Ultrafiltration devices tested for use in a free thyroxine assay validated by comparison with equilibrium dialysis.

A new ultrafiltration method for measuring serum free thyroxine (FT4) is presented. The method makes use of disposable ultrafiltration devices, which were selected on grounds of their ability to produce protein-free serum ultrafiltrates necessary for accurate determination of FT4. Ultrafiltrate thyroxine was measured by radio-immunoassay. As measured by this method, the mean serum FT4 concentration in reference subjects (n = 17) was 25.4 pmol/l (SD 7.8). Two patient groups were studied: hyperthyroid (n = 20, mean serum FT4 138 pmol/l, SD 69) and hypothyroid (n = 18, mean serum FT4 14.8 pmol/l, SD 6.5). These results were compared with serum FT4 concentrations, as measured by an ultrafiltration method based on the use of dialysis-tubing bags (r = 0.97), and by an equilibrium dialysis method (r = 0.95). The mean level of FT4, as measured by the ultrafiltration methodologies, was about twice as high as that measured by equilibrium dialysis. This new FT4 method is more practical than earlier ultrafiltration methods and, theoretically, analytically more accurate than equilibrium dialysis methods.