinology, Endocrine Related Cancer, Endocrine Connections, Bone Abstracts and our latest new product, Endocrinology, Diabetes & Metabolism Case Reports. Bioscientifica is a wholly-owned commercial subsidiary of the Society for Endocrinology.