**Abstract**

Primary hypothyroidism is a common endocrine disorder that is effectively treated with l-thyroxine (T4) replacement. Preclinical and limited clinical evidence, however, indicates that T4 is a growth factor for a variety of cancers, acting at the thyroid hormone receptor on plasma membrane integrin αvβ3. T4 is the primary ligand for this receptor, whereas 3,5,3′-triiodo-l-thyronine (T3) is the principal intracellular thyroid hormone analogue. The evidence is reviewed here that T4 is a proliferative for breast, lung, kidney and prostate cancers and for glioblastoma, regulates cancer cell respiration and is a pro-angiogenic factor in established tumors. The recommendation is made that T3 be considered alternative replacement treatment for patients with primary hypothyroidism who also have cancer.

**Keywords:** thyroid hormone receptors, integrin αvβ3, breast cancer, lung cancer, glioblastoma, renal cell carcinoma
