Hypothyroidism in Horses
True hypothyroidism is uncommon in horses, with only very occasional reported cases. Nevertheless, single serum T4 concentrations below the reference range are common. This is usually due to the presence of nonthyroidal illness or medication The test recommended to further evaluate the hypothalamic-pituitary-thyroid axis in horses is the TSH stimulation test. In horses in which an initial low serum T4 concentration is seen, a second T4 measurement a couple of weeks later after medication has ceased or after recovery from an illness will usually reveal a normal value within the reference range.