Does Hyperthyroidism Increase Risk of Breast Cancer?
Yes, hyperthyroidism is associated with a modestly increased risk of breast cancer, particularly in women under age 55 and those with toxic nodular goiter, though the absolute risk increase is small and does not warrant enhanced screening beyond standard age-appropriate mammography.
Evidence for Increased Risk
The most recent and highest quality evidence demonstrates a clear but modest association:
A 2020 Swedish national cohort study of 3.8 million women found a 23% increased risk of breast cancer in women with hyperthyroidism (IRR = 1.23,95% CI 1.12-1.36), with the highest risk observed in those with toxic nodular goiter (IRR = 1.38,95% CI 1.16-1.63). 1
This same study revealed that hyperthyroidism was associated with higher mammographic density and a high polygenic risk score for breast cancer (OR = 1.98,95% CI 1.09-3.60), suggesting shared genetic and hormonal pathways between the two conditions. 1
A 2018 Taiwanese population-based case-control study of over 100,000 women confirmed significantly increased breast cancer risk specifically in women with hyperthyroidism under age 55 (age <45: OR 1.16; age 45-55: OR 1.15), with no protective effect from thyroid treatment. 2
A 2018 French systematic review of 22 studies (including 8 cohort studies and 2 meta-analyses) found that 15 of 22 studies established a significant link between hyperthyroidism and breast cancer, with particular risk within three years of thyroid disease diagnosis. 3
Age-Specific Risk Patterns
The risk appears concentrated in specific age groups:
Premenopausal and perimenopausal women (under age 55) with hyperthyroidism face the highest relative risk, while postmenopausal women show less consistent associations. 2
The increased risk is most pronounced in the first 3 years following hyperthyroidism diagnosis, suggesting active thyroid hormone excess drives the association. 3
Clinical Screening Recommendations
Despite the demonstrated association, current guidelines do not recommend enhanced breast cancer screening for women with hyperthyroidism:
The National Comprehensive Cancer Network (NCCN) recommends standard age-appropriate breast cancer screening beginning at age 40 with annual mammography for women with thyroid disorders, as hyperthyroidism does not meet criteria for high-risk screening protocols. 4
The American College of Radiology confirms that enhanced surveillance with breast MRI is not indicated based solely on hyperthyroidism diagnosis. 4
Hyperthyroidism should not be incorporated into formal breast cancer risk assessment models, which should focus on established high-risk factors including BRCA1/2 mutations, PTEN mutations, prior thoracic radiation, and atypical hyperplasia. 4
Practical Management Approach
For women with hyperthyroidism:
Ensure annual clinical breast examination and standard mammographic screening every 1-2 years starting at age 40 (or age 50 per some guidelines). 3
Avoid over-screening based solely on hyperthyroidism diagnosis, as the relative risk increase (RR <2) does not justify intensive surveillance. 4, 3
Consider that the association may be stronger in women with additional risk factors such as obesity, early menarche, or family history. 3, 1
Do not confuse hyperthyroidism with PTEN mutations (Cowden syndrome), which carries dramatically higher breast cancer risk (77-85% lifetime risk) and requires intensive screening starting at age 30-35. 5, 4
Important Caveats
The absolute risk increase from hyperthyroidism is modest (approximately 20-40% relative increase translating to small absolute risk changes), and the biological mechanisms remain incompletely understood. 1 The association appears mediated through shared hormonal pathways, higher mammographic density, and possibly pleiotropic genetic variants affecting both conditions. 1
Treatment of hyperthyroidism does not appear to eliminate the breast cancer risk, suggesting the association may reflect underlying biological susceptibility rather than simply thyroid hormone excess. 2