This study was supported by funds from the Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC.
Measurement of Free Thyroxine Concentration in Horses by Equilibrium Dialysis
Article first published online: 5 FEB 2008
© 2006 American College of Veterinary Internal Medicine
Journal of Veterinary Internal Medicine
Volume 20, Issue 2, pages 371–376, March 2006
How to Cite
Breuhaus, B. A., Refsal, K. R. and Beyerlein, S. L. (2006), Measurement of Free Thyroxine Concentration in Horses by Equilibrium Dialysis. Journal of Veterinary Internal Medicine, 20: 371–376. doi: 10.1111/j.1939-1676.2006.tb02870.x
- Issue published online: 5 FEB 2008
- Article first published online: 5 FEB 2008
- Revised July 25, 2005; Accepted September 6, 2005.
- Assay validation;
- Equilibrium dialysis;
- Free thyroxine;
- Nonthyroidal illness syndrome
The purpose of the study reported here was to validate measurement of free thyroxine (fT4) concentration in equine serum by equilibrium dialysis (fT4D), and to compare values with fT4 concentration measured directly and with total T4 (TT4) concentration. The fT4D, fT74, and TT4 concentrations were measured over a range of values in euthyroid horses and horses made hypothyroid by administration of propylthiouracil (PTU). Concentrations of fT4D (<1.8–83 pmol/L) were consistently higher than those of fT4 (<1–40 pmol/L). There was a significant (P < .001) regression of fT4D on fT4 in 503 samples from normal horses (y = 2.086x - 0.430). In baseline samples from 71 healthy euthyroid horses, fT4 concentration ranged from 6- 21 pmol/L (median, 11 pmol/L; 95% confidence interval [CI]10.5–11.8 pmol/L), and fT4D concentration ranged from 7–47 pmol/L (median, 22 pmol/L; 95% CI 20.9–25.1 pmol/L). Free T4D, fT4, and TT4 concentrations were also measured in 34 ill horses. Horses consuming PTU and ill horses had significantly (P < .05) lower serum concentration of TT4, fT4, and fT4D 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, fT4D concentration was not significantly different in ill horses that lived, compared with that in healthy horses, whereas fT4 concentration was still significantly decreased in ill horses that died (P < 0.001). We conclude that measurement of fT4 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.