4 RADIOISOTOPIC IMMUNOASSAYS OF FREE-THYROXINE COMPARED

The analytical and clinical performance of three radioisotopic methods for the measurement of free thyroxine-GammaCoat(TM) (I-125) Free T-4, RIA-gnost(R) FT4 and Amerlex-MAB FT4-were assessed in comparison wit h the Sclavo two-step chromatographic method. The analytical evaluatio n indicated excellent performance of Amerlex-MAB FT4. For the other me thods, precision and sensitivity were comparable and acceptable for ro utine use. Only GammaCoat FT4 showed a significant positive intra-assa y drift. None of the methods showed a significant correlation with thy roxine-binding globulin. Amerlex-MAB FT4 results were weakly positivel y correlated with albumin. Sclavo, RIA-gnost and particularly GammaCoa t FT4 results were affected by the presence of increased concentration s of free fatty acids. All methods classified hyperthyroid patients co rrectly. Slight overlaps existed between the hypothyroid and euthyroid populations. Significant decreases in free thyroxine (FT4) during the third trimester of pregnancy were detected by all assays. In patients with chronic renal failure, serum FT4 was within reference limits mor e often when measured by Sclavo or Amerlex-MAB methods than by RIA-gno st or GammaCoat techniques. In conclusion, all assays performed well f rom both technical and diagnostic points of view.