(HealthDay News) — Women with hyperthyroidism have increased breast cancer risk, while hypothyroidism is associated with decreased risk, according to a study published in the European Journal of Endocrinology.

Mette Søgaard, PhD, from Aarhus University Hospital in Denmark, and colleagues examined the correlation between thyroid disease and breast cancer in a population-based cohort study. All women in Denmark with a first-time hospital diagnosis of hypothyroidism or hyperthyroidism in 1978 to 2013 were identified.

The excess risk for breast cancer was compared for patients with hypothyroidism or hyperthyroidism vs the expected risk in the general population. To avoid diagnostic work-up bias, breast cancer diagnoses in the first 12 months following thyroid disease diagnosis were excluded.

The researchers note that 61 873 and 80 343 women were diagnosed with hypothyroidism and hyperthyroidism, respectively. Compared with the general population, hyperthyroidism correlated with a slightly increased risk for breast cancer (standardized incidence ratio [SIR], 1.11; 95% CI, 1.07-1.16), which persisted after 5 years of follow-up (SIR, 1.13; 95% CI, 1.08-1.19). Hypothyroidism correlated with a lower breast cancer risk (SIR, 0.94; 95% CI, 0.88-1.00).

“We found an increased risk of breast cancer in women with hyperthyroidism and a slightly decreased risk in women with hypothyroidism indicating an association between thyroid function level and breast cancer risk,” the researchers wrote.

The study was partially funded by the Program for Clinical Research Infrastructure, established by the Lundbeck and Novo Nordisk Foundations.

Reference

1. Søgaard M, Farkas DK, Ehrenstein V, Jørgensen JOL, Dekkers OM, Sørensen HT. Hypothyroidism and hyperthyroidism and breast cancer risk: a nationwide cohort study. Eur J Endocrinol. 2016;174:409-414. doi:10.1530/EJE-15-0989.