The occurrence of low thyroxine concentrations and response to thyrotropin-releasing hormone using equine and canine assays in a population of Standardbred racehorses in Prince Edward Island, Canada

Decreased total T4 (tT4) concentrations are frequently observed in racing Standardbred horses lacking clinical evidence of hypothyroidism. This poses a diagnostic challenge as primary hypothyroidism is reported to be rare in adult horses. Despite this, owners frequently wish to administer thyroid supplementation to horses having low tT4 concentrations. Forty racing Standardbred horses were sampled. Baseline tT4 concentrations were determined using human-based (equine) and canine-based chemiluminescent assays. Dynamic evaluation of tT4 was performed using a thyrotropin-releasing hormone (TRH) stimulation test using both assays. Variability between tests was assessed with correlation coefficients and Bland-Altman plots. Mean baseline tT4 concentration using the equine assay was 15.1 nmol/L with 31/40 (77.5 %) concentrations falling below the reference interval. Those 31 horses had a mean post-TRH tT4 concentration of 27.5 nmol/L (SD = 6.1), and mean percentage increase of 113.6 % (SD = 47.4). Although all horses had post-TRH tT4 concentration higher than baseline indicating normal thyroid function, 3 horses did not reach a 50 % increase. Mean baseline tT4 concentration using the canine assay was 17.9 nmol/L with 12 of 40 (30 %) concentrations falling below the reference interval. Those 12 horses had a mean post-TRH tT4 concentration of 36.7 nmol/L (SD = 5.8), and a mean percentage increase of 242.7 % (SD = 91.7). The 31 horses identified with low baseline tT4 concentrations using the equine assay achieved a 50 % or greater increase from baseline using the canine assay. Equine and canine assay-derived values (for baseline and post-TRH tT4) were strongly correlated, with weak concordance correlation coefficients. Results from this study support that a single low tT4 concentration cannot be used to accurately diagnose hypothyroidism in horses and dynamic testing is required. While further evaluation is required, use of a canine T4 assay calibrated to achieve detection of lower tT4 concentrations in horses is promising.