1. Hyperthyroidism was associated with a higher rate of breast cancer among women.
2. Risk was particularly elevated for those with toxic nodular goiter.
Evidence Rating Level: 2 (Good)
Breast cancer is the most common cancer among women across the world as well as the leading cause of death among women. While much is known about the roles of thyroid hormones in cell proliferation within breast tissue, less is known about the relationship between hyperthyroidism and mammographic features of breast tissue. This national cohort study of women over the age of 20 years in Sweden (n = 3,793,492) included individuals assessed between 2002 and 2011 and sought to investigate the odds ratios of hyperthyroidism based on mammographic and genetic risk predictors. Participants had a main diagnosis of hyperthyroidism and follow-ups ended at breast cancer diagnosis, death, emigration, or final follow-up. Another 68,598 participants joined the Karolinska Mammography Project for Risk Prediction of Breast Cancer (KARMA, 2002-2017) for genotyping, with blood samples being obtained from a subset of 11,991 women who did not have breast cancer when they joined. Findings suggested an increase in breast cancer among patients with hyperthyroidism (incidence rate ratio [IRR] = 1.23, 95% CI 1.12 to 1.36), which was higher for toxic nodular goiter (IRR = 1.38, 95% CI 1.16 to 1.63). Hyperthyroidism was also associated with lower breastfeeding duration, higher body mass index, and early age at first birth. Higher mammographic density was found in women with toxic nodular goiter compared to those without hyperthyroidism. Among those in the KARMA group, hyperthyroidism was associated with a higher polygenic risk score overall (OR = 1.98, 95% CI 1.04 to 3.43) and estrogen receptor-positive specific PRS (OR = 1.90, 95% CI 1.04 to 3.43). Overall, this study found that hyperthyroidism was associated with an increased risk of breast cancer, with particular risk being among those with toxic nodular goiter. Further studies may explore mammographic density and genetic variants of these conditions.
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