Do I ever need to have my horse’s thyroid hormone levels checked?
Hypothyroidism (a deficiency of thyroid hormones) is a rare condition in adult horses. The most common cause of it in foals is nutritional, and is related to the iodine intake of the mare. Hyperthyroidism (an excess of thyroid hormones) is also rare and is a poorly documented condition in horses. Thyroid hormone levels can change due to various factors such as age, season, pregnancy, level of activity, and overall physiological status. Thyroid hormone levels can be low due to illness and stress; however, in these cases the thyroid gland itself is normal and the horse is not truly hypothyroid.

A common misconception is that abnormal thyroid hormone levels are the cause of equine metabolic syndrome (EMS), a disease defined by obesity, laminitis, and insulin resistance. Although thyroid hormone levels can be low in this disease, the actual thyroid gland is normal and has normal function. In fact, experimental removal of the thyroid gland in horses has resulted in ill thrift, a poor haircoat, and decreased weight gain, which are very different symptoms from what is seen in equine metabolic syndrome. One reason this misconception may persist is because frequently a horse with EMS will be placed on a thyroid hormone supplement; however, this is not done to correct any true hypothyroidism but is instead done to promote weight loss and improve sensitivity to insulin.