Use of thyroid supplements in relation to the risk of breast cancer

We conducted a case-control study to determine whether the use of thyroid supplements increases the risk of breast cancer. We compared 659 women with breast cancer and 1,719 control subjects. The rates of use of thyroid supplements were 9.1% and 8.7%, respectively (age-standardized rate ratio, 1.0; 95% confidence interval, 0.7 to 1.3). There was no evidence of an association when women known to be at increased risk for breast cancer (eg, nulliparae) were examined, even when thyroid supplements were taken for more than 15 years. Overall, there were no grounds to suggest that the long-term use of thyroid supplements increases the risk of breast cancer.