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

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 (V La4 ) and heart rate was over 150 (V 150 ) and 200 (V 200 ) 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.

Diagnosis and management of thyroid disorders and thyroid hormone supplementation in adult horses and foals

“…However, atrial fibrillation can also develop in horses treated with levothyroxine at high doses and in people, over-supplementation is associated with additional cardiovascular disorders including myocardial infarction and congestive heart failure. [69][70][71][72][73][74][75] If a horse has been on prolonged course of thyroid hormone supplementation for vague clinical complaints, gradual weaning over 2-4 weeks is recommended when therapy is to be discontinued.…”

“…Of interest, a recent study demonstrated that performance of Standardbred racehorses was reduced with thyroid hormone supplementation, possibly due to T4-induced decrease in cardiac function. 74 High T4 concentration, possibly due to excessive T4 supplementation, also increases the risk of adverse effects, including atrial fibrillation and other cardiac arrhythmias. 75 For these reasons, levothyroxine supplementation is not recommended for the treatment of poor performance in horses; rather, horses displaying poor performance should undergo a thorough clinical examination to identify underlying health issues that might be causing suboptimal performance.…”

“…Owners observing their horse when treated with levothyroxine might have a misleading impression of improvement due to the associated increase in appetite and liveliness, as well as a placebo effect. Of interest, a recent study demonstrated that performance of Standardbred racehorses was reduced with thyroid hormone supplementation, possibly due to T4‐induced decrease in cardiac function 74 . High T4 concentration, possibly due to excessive T4 supplementation, also increases the risk of adverse effects, including atrial fibrillation and other cardiac arrhythmias 75 .…”

A one-health lens offers new perspectives on the importance of endocrine disorders in the equine athlete

“…40 Of note for equine and human athletes, treatment with levothyroxine for purported low thyroid levels has been associated with increased incidence of arrhythmias, both in humans 41,42 and horses. 43…”

A one-health approach to identifying and mitigating the impact of endocrine disorders on human and equine athletes

“…Traditional dietary medications and supplements for treating EMS (above and beyond exercise and dietary change) include metformin, levothyroxine, and a resveratrol + amino acid blend, with the first and last also being used for humans with MetS. [30][31][32][33][34][35][36][37][38][39][40][41] However, a new promising class of oral treatment options, sodium-glucose cotransporter 2 inhibitors (ertugliflozin and canagliflozin), which modulate the sodium and glucose transport in the kidney by increasing urinary glucose excretion, have been described. These drugs have been shown to have efficacy in treating refractive cases of EMS/ID, ones where the typical diet, exercise, and supplementation with metformin or levothyroxine has failed; limit insulin peaks after eating; and improve Obel scores.…”