Abstract
Thyroid hormone as L-thyroxine (T4) at physiological concentrations acts at its cell surface receptor on integrin avb3 to stimulate cancer cell proliferation1. These proliferation studies have been conducted in vitro, but pharmacological reduction of T4 and substitution of nuclear receptor ligand 3,3’,5-triiodo-L-thyronine (T3) is a state of euthyroid hypothyroxinemia that has been shown clinically to arrest tumor growth in patients with cancer. T3 is inactive at physiological levels at the plasma membrane integrin receptor. A preclinical study of human basal cell carcinoma (BCC) cells has shown that the integrin thyroid hormone receptor regulates BCC radiosensitivity. While the large majority of BCCs are very manageable clinically, a small number of such tumors are aggressive. In this review of documented and proposed effects of T4 on BCC cells, we raise the possibility that BCC aggressiveness reflects T4 actions on its thyrointegrin target. The functions affected by T4 at the integrin in other human cancers include enhanced cell proliferation, anti-apoptosis, immune checkpoint regulation and metastasis, as well as state of radiosensitivity. The importance of investigating this possible pathophysiology is that euthyroid hypothyroxinemia may be tested as a treatment option.