FREE SHIPPING over $40
OVER 9000 FIVE-STAR REVIEWS
5% OFF ALL SUBSCRIPTIONS
100% HAPPINESS GUARANTEE
Analyze Diet
0
Home / Equine Research Bank / Equine Vet J / A randomised, controlled trial to determine the effect of le...
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.
Get Full Text
Publication Date: 2021-07-28 PubMed ID: 34101895DOI: 10.1111/evj.13480Google Scholar: Lookup
The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
LinkedInCopy
  • 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

References

This article includes 30 references
  1. Kritchevsky JE, Bertin FR. Endocrine system. In: Munro G, editor. Equine clinical medicine, surgery and reproduction. Boca Raton, FL: CRC Press; 2020. p. 901-22.
  2. Ciloglu F, Peker I, Pehlivan A, Karacabey K, Ilhan N, Saygin O. Exercise intensity and its effects on thyroid hormones. Neuroendocrinol Lett 2005;26:830-4.
  3. Graves EA, Ii HCS, Marteniuk JV, Refsal KR, Nachreiner RF. Thyroid hormone responses to endurance exercise. Eq Vet J 2006;38(S36):32-6.
  4. Hyyppa S. Endocrinal responses to exercising horses. Livestock Production Sci 2005;92:113-21.
  5. Fazio E, Medica P, Cravana C, Ferlazzo A. Effects of competition experience and transportation on the adrenocortical and thyroid responses of horses. Vet Rec 2008;163:713-6.
  6. Irving CHG. Effect of exercise on thyroxine degradation in athletes and non-athletes. J Clin Endo Metab 1968;28:942-8.
  7. Blackmore DJ, Greenwood RES, Johnson C. Observations on thyroid hormones in the blood of thoroughbreds. Res Vet Sci 1978;25:294-7.
  8. Medica P, Cravana C, Fazio E, Ferlazzo A. Hormonal responses of quarter horses to a 6-week conventional western-riding training program. Livestock Sci 2011;140:262-7.
  9. Hill RC, Fox LE, Lewis DD, Beale KM, Nachreiner RF, Scott KC. Effects of racing and training on serum thyroid hormone concentrations in racing Greyhounds. Am J Vet Res 2001;62:1969-72.
  10. Kaplan MM. Clinical and laboratory assessment of thyroid abnormalities. Med Clin North Am 1985;69:863-80.
  11. Duhig TJ, McKeag D. Thyroid disorders in athletes. Curr Sports Med Rep 2009;8:16-9.
  12. Vischer CM, Foreman JH, Constable PD, Benson GJ, Kline KH, Freeman DE. Hemodynamic effects of thyroidectomy in sedentary horses. Am J Vet Res 1999;60:14-21.
  13. Vischer CM, Foreman JH, Constable PD, Benson GJ, Kline KH, Freeman DE. Hypothyroidism and exercise intolerance in horses. J Vet Intern Med 1996;10:151.
  14. Bertin FR, Eichstadt L, Kritchevsky JE. Effects of high doses of levothyroxine sodium on serum concentrations of triiodothyronine and thyroxine in horses. Am J Vet Res 2019;80:565-71.
  15. Perez-Moreno CI, Couetil LL, Pratt SM, Ochoa-Acuna HG, Raskin RE, Russell MA. Effect of furosemide and furosemide-carbazochrome combination on exercise-induced pulmonary hemorrhage in Standardbred racehorses. Can Vet J 2009;50:821-7.
  16. Tennent-Brown BS, Wilkins PA, Lindborg S, Russell G, Boston RC. Assessment of a point-of-care lactate monitor in emergency admissions of adult horses to a referral hospital. J Vet Intern Med 2007;21(5):1090-8.
  17. Dubreucq C, Chatard JC, Couroucet A, Auvinet B. Reproducibility of a standardised excercise test for Standardbred trotters under field conditions. Equine Vet J 1995;18:108-12.
  18. Alberts MK, McCann JP, Woods PR. Hemithyroidectomy in a horse with confirmed hyperthyroidism. J Am Vet Med Assoc 2000;217(7):1051-4.
  19. Ramirez S, McClure JJ, Moore RM, Wolfsheimer KJ, Gaunt SD, Mirza MH. Hyperthyroidism associated with a thyroid adenocarcinoma in a 21-year-old gelding. J Vet Intern Med 1998;12:475-7.
  20. Frank N, Buchanan BR, Elliott SB. Effects of long-term oral administration of levothyroxine sodium on serum thyroid hormone concentrations, clinicopathologic variables, and echocardiographic measurements in healthy adult horses. Am J Vet Res 2008;69:68-75.
  21. Chem CL, McNulty ME, McNulty PK, Asbury AC. Serum levels of thyroxine and triiodothyronine in mature horses following oral administration of synthetic thyroxin (Synthroid®). Equine Vet Sci 1984;4:5-6.
  22. Jesty SA, Kraus M, Gelzer A, Rishniw M, Noise NS. Effect of transvenous electrical cardioversion on plasma cardiac troponin I concentrations in horses with atrial fibrillation. J Vet Intern Med 2009;23:1103-7.
  23. Vargas-Uricoechea H, Sierra-Torres CH. Thyroid hormones and the heart. Horm Mol Biol Clin Invest 2014;18:15-26.
  24. Davis P, Davis FB. Nongenomic actions of thyroid hormone on the heart. Thyroid 2002;12:459-66.
  25. Leleu C, Cotrel C, Courouce-Malblanc A. Relationships between physiological variables and race performance in French Standardbred trotters. Vet Rec 2005;156(11):339-42.
  26. Kahaly GJ, Nieswandt J, Wagner S, Schlegel J, Mohr-Kahaly S, Hommel G. Ineffective cardiorespiratory function in hyperthyroidism. J Clin Endocrinol Metab 1998;83:4075-8.
  27. Karga H, Papapetrou PD, Korakovouni A, Papandroulaki F, Polymeris A, Pampouras G. Bone mineral density in hyperthyroidism. Clin Endocrinol 2004;61:466-72.
  28. Blum MR, Bauer DC, Collet T-H, Fink HA, Cappola AR, da Costa BR. Subclinical thyroid dysfunction and fracture risk: a meta-analysis. JAMA 2015;313:2055-65.
  29. Vestergaard P, Rejnmark L, Mosekilde L. Influence of hyper- and hypothyroidism, and the effects of treatment with antithyroid drugs and levothyroxine on fracture risk. Calcif Tissue Int 2005;77:139-44.
  30. Oliva F, Berardi AC, Misiti S, Maffulli N. Thyroid hormones and tendon: current views and future perspectives. Concise review. Muscle Ligaments Tendons J 2013;3:201-3.

Citations

This article has been cited 0 times.
Subscribe to our newsletter
Join 99,359 horse owners like you who receive our email updates on horse health and nutrition.