Thyroid Disease and Breast Cancer Risk in the Sister Study

Thyroid disease and its treatments are suspected risk factors for breast cancer, attributed primarily to alterations in thyroid hormone-related metabolism. Prior literature is inconsistent, in part, due to small sample sizes. We, therefore, assessed the effects of hyper- and hypothyroidism on incident breast cancer in a large cohort of US women.

Data are from the Sister Study, a prospective cohort of 50,884 US women with no breast cancer history at enrollment (2003-2009; ages 35-74 years) and follow-up through September 2021. We used time-varying Cox proportional hazards regression to estimate hazard ratios (HR) and 95% confidence intervals (CI) for the association between self-reported thyroid diseases and treatments and incident breast cancer.

Of 45,783 eligible participants, 11,567 reported thyroid disease at baseline and over follow-up (hyperthyroidism: n = 1,710; hypothyroidism: n = 9,857), and 3,897 participants subsequently developed breast cancer. Breast cancer HR was 1.15 (95% CI, 0.95-1.39) for those with hyperthyroidism and 0.93 (95% CI, 0.85-1.01) for those with hypothyroidism, compared with no thyroid disease, with similar patterns observed across racial and ethnic groups. Hyperthyroidism was associated with greater premenopausal breast cancer hazard (HR = 1.85; 95% CI, 1.11-3.07) and differed from breast cancer hazard for hypothyroidism, which seemed reduced (HR = 0.75; 95% CI, 0.54-1.05) but did not reach statistical significance (Pheterogeneity = 0.06). Breast cancer hazards were higher, albeit not statistically significant, for those treated for hyperthyroidism but not hypothyroidism.

Women diagnosed with hyperthyroidism or receiving related treatment may have an elevated breast cancer risk, particularly for premenopausal breast cancer.

Although more research is needed, premenopausal women treated for hyperthyroidism may benefit from enhanced breast cancer screening.