[A comparison of measurements of serum free thyroxine concentration by equilibrium dialysis and ultrafiltration using undiluted serum].

We have examined the clinical usefulness for free thyroxine (FT4) measurement by equilibrium dialysis of undiluted serum and by radio-immunoassay of T4 concentration in the dialysate (ED/RIA). The results were compared with those by ultrafiltration of undiluted serum (UF/RIA), which is believed to offer the best possible approach towards estimation of in vitro serum level of FT4. The precision with both methods was excellent. There was a high degree of correlation of FT4 concentration by both methods (r = 0.831, p less than 0.001, n = 99). FT4 concentration by ED/RIA and UF/RIA methods agreed well for hyperthyroidism, hypothyroidism, normal pregnancy, and for low T4-binding globulin concentration. In nonthyroidal illness (NTI), FT4 levels were all within or above normal for UF/ED method, while some FT4 values in low T4 syndrome were below normal for ED/RIA method. The effect of progressive serum dilution on FT4 concentration was also examined for both methods. There was no fall in FT4 concentration in sera from normal, hyperthyroid patient and in serum with low T4 syndrome for both methods, but in a serum from low T3 syndrome, there was a fall in FT4 concentration by dilution with UF/RIA method, indicating that serum dilution with ED/RIA method might underestimate FT4 concentration in some NTI sera. From these results it is concluded that the present ED/RIA method is adaptable to clinical laboratory use as a reference standard method for FT4 measurement.