Equine hypothyroidism is a endocrine condition which occurs in older horses, in which the thyroid gland does not produce enough thyroid hormone (TH). It is the result of diseases that affect thyroid function, exogenous compounds that reduce TH synthesis, and disorders of the hypothalamus or pituitary gland. Neoplasia is the most common cause of hypothyroidism in older horses.

The diagnosis can be a challenge since many variables can influence diagnostic test results. Horses treated with phenylbutazone and glucocorticoids can often be misdiagnosed as hypothyroid.