Abstract: The synthesis and the physicochemical characterization of a new contrast agent for magnetic resonance imaging (MRI), Gd-C4-thyroxin-DTPA, which has a high affinity for human serum albumin (HSA), are reported. The results show that this chelate is characterized by a relatively high relaxivity, which increases moreover with the concentration. This reflects an aggregation of the molecules in solution. It is also characterized by a better stability versus the transmetallation with the zinc ion than the parent compound, the Gd-DTPA (Magnevist®, Bayer Healthcare). The study of its interaction with human serum albumin was performed by the proton relaxometry technique, which has revealed a relatively high affinity (Ka of the order of 10000 M-1, with 2 binding sites). Finally, competition experiments with ibuprofen and salicylate, of which the binding sites on HSA are known, were performed by the NMR diffusometry method. The results suggest that the chelate shares one of the binding site of ibuprofen.