Equine hypothyroidism: the difficulties of diagnosis

SUMMARY In summary, there is substantial evidence suggesting that hypothyroidism exists in the foal and may be occasionally encountered in practice. However, there is limited evidence suggesting that hypothyroidism exists in the adult horse and the selection of cases for further investigation will be exceptionally rare. In the evaluation of such cases, nonthyroidal illnesses should be excluded and if possible drug therapy stopped for at least 10 days prior to testing (Beech 1987). If serum total T4 and T3 concentrations are abnormal, confirmation of the diagnosis should be attempted by a TSH/TRH stimulation test. If a normal response is elicited, primary hypothyroidism does not exist and an alternative diagnosis should be considered. In the exceptional case of the diagnosis being confirmed, treatment can be instituted using L‐thyroxine at a daily dose of 20 μg/kg with dosage adjustments depending on serum thyroid hormone concentrations (Chen et al. 1984). On hearsay evidence, thyroid hormone supplementation of horses has been advocated as a means of increasing performance but such a response does not confirm a diagnosis of hypothyroidism. On current and more scientific evidence, poor performance is unlikely to relate to thyroid dysfunction.