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.
Authors: MacMillan K M, Burns J J, John E, Clancey N, Stull J W
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Low thyroxine (T4) concentrations are commonly detected in racing Standardbred horses despite the absence of clinical hypothyroidism, creating uncertainty about which animals genuinely require thyroid supplementation. MacMillan and colleagues examined 40 racing Standardbreds using two different T4 assays (one calibrated for humans/horses, one for dogs) at baseline and after thyrotropin-releasing hormone (TRH) stimulation to assess whether assay choice and dynamic testing could improve diagnostic accuracy. The equine-calibrated assay identified 77.5% of horses as having low baseline T4 (mean 15.1 nmol/L), whilst the canine assay flagged only 30% (mean 17.9 nmol/L); crucially, all 31 horses with low T4 on the equine assay demonstrated appropriate thyroid responsiveness post-TRH stimulation, with mean increases of 113.6%, indicating normal thyroid function. Whilst the two assays showed strong correlation overall, the canine assay demonstrated superior concordance and identified a smaller population requiring further investigation. These findings underscore that baseline T4 concentration alone is insufficient for diagnosing hypothyroidism in horses and support the use of TRH stimulation testing as standard practice; the canine assay's improved sensitivity at lower T4 ranges may prove particularly valuable in ruling out true hypothyroidism before unnecessary supplementation is initiated.
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Practical Takeaways
- •Do not supplement thyroid hormone based on a single low tT4 result—request dynamic TRH stimulation testing to confirm hypothyroidism before treatment
- •Be aware that different assay platforms (equine vs. canine) produce substantially different reference intervals and diagnostic conclusions in horses; discuss assay method with your laboratory
- •Low tT4 in racing Standardbreds may reflect training stress, nutrition, or assay-related factors rather than true hypothyroidism; normal TRH response typically indicates adequate thyroid function
Key Findings
- •77.5% of Standardbred racehorses had baseline tT4 below reference interval using equine assay, but only 30% using canine assay
- •All 40 horses demonstrated normal thyroid function on TRH stimulation testing despite low baseline tT4 on equine assay
- •Equine and canine assays were strongly correlated but showed weak concordance, indicating systematic differences between assays
- •A single low tT4 measurement cannot diagnose hypothyroidism in horses; dynamic testing with TRH stimulation is required for accurate assessment