Evaluation of five commercial kits for free thyroxine estimation by comparison with equilibrium dialysis method in nonthyroidal illness.

Five commercial kits for estimating FT4 in serum of 59 euthyroid control and 38 patients with severe NTI were studied: one non analog method (Gammacoat two step RIA, Clinical Assay) and four different analog methods (Amerlex-M RIA, Amersham; Enzymun test competitive enzyme immunoassay, Boehringer Mannheim; Amerlite chemiluminescence immunoassay, Kodak Clinical Diagnostics; Berilux chemiluminescence immunoassay, Behring) compared with equilibrium dialysis (Eq) method. Serum FT4 estimates in NTI patients measured by all commercial kits in this study yielded results comparable with those by equilibrium dialysis. The proportions of serum FT4 values concordant with FT4 (Eq) in each kit were 76.3, 76.3, 76.3, 68.4 and 78.9 per cent respectively. The percentage of NTI patients who had serum FT4 values lower than the reference levels of the methods used were 21.2, 26.3, 7.9, 15.8, 18.4 and 18.4 per cent respectively. No patient with low serum FT4 (Eq) level had subnormal or high serum TSH value. However, 4 out of 6 patients with high serum FT4 (Eq) values had depressed serum TSH values. All of them also had elevation of serum FT4 estimates measured by all kits. Serum FT4 estimates measured by all methods correlated well with FT4 (Eq) levels within the NTI group.