Gd-C-4-thyroxin-DTPA, a potential MRI contrast agent, was synthesized front Gd-DTPA and thyroxine, which interacts strongly with human serum albumin (HSA). It was characterized in water by its relaxometric properties and its stability Vel-SUS Zinc transmetalation. The affinity of the complex for HSA was Studied by using three different methods: proton relaxometry, NMR diffusometry, and electrospray mass spectrometry. From the results, it appears that Gd-C-4-thyroxin-DTPA exhibits a relatively high relaxivity (r(1) = 9.01 s(-1) mM(-1) at 1.5 T and 310 K), a good stability versus Zinc transmetalation, and a strong interaction with HSA (K-a similar to 10 000 W with two binding sites). The kinetics of the exchange between the bound and the free Form of the complex was evaluated by the NMR diffusometry technique. Competition experiments have allowed the assignment of the chelate's binding site on HSA.