The DRG T4 Total ELISA is an enzyme immunoassay for the quantitative in vitro diagnostic measurement of total Thyroxin (T4) in serum or plasma. Measurement of serum thyroxine concentration is generally regarded as an important in-vitro diagnostic test for assessing thyroid function. This importance has provided the impetus for the significant improvement in assay methodology that has occurred in the last three decades. This procedural evolution can be traced from the empirical protein bound iodine (PBI) test (1) to the theoretically sophisticated radioimmunoassay (2). NOT INTENDED FOR NEWBORN SCREENING. The DRG T4 Total ELISA Kit 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 an antigenic site of the T4 molecule. Endogenous T4 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 T4 in the sample. After addition of the substrate solution, the intensity of colour developed is inversely proportional to the concentration of T4 in the patient sample.