Background: Tetraiodothyroacetic acid (tetrac) is a deaminated analogue of L-thyroxine that blocks the actions of L-thyroxine and 3,5,3'-triiodo-L-thyronine at the cell surface receptor for thyroid hormone on integrin alpha v beta 3. Tetrac blocks the proliferative effects of thyroid hormone on tumor cells and the proangiogenesis actions of the hormone. In the absence of thyroid hormone, tetrac also blocks angiogenesis induced by various growth factors. Covalently linked to poly(lactide-co-glycolide), tetrac nanoparticles (tetrac NP) do not gain access to the cell interior and act exclusively at the integrin receptor. Here, the activity of tetrac and tetrac NP against follicular thyroid carcinoma (FTC)-236 cells was studied in two models: (1) tumor cell implants in the chick chorioallantoic membrane (CAM) system and (2) xenografts in the nude mouse.