ENDOGENOUS SERUM ANTIBODIES THAT INTERFERE WITH A COMMON THYROID-HORMONE UPTAKE ASSAY - CHARACTERIZATION AND PREVALENCE

We identified individuals whose serum contained a substance that produ ced falsely decreased thyroid hormone m-uptake values determined by th e Emit(TM) (Syva) procedure. Investigation of this interference was pr ompted by identification of a patient with T-uptake values inconsisten t with clinical assessment. IgG depletion and supplementation studies with this patient's serum suggested that the interference was due to e ndogenous antibodies with specificity for the thyroxine-glucose-6-phos phate dehydrogenase conjugate in the Emit T-uptake assay. The prevalen ce of the interference was examined by prospectively comparing routine Emit T-uptake values of 1710 patients' samples to T-uptake values obt ained by another method. Discrepant samples were also assayed by a rad ioactive binding triiodothyronine-uptake assay. We identified eight sa mples that had falsely decreased T-uptake values by Emit, for an overa ll prevalence of 0.46%. Among 45 consecutive patients with a T-uptake value <20%, five patients, or 11%, were falsely decreased by Emit and three of these were clearly due to an interfering IgG. We suggest that samples with abnormally low T-uptake values determined by the Emit me thod be confirmed by an alternative method.