Anomalous serum thyroxin measurements with the Abbott TDx procedure.

We describe a patient whose serum gave consistently low results for thyroxin as measured with the Abbott TDx but normal results by radioimmunoassay. Further clinical and laboratory studies did not support the low TDx results. Anti-thyroxin antibodies did not explain the discordant results, and we found no evidence of heterophilic antibodies in the patient's serum. We were unable to identify the cause of the problem.