Meta-analyses show an increased risk of thyroid cancer following breast cancer (OR 1.55) and vice versa (OR 1.32).1Shared genetic mutations (PTEN, BRCA1/2) and hormonal influences (estrogen receptor expression) contribute to this link.15, 17Cancer treatments, including radiation therapy and radioactive iodine-131, may further impact risk. The gene discussed is BRCA1; the disease is breast cancer.