SUMMARY. The analytical and clinical performance of three radioisotopic methods for the measurement of free thyroxine-c-GammeCoat" (1 25 1) Free T 4 , RIA-gnost® FT 4 and Amerlex-MAB* FT 4-were assessed in comparison with the Sclavo twostep chromatographic method. The analytical evaluation indicated excellent performance of Amerlex-MAB FT 4 . For the other methods, precision and sensitivity were comparable and acceptable for routine use. Only GammaCoat FT 4 showed a significant positive intra-assay drift. None of the methods showed a significant correlation with thyroxine-binding globulin. Amerlex-MAB FT 4 results were weakly positively correlated with albumin. Sclavo, RIA-gnost and particularly GammaCoat FT 4 results were affected by the presence of increased concentrations of free fatty acids. All methods classified hyperthyroid patients correctly. Slight overlaps existed between the hypothyroid and euthyroid populations. Significant decreases in free thyroxine (FT 4 ) during the third trimester of pregnancy were detected by all assays. In patients with chronic renal failure, serum FT 4 was within reference limits more often when measured by Sclavo or Amerlex-MAB methods than by RIA-gnost or GammaCoat techniques. In conclusion, all assays performed well from both technical and diagnostic points of view. Additional key phrases: imprecision; effect of thyroxine-binding globulin, albumin and free fatty acids; reference interval; diagnostic efficiencyThyroid hormones circulate in the blood in free and bound forms, which are in equilibrium with one another. Thyroid hormones bind to thyroxine-binding globulin (TBG), thyroxine-binding prealbumin and albumin. Approximately 0·03% of thyroxine (T 4 ) is present in its free form and only this fraction is metabolically active. 1.2 Therefore, an exact assessment of the metabolic status requires determination of the biologically active free thyroid hormone.Several simple, fast and economical commercial free thyroxine (FT 4 ) kits have been developed. Our aim here was to evaluate three of these FT 4 assays, GammaCoat" (1 25 1) Free T 4 (GFT 4 ) , RIA-gnost® FT 4 (RFT 4 ) and Arnerlex-MAB*FT 4 (AFT 4 ) , in comparison with the twostep chromatographic method of Sclavo (SFT 4 ) , which we currently use and which is a reliable but expensive and labour intensive technique.Correspondence: M Van Blerk MD.We compared these assays for precision, reference values in euthyroid adults and pregnant women, influence of binding proteins, performance in thyroid disease and severe illness and practicability in a routine hormone laboratory. MATERIALS AND METHODSFree thyroxine assays Sclavo SFT 4 was based on a chromatographic separation on Sephadex LH-20 columns followed by a radioimmunoassay (RIA) on Liso-Phase columns. Five hundred microlitres of sample was added into LH-20 columns containing Sephadex LH-20 gel and about 0'5mL 0'2mol/L Tris buffer, pH 7-4. The samples were allowed to pass through the Sephadex gel during 15min. Protein fractions were then eliminated by washing the columns w...