Nonthyroidal Illness Syndrome (Low T3 syndrome) and Thyroid Hormone Actions at Integrin αvβ3(review) Aleck Hercbergs Shaker A Mousa Paul J Davis The Journal of Clinical Endocrinology & Metabolism, jc.2017-01939, https://doi.org/10.1210/jc.2017-01939 Published: 01 February 2018 Abstract Context The nonthyroidal illness syndrome (NTIS) is a constellation of changes in circulating thyroid hormone levels that occur in euthyroid patients with acute or chronic systemic diseases. The changes that occur include reduction in serum T3, increase in serum rT3 and variable changes in circulating T4 levels. No consensus yet exists on therapeutic intervention on NTIS. Methods We briefly review the published literature on the physiological actions of T4 and of rT3—hormones that until recently have been seen to have little or no bioactivity—and analyze specifically the apparent significance of changes in circulating T4 and T3 encountered in the setting of NTIS in cancer patients. In the case of T4, these actions may be initiated at a cancer or endothelial cell plasma membrane receptor on integrin αvβ3 or at the cytoskeleton. Results The possible utility emerges in this review of therapeutic intervention in NTIS in cancer patients in terms of T4 reduction and of T3 support. Evidence also exists that rT3 may be a cancer support hormone. Conclusions Prospective study is proposed of pharmacological reduction of normal or elevated T4 in cancer-associated NTIS and of support to normal of circulating levels of T3 in such patients.