Hypothyroidism May Lower Breast Cancer Risk, Especially After Menopause: Study

• The main conclusion of the study was that hypothyroidism was linked to a statistically significant decrease in the risk of breast cancer.

• That is, the hazard ratio for women with hypothyroidism was 0.85 (95% CI: 0.78–0.93), which means 15% lower risk compared to women without thyroid conditions.

• For postmenopausal women, the protective effect looked even more pronounced, with a hazard ratio of 0.78 (95% CI: 0.68–0.90).

• In contrast, no risk association was seen between hyperthyroidism and the risk of breast cancer.

• The hazard ratio in this cohort was 1.00 (95% CI: 0.89–1.11), indicating no increase or decrease in risk.

• These results were similar for different histological subtypes of breast cancer and were not importantly different by time since the first diagnosis of thyroid disease.

This large multicenter national study found that hypothyroidism is associated with a reduced risk of breast cancer, and the protective effect was most evident in postmenopausal women. Conversely, hyperthyroidism was not found to be associated with an altered risk of breast cancer. These results prompt further investigation into the biological rationale for this association and into whether thyroid function should be considered as an element of breast cancer prevention in the future.

Reference:

Jensen, A., Gottschau, M., Christensen, J., Lindquist, S., Aalborg, G. L., Rosthøj, S., Grand, M. K., Pedersen, J., Kjær, S. K., & Nøhr, B. (2025). Risk of breast cancer among women with hypo- and hyperthyroidism: Results from a large nationwide cohort study. International Journal of Cancer. Journal International Du Cancer. https://doi.org/10.1002/ijc.70007