Effect of levothyroxine on Standardbred racehorses

Although this focuses on racehorses the results can be looked at in a more general
way. Inducing a hyperthyroid state through the supplementation of levothyroxine (thyroid hormone) was associated with a rise in heart rate and arrythmias when the horses were exercised. Casual use of thyroid hormone supplementation has been common as an adjunct to treating obesity in horses. Unlike other species that suffer ill effects from over-supplementation, it has been assumed this practice was safe in horses. This paper brings this into question.
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A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses
Equine Vet J. 2021 Jun 8. doi: 10.1111/evj.13480. Online ahead of print.
Authors
Janice Kritchevsky 1 , Carla Olave 1 , Stacy Tinkler 1 , Melissa Tropf 1 , Kathleen Ivester 1 , Lauren Forsythe 2 , Laurent Couetil 1
Affiliations

1 Department of Veterinary Clinical Sciences, Purdue University College of Veterinary Medicine, West Lafayette, Indiana, 47907, USA.
2 University of California Davis School of Veterinary Medicine, Davis, California, 95616, USA.

PMID: 34101895
DOI: 10.1111/evj.13480

Abstract

Background: The use of thyroid supplement is pervasive in athletic horses although its effects on measures of performance are not known.

Objectives: 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.

Study design: A randomised, crossover, trial was performed in 6 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 < 0.05 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, and10 m/s, respectively) and recovery (144±20 and 119±15 at 5 and15 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 min, respectively). Three of six horses developed cardiac arrhythmias including atrial fibrillation. Main limitations: 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. Keywords: atrial fibrillation; horse; standardised exercise test; thyroid supplementation hyperthyroidism.