A source of error in the automated serum triiodothyronine-uptake test.

N AUTOMATED TECHNIQUE for the determination of serum T31 uptake, based on the AutoAnalyzer, was proposed by Pollard et al. (1); a report of the clinical application of this method followed (2) and minor technical improvements were reported by Webber et al. (3). The latter showed an excellent correlation between the T3 uptake as measured by the automated technique and serum PBI. Continued experience with the automated method in our department has not confirmed the initial promise. The diagnostic efficiency of the test was variable when comparisons were made with FBI, measurements of uptake of radioiodine in the neck, and clinical assessment (4). Extremes of thyroid status were well demonstrated by all these techniques, but in intermediate cases the automated T3 test frequently gave results within the normal range when the corresponding PBI and the results of measurements of uptake in the neck were clearly abnormal. These discrepancics remained after all the known causes of abnormal T3-uptake results-pregnancy, contraceptive pills, peculiarities of serum proteins-had been eliminated. We noted that these discrepancies were more frequently associated with PBI and neckuptake results in the hypothyroid range than with those in the thyrotoxic range.