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Equine veterinary journal2021; 54(3); 584-591; doi: 10.1111/evj.13480

A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses.

Abstract: The use of thyroid supplement is pervasive in athletic horses although its effects on measures of performance are not known. Objective: One purpose of this study was to determine whether supra-physiologic doses of levothyroxine affect the velocities at which blood lactate was greater than 4 mmol/L (VLa4 ) and heart rate was over 150 (V150 ) and 200 (V200 ) beats per minute respectively. Additionally, a survey of post-race blood samples was also conducted to determine whether high thyroxine concentrations were common in racehorses. Methods: A randomised, crossover, trial was performed in six healthy Standardbred racehorses. Methods: Study 1: T4 was determined in 50 post-race samples from a single Standardbred meet. Study 2: Research horses were trained to fitness and then randomised to one of three treatments: carrier, 0.1 mg/kg thyroxine or 0.25 mg/kg thyroxine for 2 weeks. Horses completed a standardised exercise treadmill test (SET) to fatigue on the last day of treatment. Serum free and total thyroxine and triiodothyronine were determined on the day of SET testing. Blood lactate and ECG data were collected during the SET at 6, 8, 10, 11, and 12 m/s and during recovery. The effect of treatment and SET on heart rate and blood lactate was examined using generalised linear mixed models. Post hoc analysis was adjusted for multiple comparisons using Tukey's Test. Data were expressed as mean ± standard deviation and P < .05 was considered significant. Results: Study 1: The median T4 value in this population of horses was 2.00 µg/dL (laboratory's normal range 1.5-4.5 µg/dL) and 3 of 50 racehorses had values above the laboratory reference range. Study 2: Levothyroxine at 0.25 mg/kg resulted in higher heart rates during SET (199 ± 30, 223 ± 17 and 239 ± 9 bpm at 6, 8 and 10 m/s respectively) and recovery (144 ± 20 and 119 ± 15 at 5 and 15 min) as compared to placebo (176 ± 18, 203 ± 10 and 219 ± 6 bpm at 6, 8, and10 m/s and 126 ± 5, 102 ± 11 at 5-15 minutes respectively). Three of six horses developed cardiac arrhythmias including atrial fibrillation. Conclusions: A relatively small number of animals were used and a SET is not identical to actual racing conditions. Conclusions: Supra-physiologic thyroxine supplementation caused a decreased V200 during a standard exercise test and may result in cardiac arrhythmias.
© 2021 EVJ Ltd.
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Publication Date: 2021-07-28 PubMed ID: 34101895DOI: 10.1111/evj.13480Google Scholar: Lookup
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  • Journal Article
  • Randomized Controlled Trial
  • Veterinary

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

This research investigates the impact of the thyroid supplement, levothyroxine, on the performance indicators in athletic horses. The study utilizes a randomized controlled trial involving six Standardbred racehorses as well as additional data from 50 post-race blood samples. The results suggest that high doses of levothyroxine can affect heart rates and may potentially contribute to cardiac arrhythmias.

Study objectives and methods

  • The primary purpose of this research was to establish whether extraordinarily high doses of levothyroxine influence the speed at which blood lactate levels exceed 4 mmol/L and heart rates surpass 150 and 200 beats per minute.
  • Post-race blood sample analysis comprised an important part of the study, which aimed to understand if high thyroxine concentrations were common among racehorses.
  • In a randomised, crossover trial, six healthy Standardbred racehorses were assessed under different conditions. Also, 50 post-race samples drawn immediately after a single Standardbred meet were examined.

Research methodology

  • The horses in the study were trained to fitness and then assigned one of three treatments randomly: a carrier, 0.1 mg/kg of thyroxine, or 0.25 mg/kg of thyroxine for two weeks.
  • On the final day of treatment, a standardised exercise treadmill test (SET) was administered to the horses, up to the point of fatigue. Free and total thyroxine, along with triiodothyronine in the serum, were assessed on the test day.
  • Blood lactate and ECG data were collected during the SET at different velocities, specifically 6, 8, 10, 11, and 12 m/s, as well as during the recovery process.

Results and analysis

  • Post-race blood samples showed that the median T4 value in the horse population was 2.00 µg/dL, with 3 of 50 racehorses having values above the normal lab reference range.
  • In the second part of the study, levothyroxine at the dose of 0.25 mg/kg led to elevated heart rates during both the SET and recovery periods in comparison to the placebo.
  • Of the six horses trialled, three developed cardiac arrhythmias, including a condition known as atrial fibrillation.

Study limitations and conclusion

  • The study acknowledges certain limitations, specifically the small number of animals used in the trial, and that a standard exercise treadmill test does not perfectly replicate real racing conditions.
  • The research concludes that administering higher than normal thyroxine levels can potentially decrease the horse’s performance during a standard exercise test and may result in cardiac arrhythmias.

Cite This Article

APA
Kritchevsky J, Olave C, Tinkler S, Tropf M, Ivester K, Forsythe L, Couetil L. (2021). A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses. Equine Vet J, 54(3), 584-591. https://doi.org/10.1111/evj.13480

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English
Volume: 54
Issue: 3
Pages: 584-591

Researcher Affiliations

Kritchevsky, Janice
  • Department of Veterinary Clinical Sciences, Purdue University College of Veterinary Medicine, West Lafayette, IN, USA.
Olave, Carla
  • Department of Veterinary Clinical Sciences, Purdue University College of Veterinary Medicine, West Lafayette, IN, USA.
Tinkler, Stacy
  • Department of Veterinary Clinical Sciences, Purdue University College of Veterinary Medicine, West Lafayette, IN, USA.
Tropf, Melissa
  • Department of Veterinary Clinical Sciences, Purdue University College of Veterinary Medicine, West Lafayette, IN, USA.
Ivester, Kathleen
  • Department of Veterinary Clinical Sciences, Purdue University College of Veterinary Medicine, West Lafayette, IN, USA.
Forsythe, Lauren
  • Davis School of Veterinary Medicine, University of California, Davis, CA, USA.
Couetil, Laurent
  • Department of Veterinary Clinical Sciences, Purdue University College of Veterinary Medicine, West Lafayette, IN, USA.

MeSH Terms

  • Animals
  • Atrial Fibrillation / veterinary
  • Exercise Test / veterinary
  • Heart Rate
  • Horse Diseases / epidemiology
  • Horses
  • Lactic Acid
  • Physical Conditioning, Animal / physiology
  • Thyroxine

Grant Funding

  • NA / State of Indiana and Purdue University College of Veterinary Medicine research account funded by the total wager tax
  • EG2013-08 / Equine Guelph

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