Thyroid Disease and Breast Cancer Risk in the Sister Study

Area of Science:

• Endocrinology
• Oncology
• Epidemiology

Background:

• Thyroid disease (TD) and its treatments are potential risk factors for breast cancer (BC) due to hormonal metabolism changes.
• Previous studies on TD and BC risk have yielded inconsistent results, often due to small sample sizes.

Purpose of the Study:

• To investigate the association between hyperthyroidism and hypothyroidism and the risk of developing breast cancer in a large cohort of US women.
• To assess the impact of thyroid disease treatments on breast cancer incidence.

Main Methods:

• Utilized data from the prospective Sister Study cohort, including 50,884 US women without a history of BC at enrollment (2003-2009).
• Employed time-varying Cox proportional hazards regression analysis to evaluate the relationship between self-reported thyroid diseases, treatments, and incident BC over follow-up through September 2021.

Main Results:

• Among 45,783 eligible participants, 11,567 reported TD. Hyperthyroidism was associated with a non-significant increased hazard of BC (HR=1.15), while hypothyroidism showed a non-significant decreased hazard (HR=0.93).
• Hyperthyroidism showed a significantly higher hazard for premenopausal BC (HR=1.85).
• Treatments for hyperthyroidism were linked to higher, though not statistically significant, BC hazards, whereas hypothyroidism treatments did not show this association.

Conclusions:

• Women diagnosed with hyperthyroidism or undergoing related treatments may have an elevated risk of breast cancer, particularly premenopausal breast cancer.
• Premenopausal women treated for hyperthyroidism might benefit from enhanced breast cancer screening protocols.