A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses
A portion of the data was presented as a poster at the 65th American College of Veterinary internal Medicine Forum, Phoenix, Arizona, USA 2019.
Funding information
This study was funded by Equine Guelph and the State of Indiana and Purdue University College of Veterinary Medicine research account funded by the total wager tax.
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 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.
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.
CONFLICT OF INTERESTS
No competing interests have been declared.
Open Research
PEER REVIEW
The peer review history for this article is available at https://publons.com/publon/10.1111/evj.13480.