Several clinical and experimental studies have implicated thyroid hormones in cancer progression. Cancer-relevant effects, including stimulation of tumor growth and new blood vessel formation by angiogenesis, are thought to be mediated by a nonclassical signaling pathway initiated through integrin αvβ3 expressed on cancer cells and proliferating endothelium. In an earlier study, we established mesenchymal stem cells (MSCs), important contributors to the fibrovascular network of tumors, as new thyroid hormone-dependent targets. Here, we evaluated the effects of the thyroid hormones triiodothyronine (T3) and thyroxine (T4) versus Tetrac, an integrin-specific inhibitor of thyroid hormone action, on MSCs in tumor angiogenesis. Modulation of the expression and secretion of angiogenesis-relevant factors by thyroid hormones in primary human MSCs and their effect on endothelial cell tube formation were tested . We further engineered MSCs to express the sodium iodide symporter () reporter gene under control of a hypoxia-responsive promoter and the vascular endothelial growth factor (VEGF) promoter to test effects on these pathways and, for VEGF, in an orthotopic hepatocellular carcinoma (HCC) xenograft mouse model by positron emission tomography imaging. T3 and T4 increased the expression of pro-angiogenic genes in MSCs and NIS-mediated radioiodide uptake in both NIS reporter MSC lines in the presence of HCC cell-conditioned medium. Supernatant from thyroid hormone-treated MSCs significantly enhanced endothelial cell tube formation. Tetrac and/or inhibitors of signaling pathways downstream of the integrin reversed all these effects. Tumoral radioiodide uptake demonstrated successful recruitment of MSCs to tumors and VEGF promoter-driven NIS expression. Hyperthyroid mice showed an increased radioiodide uptake compared with euthyroid mice, while tracer uptake was markedly reduced in hypothyroid and Tetrac-treated mice. Our data suggest that thyroid hormones influence angiogenic signaling in MSCs via integrin αvβ3 and further substantiate the anti-angiogenic activity of Tetrac in the tumor microenvironment.