Levothyroxine Use in Performance Horses and Racehorses

Levothyroxine Use in Performance Horses and Racehorses

Levothyroxine, also known as L-thyroxine or T4, is a synthetic form of the thyroid hormone thyroxine, which is normally secreted by the follicular cells of the thyroid gland.

In the case of race and performance horses, levothyroxine is commonly supplemented to help strip weight off horses and/or to help horses ‘perk up’. Yet research indicates that in fit horses- supplementing with thyroxine actually resulted in the horses fatiguing quicker. Its use was also associated with an increased risk of cardiac arrhythmia- hardly performance enhancing! In cases of respiratory infection, the use of levothyroxine may actually prolong the infection.

Supplementation of synthetic hormones in horses is not something which should be taken lightly. True hypothyroidism in horses is a rare condition- and when it does occur, reasons as to WHY it has occurred should be addressed first, or in conjunction with appropriate treatment. Thorough assessment of thyroid function MUST extend beyond just testing thyroid stimulating hormone (TSH). Testing T4 levels alone (or in conjunction with TSH) is also insufficient to provide reliable information to determine the true state of thyroid function. T3 also needs to be included. As does a thorough physical assessment and case history, analysis of the horse’s diet/nutritional intake, current medication and/or supplement use.

It also needs to be remembered that abnormal thyroid hormone concentrations can occur after a high grain diet meal, after extensive travel, training stress, or if a horse is ill.

In cases of sub-clinical hypothyroidism, specific nutritional insufficiencies/deficiencies are commonly implicated. Similarly overtraining without sufficient rest/recovery/adaptation periods can lead to impaired thyroid function. No amount of levothyroxine supplementation can rectify these performance and health limiting factors- thus without addressing the causes, the problems remain.