Determination of Free Thyroxine in Serum by Low-Temperature Equilibrium Dialysis

Abstract

Abstract Equilibrium dialysis of thyroxine (T4) in diluted serum at low temperature gives consistent and reproducible values for the dialyzable fraction (DF) of this hormone. The quantity of serum required for the determination is minimized by dilution, which is technically advantageous. Variations caused by temperature, dialysis time, dilution of serum, buffer system, and stability of 125I-T4 could be controlled by parallel use of a normal reference serum in each experiment. The reference serum was assigned a DF value of 0.04%, which was corrected by a factor derived from the ratio of assigned DF to the observed DF calculated from the experimental data for this reference serum. This correction factor permits the observed DF results for concurrent unknown samples to be related to the values for the DF of the normal reference serum. The free thyroxine concentration is calculated from the product of the DF and the total serum thyroxine concentration as determined by competitive binding analysis. The method is convenient for use in clinical laboratories, and data for free thyroxine obtained from patients with metabolic thyroid disorders are diagnostically discriminatory.