Background: Breast cancer and thyroid cancer are two prevalent malignancies in women, and a potential association between the two diseases has been suggested.
Methods: This retrospective case-control study was conducted involving 97 patients with breast cancer and thyroid cancer (BC-TC group) and 97 age-matched patients with breast cancer alone (BC group). Thyroid hormone levels, including triiodothyronine (T3), thyroxine (T4), free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone (TSH), were analyzed in healthy controls, BC patients, and BC-TC patients.
Results: BC-TC patients exhibited a higher rate of estrogen receptor (ER) and progesterone receptor (PR) positivity compared to BC patients. Serum T3 levels were significantly decreased in BC and BC-TC patients compared to healthy controls. However, there was no significant difference in T3 levels between BC and BC-TC patients. Serum TSH levels were significantly higher in BC-TC patients compared to BC patients.
Conclusion: ER positivity, PR positivity, and serum TSH levels greater than 4.45 mU/L were independent risk factors for primary thyroid cancer in breast cancer patients.

Keywords: breast cancer, thyroid cancer, estrogen receptor, ER