Endogenous Serum Antibodies that Interfere with a Common Thyroid Hormone Uptake Assay: Characterization and Prevalence

We identified individuals whose serum contained a sub-stance that produced falsely decreased thyroid hormone (T)-uptake values determined by the Emit ” (Syva) pro-cedure. Investigationof this interference was prompted by identification of a patient with T-uptake values inconsis-tent with clinical assessment. lgG depletion and supple-mentation studies with this patient’s serum suggested that the interference was due to endogenous antibodies with specificityfor the thyroxine-glucose-6-phosphate dehy-drogenase conjugate in the Emit T-uptake assay. The prevalence of the interference was examined by prospec-tively comparing routine Emit 1-uptake values of 1710 patients ’ samples to 1-uptake values obtained by another method. Discrepant samples were also assayed by a radioactive binding triiodothyronine-uptake assay. W