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.