A cell surface receptor for thyroid hormone has recently been identified on the extracellular domain of integrin αvβ3. In a variety of human and animal cell lines this hormone receptor mediates activation by thyroid hormone of the cellular mitogen-activated protein kinase (MAPK) signal transduction cascade. An arginine-glycine-aspartate (RGD) recognition site on the heterodimeric integrin is essential to the binding of a variety of extracellular matrix proteins. Recent competition data reveal that RGD peptides block hormone-binding by the integrin and consequent MAPK activation, suggesting that the hormone interaction site is located at or near the RGD recognition site on integrin αvβ3. A deaminated thyroid hormone (l-thyroxine, T4) analogue, tetraiodothyroacetic acid (tetrac, T4ac), inhibits binding of T4 and 3,5,3′-triiodo-l-thyronine (T3) to αvβ3, but does not activate MAPK. Structural data show that the RGD cyclic peptide binds at the interface of the propeller of the αv and the B domains on the integrin head [Xiong JP, Stehle T, Zhang R, Joachimiack A, Frech M, Goodman SL, et al. Crystal structure of the extracellular segment of integrin αvβ3 in complexing with an Arg-Gly-Asp ligand. Science 2002;296:151–5]. To model potential interactions of thyroid hormone analogues with integrin, we mapped T4 and T4ac to the binding site of the RGD peptide. Modeling studies indicate that there is sufficient space in the cavity for the thyroid hormone to bind. Since the hormone is smaller in overall length than the RGD peptide, the hormone does not interact with the Arg recognition site in the propeller domain from αv. In this model, most of the hormone interactions are with βA domain of the integrin. Mutagenic studies can be carried out to validate the role of these residues in directing hormone interactions.