Equine hyperthyroidism is an endocrine condition in which the thyroid gland makes too much thyroid hormone. The condition is somewhat uncommon in horses and is caused by exposure to excessive amounts of iodine-containing compounds (leg paints, povidone-based products, counterirritants, drugs, expectorants), inflammation of the thyroid, and noncancerous growths of the thyroid gland or pituitary gland.