Investigation of 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.

The research focuses on analyzing the occurrence of low thyroxine (tT4) concentrations in racing Standardbred horses and their response to Thyrotropin-Releasing Hormone (TRH), using both equine and canine assays. The study concludes that a single low tT4 concentration cannot be used to diagnose hypothyroidism in horses accurately, underlining the need for dynamic testing.

Research Methodology

• The research involved the participation of 40 racing Standardbred horses. These horses were included in the study to investigate the frequent detection of low tT4 concentrations in such horses, despite the absence of clinical signs of hypothyroidism.
• Baseline tT4 concentrations were measured using chemiluminescent assays, one developed for humans (e.g., equine) and one for dogs (canine).
• Dynamic evaluation of tT4 was done through a TRH stimulation test conducted using both assays, and the variability between the two tests was determined through correlation coefficients and Bland-Altman plots.

Findings of the Study

• Using the equine assay, the mean baseline tT4 concentration was found to be 15.1 nmol/L, with about 77.5% (31 out of 40) of the horses having concentrations below the reference range. However, following exposure to TRH, all of these horses demonstrated a higher tT4 concentration than at baseline, indicative of normal thyroid function.
• When the dogs’ assay was used, the mean baseline tT4 concentration was slightly higher (17.9 nmol/L), and only 30% (12 out of 40) of the animals had readings below the reference interval. Following the TRH test, these 12 horses showed a marked increase in tT4 concentration.
• The study revealed that the equine and canine assay values were strongly correlated, with concordance correlation coefficients showing weak concordance. This points at potential differences in the sensitivity or specificity of the two assays, which implies that a single test may not be sufficient to diagnose the condition.

Conclusions and Implications

• The findings of the study concluded that a single low tT4 concentration is insufficient to accurately diagnose hypothyroidism in horses, and a dynamic evaluation involving a TRH test is necessary.
• The research also emphasized that the use of a canine T4 assay calibrated to detect lower tT4 concentrations in horses holds promise for future studies, but it requires further investigation. Moreover, it could offer an alternative or supplementary method for more accurately diagnosing hypothyroidism in horses.