There is preclinical and recent epidemiological evidence that thyroid hormone supports breast cancer. These observations raise the issue of whether management of breast cancer in certain settings should include consideration of reducing the possible contribution of thyroid hormone to the advancement of the disease. In a preliminary experience, elimination of the clinical action of endogenous L-thyroxine (T4) in patients with advanced solid tumors, including breast cancer, has favorably affected the course of the cancer, particularly when coupled with administration of exogenous 3,5,3'-triiodo-L-thyronine (T3) (euthyroid hypothyroxinemia). We discuss in the current brief review the possible clinical settings in which to consider whether endogenous thyroid hormone-or exogenous thyroid hormone in the patient with hypothyroidism and coincident breast cancer-is significantly contributing to breast cancer outcome.