(HealthDay News) — Women with hyperthyroidism have increased breast cancer risk, while hypothyroidism is associated with decreased risk, according to a study published in the April 1 issue of the European Journal of Endocrinology.

Mette Søgaard, Ph.D., 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 of breast cancer was compared for patients with hypothyroidism or hyperthyroidism versus 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 of breast cancer (standardized incidence ratio [SIR], 1.11; 95 percent confidence interval [CI], 1.07 to 1.16), which persisted beyond five years of follow-up (SIR, 1.13; 95 percent CI, 1.08 to 1.19). Hypothyroidism correlated with a lower breast cancer risk (SIR, 0.94; 95 percent CI, 0.88 to 1.00).

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“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 authors write.

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

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