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 (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 5 2.086x 2 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.

Nonthyroidal Illness Syndrome in Adult Horses

“…Various drugs can affect TH concentrations in this way. Salicylates alter TH binding to carrier proteins and, in humans, result in increased fT4D . Carrier proteins such as TBG, albumin, and transthyretin, all of which are negative acute‐phase reactants, decrease in acute tissue injury and inflammation, enhancing the activity of low‐affinity competitors of THs on binding to carrier proteins in disease .…”

“…Serum concentrations of TT3, fT3, fT4D, and TSH were measured with commercially available radioimmunoassay (RIA) kits (Clinical Assays Gammacoat free T3 125I RIA Kit) , or RIA as previously described and validated in the horse . TT3 assay sensitivity was 0.3 nmol/L.…”

“…In the horse, phenylbutazone, dexamethasone, dietary imbalances, and fasting decrease TT4 without the presence of thyroid gland disease . Nonthyroidal illness syndrome (NTIS) also may result in altered TH concentrations, but NTIS has only been studied minimally in horses . In humans, dogs, and cats suffering from systemic illnesses, NTIS is characterized first by low TT3 with or without decreases in TT4 .…”

Thyroid hormone concentrations differ between donkeys and horses

“…Numerous techniques have been validated for the determination of thyroid hormones in animals, notably in horses [38,40–43]. However, although all the radioimmunoassay kits used in this study have been previously validated for horses [8], a species‐dependent effect cannot be ruled out.…”

Quantitative Pertechnetate Thyroid Scintigraphy and the Ultrasonographic Appearance of the Thyroid Gland in Clinically Normal Horses

“…9 Thyroid dysfunction in horses Thyroid dysfunction in horses is incompletely understood, 10 difficult to diagnose and poorly documented in the literature. 11 Particular confusion surrounds the diagnosis and prevalence of primary hypothyroidism in adult horses. Thyroid disorders reported in the literature include nodular hyperplasia, 12 non-functional C-cell adenoma, [13][14][15] hyperthyroidism associated with thyroid neoplasia, [16][17][18] non-functional thyroid carcinoma, [19][20][21][22][23][24][25] idiopathic hypothyroidism 26 and…”