Breast Cancer Coverage from Every Angle

Relationship Between Thyroid Dysfunction and Risk of Breast and Thyroid Cancers

By: Sarah Campen, PharmD
Posted: Thursday, June 11, 2020

Hypothyroidism and increased thyroid-stimulating hormone (TSH) levels appear to be associated with a lower risk of developing breast cancer—mainly estrogen receptor–positive tumors—and thyroid cancer, according to the findings of a study published in the International Journal of Cancer. Conversely, hyperthyroidism and increased free thyroxine levels were found to be linked to a higher risk for breast cancer.

“Along with the benefits of thyroid dysfunction treatment on cancer survival and cardiovascular diseases, it is suggested that treatment of subclinical and diagnosed hyperthyroidism may be an efficient cancer prevention strategy,” stated Susanna C. Larsson, PhD, of the Karolinska Institutet, Stockholm, and colleagues.

This two‐sample Mendelian randomization study investigated the association between genetic predisposition to thyroid dysfunction and 22 site-specific cancers. The researchers selected single-nucleotide polymorphisms associated with four traits of thyroid function from a genome-wide association meta-analysis of 72,167 European-descent individuals. Next, the presence of these polymorphisms in cancer types was evaluated, including a large cohort of patients from the Breast Cancer Association Consortium (122,977 breast cancer cases and 105,974 controls) and the U.K. Biobank (367,643 individuals).

A causal inverse association was observed between TSH levels and breast cancer; as genetically predicted TSH levels increased, the risk for breast cancer decreased (odds ratio = 0.94, P = .007). Conversely, a genetic predisposition to hyperthyroidism and genetically predicted one standard deviation increase in free thyroxine levels significantly increased the risk for breast cancer. Thyroid cancer was also inversely related to genetically predicted TSH levels and hypothyroidism, with odds ratios of 0.47 (P = .001) and 0.70 (P = .038), respectively.

“The role of TSH and hypothyroidism for thyroid cancer and the associations between thyroid dysfunction and other cancers need further exploration,” concluded the authors.

Disclosure: For full disclosures of the study authors, visit

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