[False results on elevated triiodothyronine values in radioimmunoassay]

Five patients with falsely elevated serum triiodothyronine (T3) concentrations (> 9 nmol/l) in a radioimmunoassay are reported. The high T3-values disagreed with the other thyroid variables investigated as well as with the clinical observations. In sera from all patients a normal non-specific binding of T3 was found, thus excluding abnormal serum-protein-binding of the hormone. An ethanol extraction of T3 from serum before RIA reduced the T3 content in serum from all patients to normal levels (2.0-2.4 nmol/l). These findings indicate the presence in the sera of substances, probably of protein nature, that were interfering with the assay by binding the reagent-antibody and not the antigen. Addition of non-immune rabbit serum prevented this interference and normalized the T3-values (1.8-2.4 nmol/l). Thus the interfering substance in T3-RIA could be an anti-rabbit antibody, the interaction of which can be eliminated by a minor modification of the assay making it possible to differentiate true from false T3-values.