An evaluation of a fluorescence polarization immunoassay of thyroxin and thyroxin-uptake.

We evaluated the fluorescence polarization immunoassay for total thyroxin (T4) and thyroxin-uptake (T-U) in the Abbott "TDx" Analyzer. Between-assay precision was good when we did once-fortnightly calibration and assayed samples in singleton. Measured T4 concentration was decreased in hemolyzed samples with obvious red coloration and undetectable in severely hemolyzed samples. The T-U assay was unaffected by hemolysis. Unlike the triiodothyronine-uptake methods, the T-U assay utilizes labeled T4 and measures a variable related to serum thyroxin-binding capacity rather than the concentration of unoccupied binding sites in serum. The T4 and T-U values of 422 samples correlated highly with a T4 (in-house) radioimmunoassay and a commercial assay for thyroxin-binding globulin, respectively. The free thyroxin ratio (ratio of T4 to T-U, FTI) correlated highly with free T4 concentration as measured by T4-analog-tracer radioimmunoassay (fT4 analog RIA). FTI and fT4 values were discordant in late pregnancy (normal FTI and low fT4) and euthyroid sick patients (above-normal or normal FTI and low fT4), suggesting that the FTI gives fewer misleading results in these patients.