Benign breast disease and risk of thyroid cancer.

BACKGROUND: It has been suggested that breast and thyroid diseases may be linked. The aim of this study was to investigate the association between benign breast disease and subsequent risk of thyroid cancer. METHODS: Postmenopausal women (n = 133,875) aged 50-79 years were followed up for a mean of 14 years. Benign breast disease was defined by history of biopsy. Incident thyroid cancer cases were confirmed by medical record review. Multivariable Cox proportional hazard modeling was used to estimate hazard ratios. RESULTS: There were 370 incident thyroid cancer cases during the follow-up period. Compared to women without BBD, women with BBD had a significant increased risk of thyroid cancer after adjusting for potential confounders (HR 1.38 95% CI 1.10-1.73), especially for women with more than two biopsies (HR 1.59 95% CI 1.10-2.26). There were no significant differences in thyroid tumor size, stage or histologic types between women with and without BBD. CONCLUSION: Our large prospective study observed that postmenopausal women with BBD had an increased risk for thyroid cancer compared with women without BBD. A more detailed investigation of thyroid cancer risk according to different subtypes of benign breast disease is needed to better understand the association observed between thyroid and benign breast diseases.