The interaction between thyroxine hormone and 7 hydroxycoumarin (7HC) was investigated using fluorescence quenching method. The experimental results showed that thyroxine could quench the fluorescence of 7HC by forming the 7HC-thyroxine complex with static quenching. The apparent binding constants (K) between 7HC and thyroxine were determined to be 1.51 x 10(4) (297 K) and 9.06 x 10(3) (310 K). The binding sites (n) 0.98 +/- 0.1. The thermodynamic parameters showed that the interaction between 7HC and thyroxine was driven mainly by hydrogen bonding interactions and van der Waals force. Calibration for thyroxine, based on quenching titration data, was linear in the concentration range 2.0 x 10(-8) to 3.0 x 10(-7) mol/l. The relative standard deviation was 2.58% for 2.0 x 10(-7) mol/l thyroxine (n= 4) and the 3 sigma limit of detection was 3.42 x 10(-8) mol/l in cationic surfactant CTAB medium.