Open AccessMeasurement of Free Thyroxine Concentration in Horses by Equilibrium Dialysis
Author(s) -
Breuhaus Babetta A.,
Refsal Kent R.,
Beyerlein Susan L.
Publication year - 2006
Publication title -
journal of veterinary internal medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.356
H-Index - 103
eISSN - 1939-1676
pISSN - 0891-6640
DOI - 10.1111/j.1939-1676.2006.tb02870.x
Subject(s) - medicine , euthyroid , horse , endocrinology , confidence interval , propylthiouracil , free thyroxine , reference range , dialysis , zoology , hormone , biology , paleontology , thyroid function
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 74 , 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 = 2.086 x ‐ 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.