Measurement of Free Thyroxine Concentration in Horses by Equilibrium Dialysis

The purpose of the study reported here was to validate measurement of free thyroxine (fT 4 ) concentration in equine serum by equilibrium dialysis (fT 4 D), and to compare values with fT 4 concentration measured directly and with total T 4 (TT 4 ) concentration. The fT 4 D, fT 4 , and TT 4 concentrations were measured over a range of values in euthyroid horses and horses made hypothyroid by administration of propylthiouracil (PTU). Concentrations of fT 4 D (< 1.8-83 pmol/L) were consistently higher than those of fT 4 (<1-40 pmol/L). There was a significant (P <.001) regression of fT 4 D on fT 4 in 503 samples from normal horses (y = 2.086x - 0.430). In baseline samples from 71 healthy euthyroid horses, fT 4 concentration ranged from 6-21 pmol/L (median, 11 pmol/L; 95% confidence interval [CI]10.5-11.8 pmol/L), and fT 4 D concentration ranged from 7-47 pmol/L (median, 22 pmol/L; 95% CI 20.9-25.1 pmol/L). Free T 4 D, fT 4 , and TT 4 concentrations were also measured in 34 ill horses. Horses consuming PTU and ill horses had significantly (P <.05) lower serum concentration of TT 4 , fT 4 , and fT 4 D than did clinically normal, healthy horses. If serum samples from ill horses were further subdivided into samples from horses that lived and samples from horses that died, fT 4 D concentration was not significantly different in ill horses that lived, compared with that in healthy horses, whereas fT 4 concentration was still significantly decreased in ill horses that died (P < 0.001). We conclude that measurement of fT 4 concentration by equilibrium dialysis is a valid technique in the horse, and its use may provide improved ability to distinguish nonthyroidal illness syndrome from hypothyroidism in that species.