We describe a method for free thyroxin (FT4) in serum, based on radioimmunoassay of T4 in serum dialysate, FT4(D). The method is analytically accurate, sensitive, reproducible, and suitable for routine use in the clinical laboratory. We compared results by this method with those obtained with five commercial FT4 assays (Amerlex, GammaCoat, ImmoPhase, LiquiSol, Spiria) and the free T4 index (FT4...