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

Abstract

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/1 (SD 7.8). Two patient groups were studied: hyperthyroid (n=20, mean serum FT₄ 138 pmol/1, SD 69) and hypothyroid (n = 18, mean serum FT₄ 14.8 pmol/1, 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.

Key Words:

• albumin
• radio-immunoassay
• thyroid hormone