New diagnostic standards for thyroxine use in California could be on the way after a California Horse Racing Board (CHRB) staff report suggests some trainers are ignoring board warnings about overuse of thyroxine.
A staff report released ahead of a meeting of the CHRB's Medication, Safety and Welfare Committee this week revealed that two unidentified trainers in Southern California were responsible for more than half the 256 prescriptions for thyroxine on the circuit so far this year, and three veterinarians accounted for 80 percent of the prescriptions. Those figures are based on required reporting to the CHRB which, per rule language, prohibits the board from disclosing the identities of horses or connections involved.
Thyroxine (commonly known by its trade name as Thyro-L) is product for the treatment of hypothyroidism, or underperformance of the thyroid gland, in horses. Horses with hypothyroidism are typically overweight with cresty necks, struggle to lose weight with exercise, and are often lethargic with a poor hair coat — not a typical picture of a fit, healthy racehorse. Hypothyroidism is relatively rare in horses, and is most commonly found among foals who suffered an iodine deficiency.
Thyroxine overuse in racehorses first gained attention in 2013 when the Board launched an investigation into the sudden deaths of seven horses trained by Bob Baffert between 2011 and 2013. Baffert told investigators at the time that he had all the horses in his care on thyroxine, which was given as orally in a horse's grain in a similar manner as powdered supplements. At that time, trainers would commonly reach for thyroxine as a way to help get weight off horses, particularly if they'd recently returned from a lay-off.
The tendency to think of thyroxine as a supplement rather than a drug extends well beyond one barn, however. The Racing Medication and Testing Consortium and American Association of Equine Practitioners put out an advisory earlier this year emphasizing to trainers that thyroxine is a drug which should be given based on a medical diagnosis, not a wellness product to be distributed to the whole shedrow.
Hypothyroidism may be diagnosed by measuring a horse's T3 or T4 levels in their bloodstream. Those hormones are produced by the thyroid and can be an indicator of its function. The trouble with relying on those blood tests for a diagnosis in a racehorse, according to CHRB staff, is that T3 and T4 are known to circulate in lower levels in fit horses, thanks to certain common therapeutic drugs, lots of exercise, high protein diets, or high carbohydrate hay — all of which are the norm for horses in training.
It seems some practitioners are less concerned than others to use blood tests for diagnosis.
“Veterinarians have argued a 'positive' clinical response in a thyroxine treated horse is evidence of hypothyroidism,” the report read. “That is incorrect.”
A 2018 academic study on racing Standardbreds found that horses appeared more alert after being given thyroxine, leading some horsemen to believe it was having a positive effect. That study also found thyroxine-treated horses tired more easily, did not experience improved performance, and that four of six treated study horses developed cardiac arrhythmias.
The committee was urged to consider requiring a blood test for thyrotropin-releasing hormone (TRH) as a more reliable indicator of hypothyroidism. Proposed changes to board rule 1866.4 would restrict thyroxine use to horses who had a positive hypothyroid diagnosis based on a TRH test. Test results would have to be submitted to the CHRB equine medical director for review and would have to be reauthorized every 90 days. Horses would be ineligible to race for 30 days after administration.
The staff report indicates there may be serious risks with giving thyroxine to a horse who doesn't need it.
“Thyroxine has been associated with cardiac arrhythmias and atrial fibrillation in humans and anecdotally similar cardiac arrhythmias and atrial fibrillation have been reported in horses,” the report read. “While we cannot assert a cause and effect relationship, one sudden death in 2020 occurred five days after the horse was prescribed thyroxine.”
The board will consider the new thyroxine rule language at its meeting on Nov. 19.
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