The DRG Free T4 ELISA is an enzyme immunoassay for the quantitative measurement of free Thyroxine (fT4) in serum or plasma (EDTA-, heparin- or citrate plasma) The DRG Free T4 ELISA is a solid phase enzyme-linked immunosorbent assay (ELISA) based on the principle of competitive binding. The microtiter wells are coated with a monoclonal (mouse) antibody directed towards a unique antigenic site of the Thyroxine (T4) molecule. Endogenous fT4 of a patient sample competes with a T4-horseradish peroxidase conjugate for binding to the coated antibody. After incubation the unbound conjugate is washed off. The amount of bound peroxidase conjugate is inversely proportional to the concentration of fT4 in the sample. Having added the substrate solution, the intensity of colour developed is inversely proportional to the concentration of fT4 in the patient sample.