Association Between Benign Thyroid Disorders and Breast Cancer Risk in Korean Women.

Purpose: This study aimed to investigate the potential association between thyroid disorders and breast cancer (BC) risk in a cohort of Korean women.

Materials and methods: Data for this retrospective cohort study were obtained from the Korean National Health Insurance database, including all women aged ≥ 40 who underwent BC screening from 2009 to 2010 in Korea. Thyroid disorders were identified using medical records from 2009 to 2010 and extracted using the ICD-10 codes for thyroid nodules, hypothyroidism, and hyperthyroidism. BC cases were defined using the ICD-10 codes and tracked until December 2021. A Cox regression model was used to evaluate the association between thyroid disorders and the risk of BC. Additionally, we evaluated the association between well-known risk factors of BC and thyroid disorders using logistic regression analysis.

Results: Among 5,051,633 women, the mean (standard deviation) age was 55.2 (10.7) years, and the median follow-up was 11.6 years, with 87,784 BC cases recorded. The proportions of patients with thyroid nodules, hypothyroidism, and hyperthyroidism were 2.5, 1.8, and 0.9%, respectively. The hazard ratio (HR) for BC risk associated with thyroid nodules was 1.16 (95% CI 1.11-1.20), for hypothyroidism was 0.98 (95% CI 0.93-1.03), and for hyperthyroidism was 1.13 (95% CI 1.06-1.21). In both premenopausal and postmenopausal women, an increased risk of BC was significantly associated with thyroid nodules (aHR 1.16 and 1.13) and hyperthyroidism (aHR 1.11 and 1.16). History of benign breast disease, oral contraceptive use, breastfeeding, menopausal status, and hormone replacement therapy were associated with thyroid nodules and hyperthyroidism.

Conclusion: Our findings suggest an increased risk of BC in women with a history of thyroid nodules and hyperthyroidism, whereas no such association was found in women with hypothyroidism.