T4 is a useful marker for the diagnosis of hypothyroidism and hyperthyroidism. The level of T4 is decreased in hypothyroid patients and is increased in hyperthyroid patients. BioVision’s human thyroxine (T4) kit is a solid phase competitive ELISA Kit. The samples, assay buffer and T4 enzyme conjugate are added to the wells coated with anti-T4 monoclonal antibody. T4 in the sample competes with a T4 enzyme (HRP) conjugate for binding sites. Unbound T4 and T4 enzyme conjugate are washed off by wash buffer. Upon the addition of the substrate, the intensity of color is inversely proportional to the concentration of T4 in the samples. A standard curve is prepared relating color intensity to the concentration of the T4.