Possible contributions of thyroid hormone replacement to specific behaviors of cancer.

l-Thyroxine (T4) is the principal replacement hormone for patients who have hypothyroidism. Some preclinical and clinical evidence supports the possibility that T4 can at least permissively affect certain features of established cancers and cancer-relevant angiogenesis. Thus, in the occasional patient with hypothyroidism and concomitant cancer, it appears reasonable to consider thyroid hormone replacement exclusively with 3,3',5-triiodo-l-thyronine (T3). This use of T3 has been shown to be effective and safe in early experience with medical induction of euthyroid hypothyroxinemia in patients with advanced solid tumors.