Hypothyroidism May Lower Breast Cancer Risk, Especially After Menopause: Study
Researchers have found that women diagnosed with hypothyroidism may have a lower risk of developing breast cancer, especially after menopause. This conclusion was drawn from a large Danish cohort study that followed over 1 million women for nearly two decades. The study published in the International Journal of Cancer was conducted by Allan J. and colleagues.
The cohort consisted of 1,058,939 Danish-born women from January 1, 1960, to December 31, 1997. Information was gathered from a variety of Danish national registers, such as evidence of thyroid disease and breast cancer diagnoses, demographic characteristics, migration, and death. Median follow-up was 18.8 years, offering a strong longitudinal setting in which to compare long-term risk of cancer.
Among the entire population, 49,015 women were diagnosed with hypothyroidism and 26,950 with hyperthyroidism within the duration of the study. A total of 15,703 individuals were diagnosed with breast cancer. Cox proportional hazard regression models estimated the adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) after adjusting for age, comorbidities, and menopausal status.
Key Findings
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
