Past inquiries have suggested a possible connection between breast cancer and thyroid cancer. A new study of more than 11,000 women shows the two cancers do indeed tend to occur in the same women more frequently than would be expected by chance.

Reporting their results in the December 2015 edition of the journal Thyroid, Jee Hyun An, MD, PhD, and colleagues at Seoul National University College of Medicine in Korea report that they effectively confirmed the link after performing a retrospective, case-controlled study of 4,243 patients with differentiated thyroid cancer and 6,833 patients with breast cancer.

Comparing these with age-matched control groups without second malignancies, the researchers found that, within the prior thyroid-cancer cohort, 55 patients developed subsequent breast cancer during a five-year follow-up.

Similarly, among the 6,833 patients with prior breast cancer, 81 patients developed subsequent thyroid cancer during a 6.2-year follow-up.

Those ratios represent a 4.3 percent incidence of subsequent breast cancer in women with prior thyroid cancer and a 2.6 percent incidence of subsequent thyroid cancer in women with prior breast cancer.

Both incidences are greater than double the expected rates for women in the general population, the authors point out.

The team also recorded that the expression of both estrogen and progesterone receptors was significantly higher in breast cancer patients with coexisting thyroid cancer compared with those with only breast cancer—a finding that strongly suggests an association between the respective molecular pathogeneses of thyroid cancer and breast cancer.

The authors acknowledge that early detection of the second cancer may have contributed to the finding of this latter link.

Still, they conclude, “the overall risk of second primary thyroid cancer or breast cancer is increased in patients with prior breast cancer or thyroid cancer, respectively.”