Hypothyroidism in Horses

Hypothyroidism is a deficiency of thyroid gland activity with underproduction of the thyroid hormone thyroxin.

The thyroid gland is located in the neck, below the larynx. In the horse, there are two lobes that are about the size of an egg, located on either side of the trachea or windpipe. People often mistake them for lymph nodes.

The thyroid gland produces hormones that affect your horse in many ways:

The thyroid gland has an absolute dependence on iodine in order to produce hormones, most importantly T3 and T4. Without iodine in the body, thyroid hormones cannot be made. When supplemental thyroid hormone is given, it is given in the form of T4, or thyroxine.

Hypothyroidism is probably the most over-diagnosed diseases of the equine veterinary agrworld. Oftentimes it is diagnosed based on the results of a single sampling of T3 and T4. Hypothyroidism can be primary, secondary or tertiary.

In horses, primary hypothyroidism is by far the most common. Secondary and tertiary hypothyroidism has not been well-documented in horses.

Causes of primary hypothyroidism in horses:

Many problems in horses have been attributed to hypothyroidism. However, these diseases are rarely caused by hypothyroidism, and treatment with replacement hormone (thyroxine) can be excessive, even dangerous to your horse. Some of these problems include:


Many factors can affect the level of thyroid hormone in the body without the horse actually having intrinsically impaired thyroid function.

Sick animals have a decreased metabolic rate due to an effect of the sickness to decrease thyroid levels. When the primary illness is cured, the thyroid levels return to normal. This is referred to as euthyroid (meaning normal functioning thyroid) sick syndrome.

Thyroid hormone levels vary with age, sex and even time of day. The levels are also effected by nutrition; for example, starved animals have very low thyroxine levels. Surgery or anesthesia can artificially increase thyroxine levels, as can training or high levels of activity. Young horses increase their levels of thyroid hormones when they are fed high carbohydrate meals.

Many drugs, including glucocorticoids (such as Azium), sulfa drugs, phenylbutazone ('bute'), and certain tranquilizers, can decrease the thyroid hormone level.

All of these factors can make it very difficult to interpret a single low thyroid hormone level in horses. However, finding low levels of thyroxine on a spot check does not mean the horse has hypothyroidism or that it will benefit from thyroxine supplementation.

The best way to evaluate thyroid function is to administer a drug, TSH, or thyroid stimulating hormone, which is actually a natural body compound that stimulates the thyroid gland to release thyroid hormone. If the thyroid gland has normal function, then TSH will cause the thyroid to produce hormones.

Unfortunately, at this time, TSH is not available on a regular basis, so the practitioner may have to rely on a good medical history, multiple thyroid hormone level measurements, and ruling out other diagnoses.


Thyroid hormone is a potentially toxic drug. It can speed up the metabolism, and it can also speed up the heart. There have been no long term safety studies. Thyroxine treatment in humans is avoided unless absolutely necessary to treat confirmed hypothyroidism.

Excess thyroid hormone can cause nervousness and weight loss in horses. Excessive amounts of thyroid hormone can eventually cause cardiac arrhythmias and heart failure.

There are two different syndromes of hypothyroidism in the horse; neonatal (foals) and adult onset. Foals develop hypothyroidism most commonly due to problems during the mare's pregnancy, so the effects are far more devastating than they are in the adult horse. Adult horses with hypothyroidism tend to have less clear-cut symptoms, and thus are harder to identify, but the disease is not life-threatening in the adult.

Most affected foals actually develop hypothyroidism while still inside the mare's womb, and it is almost always due to a nutritional problem – either the mare ate too much iodine or too little, or ate a goitrogenic compound. Common sources of excess iodine in the mare's diet include seaweed, which is naturally very high in iodine. Many of our equine vitamin supplements are based on kelp and other seaweeds. Some areas of western Canada have an unusually high number of foals born with low levels of thyroid hormones, and clinical signs consistent with hypothyroidism. Researchers hypothesize that goitrogenic plants may play a role in this syndrome.

Thyroid hormone is absolutely crucial for proper fetal development. The results of hypothyroidism in the neonate depend on when during development the fetus was deprived of thyroid hormone.

What to Watch For


Clinical signs are suggestive of hypothyroidism and foals may have a goiter, which is an enlarged and visible thyroid gland. Additional tests include:


Because of the myriad and devastating effects of hypothyroidism on the fetus, it may not be possible to treat the affected foal. Treatment really depends on when and how the fetus was affected by hypothyroidism. For instance, if the foal has incomplete ossification of some of the bones, which has resulted in crushing of the joints, there may not be any effective treatment.

If the foal was affected late in gestation, and most of the organ systems were well formed, then there may be some hope for treating the foal with thyroid hormone replacement therapy. However, it is important to remember that even when the foal is born appearing relatively normal, some abnormalities, especially musculoskeletal problems, may not appear for a few days or weeks.

Preventative Care

It is important to be aware that supplements should be given only in recommended amounts. Plants that are classified as brassicas are goitrogens, and should not be fed to pregnant mares. These include turnips, mustards, and cabbages.

Hypothyroidism in the adult horse is not a life-threatening condition, but if we reconsider the many aspects of the horse's system that can be affected by a lack of thyroid hormone, then it is clear that hypothyroidism might have an effect on performance – both athletic and reproductive. It is useful to consider what happens in the horse when the thyroid glands are removed.

Horses with no thyroid hormone production do not develop tying up syndrome, nor do they develop laminitis.

What to Watch For


Medical history. Your veterinarian will ask questions about vitamin and mineral supplements and drugs that your horse may be taking; your horse's diet; the level of work that your horse is doing. He may want to look at your pasture. Oftentimes, obesity and laminitis are the consequence of a diet that is inappropriately high in calories.


Remember that excessive amounts of thyroxine result in thyrotoxicosis, or toxicity due to thyroid hormone. Signs of thyrotoxicosis include:

It is important to remember that in euthyroid sick syndrome, in which the thyroid levels are low due to concomitant disease rather than to actual thyroid malfunction, it doesn't help to give extra thyroid hormone, and it can hurt your horse.