A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses.
Authors: Kritchevsky Janice, Olave Carla, Tinkler Stacy, Tropf Melissa, Ivester Kathleen, Forsythe Lauren, Couetil Laurent
Journal: Equine veterinary journal
Summary
# Editorial Summary: Levothyroxine Supplementation in Standardbred Racehorses Despite widespread use of thyroid supplements in athletic horses, robust evidence supporting their efficacy remains scarce. Researchers conducted a randomised crossover trial in six Standardbred racehorses, supplementing them with supra-physiologic doses of levothyroxine (0.1 and 0.25 mg/kg) or placebo for two weeks, then measuring performance markers including velocity at blood lactate threshold (VLa4) and heart rate thresholds (V150, V200) via standardised treadmill exercise testing. The higher dose (0.25 mg/kg) significantly elevated heart rates across all exercise intensities—for instance, heart rates at 10 m/s reached 239±9 bpm versus 219±6 bpm on placebo—and critically, three of six horses developed cardiac arrhythmias including atrial fibrillation, with slower heart rate recovery observed post-exercise. Survey data from 50 post-race samples revealed naturally elevated thyroxine concentrations were uncommon in this population (median 2.00 µg/dL, within reference range), suggesting widespread supplementation may be addressing a non-existent deficiency. For equine practitioners, these findings suggest levothyroxine supplementation offers no performance advantage and poses genuine arrhythmia risk; therapeutic thyroid supplementation should be reserved for horses with confirmed hypothyroidism rather than used empirically in healthy athletic animals.
Read the full abstract on PubMed
Practical Takeaways
- •Levothyroxine supplementation at performance doses does not improve athletic performance and carries significant cardiac risk; reconsider use in racing programs
- •Horses receiving thyroid supplements should be monitored with baseline and post-supplementation ECGs due to arrhythmia risk, particularly atrial fibrillation
- •Most racehorses maintain adequate thyroid function naturally; supplementation is rarely indicated and may be contraindicated in athletic horses
Key Findings
- •Supra-physiologic levothyroxine at 0.25 mg/kg resulted in significantly higher heart rates during standardized exercise testing compared to placebo (239±9 vs 219±6 bpm at 10 m/s)
- •Three of six horses (50%) receiving high-dose levothyroxine developed cardiac arrhythmias including atrial fibrillation
- •Levothyroxine supplementation decreased V200 (velocity at 200 bpm heart rate) during exercise testing, contrary to intended performance benefits
- •Survey of 50 racehorses showed median T4 of 2.00 µg/dL within normal range with only 3 horses (6%) exceeding laboratory reference limits