BHA recognises that equine endocrinology is a complex, relatively poorly researched, but fastgrowing field. It is aware of the administration of levothyroxine sodium to horses in training andwould like to clarify whether, when and how that may be done under BHA Rules of Racing(Rules).Levothyroxine sodium is a synthetic homologue of the naturally occurring thyroid hormone,thyroxine (T4). It is a prohibited substance therefore it must not be present in the horses systemon raceday.The Rules - Rule (C)28. Veterinary treatment and medication clearly state that “everytreatment must be fully justifiable by the medical condition of the horse receiving the treatment”.In order for BHA to be satisfied that administration of levothyroxine sodium to a horse in trainingis justified the expectation is that the horse would have undergone dynamic endocrine testsshowing good evidence of either equine metabolic syndrome or hypothyroidism.