Measurement of Free Thyroxine Concentration in Horses by Equilibrium Dialysis

Abstract

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, fT4, 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.

Medición de hormonas tiroideas y cortisol en caballos mediante un analizador de inmunoensayo automatizado

“…The selected reference interval for TT4 was 14.16 -46.33 nmol/L, which is comparable to the range reported by several authors (Morris & García 1985;Meredith & Dobrinski 2004;Breuhaus et al 2006;Graves et al 2006;Kheirkhah & Hassanpour 2013;Mendoza et al 2013;Hilderbran et al 2014). In addition, the selected reference interval for FT4 was 3.60 -16.09 pmol/L (Breuhaus et al 2006;Graves et al 2006;Mendoza et al 2013).…”

“…In addition, the selected reference interval for FT4 was 3.60 -16.09 pmol/L (Breuhaus et al 2006;Graves et al 2006;Mendoza et al 2013). Besides, the selected reference range for cortisol, 23.45 -166.64 nmol/L, coincides with previous reports using radioimmunoassay (Haritou et al 2008;Mair & Sherlock 2009;Schenk et al 2012;Liburt et al 2013).…”

“…Even though thyroid gland dysfunction is fairly uncommon in adult horses, some clinicians consider important to exclude thyroid malfunction as a main cause of clinical manifestations (Hilderbran et al 2014). In addition, hypothyroidism in horses has been related to conditions such as laminitis, poor reproduction performance, and chronic myositis (Breuhaus et al 2006). However, the information related to thyroid function in horses is scarce (Breuhaus et al 2006), specially under tropical conditions.…”

Thyroid Function and Dysfunction in Term and Premature Equine Neonates

“…This study provides further evidence of NTIS in both term and premature foals that are ill. NTIS has been well described in humans, dogs, and cats suffering from systemic diseases, and it has been described preliminarily in adult horses19, 20 and foals 9, 10, 15. TT3 concentration decreases even in mild disease, with or without decreased fT3 concentration, followed by decreased TT4 concentration as disease severity increases 32, 33, 34.…”

“…Serum concentrations of TT3,17 fT3,4 , 18 TT4,5 , 19 fT4D,6 , 20 and TSH21 were measured by radioimmunoassay, either with commercially available kits4 , 5 , 6 or as previously described and validated in the horse 16, 17, 18, 19, 20. The TT3 assay sensitivity was 0.3 nmol/L; normal reference range in adult euthyroid horses, 0.7–2.5 nmol/L.…”