An immunochemiluminometric assay for serum free thyroxine

An immunochemiluminometric assay has been developed for the measurement of free T4 concentrations in serum. The assay uses chemiluminescent acridinium ester labelled monoclonal antibodies which react with free T4 in the sample. A T4-rabbit immunoglobulin G conjugate competes for antibody binding sites, immune-complexes containing this being isolated using an anti-immunoglobulin G antibody coupled to paramagnetic particles. Associated chemiluminescence intensity is thus dependent on the free T4 concentration. The assay distinguishes patients with primary thyroid disease from euthyroid subjects and is unaffected by abnormal binding proteins which compromise the diagnostic accuracy of radiolabelled analogue immunoassays. the test yields results which accurately reflect the clinical thyroid status of euthyroid patients with a variety of acute and chronic non-thyroid illnesses. This is again in marked contrast to the aberrant results seen using certain radiolabelled analogue procedures.