Hypothyroidism in Horses

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:

  • Thermogenesis, or the ability to regulate body temperature
  • The body's ability to use protein
  • Heart rate and strength
  • Production of red blood cells
  • Reproduction
  • Hair coat
  • Energy levels
  • Appetite
  • Milk production
  • The immune system
  • In foals, thyroid hormone is absolutely essential for the normal development of the musculoskeletal and nervous systems.

    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 world. Oftentimes it is diagnosed based on the results of a single sampling of T3 and T4. Hypothyroidism can be primary, secondary or tertiary.

  • Primary hypothyroidism. This form results from disease of the thyroid gland. For a variety of reasons the thyroid gland does not produce adequate amounts of thyroxin.
  • Secondary hypothyroidism. This form is caused by a deficiency of thyroid-stimulating hormone, usually as a result of a lesion in the pituitary gland. The thyroid gland is capable of producing thyroid hormone, but is not receiving the correct stimuli from higher levels.
  • Tertiary hypothyroidism. This form is caused by a lack of synthesis or release of thyrotropin releasing hormone. Thyroid hormone is being produced, but the body, for various reasons, is unable to use the thyroid hormone.

    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:

  • Iodine deficiency or excess (goiter). Iodine deficiency is usually due to horses living in areas where the soil is naturally low in iodine, such as the areas around the Great Lakes in the United States. Iodine excess is usually the fault of the owners such as giving a few too many scoops of an iodine-containing mineral supplement. It may also be the result of using topical iodine found in leg paints, blisters, and in medications that are used for treating diseases such as rain rot. Some expectorants that help horses to produce a cough contain iodine. It is also important to remember that any substance that is applied with DMSO will be carried into the body and can lead to toxicity.
  • Inflammation of the thyroid gland
  • Cancer of the thyroid gland
  • Developmental failure of the thyroid to grow
  • Ingestion of goitrogenic compounds. This refers to plants or other toxins that interfere with the body's ability to make thyroid hormone.

    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:

  • Fat horses with cresty necks
  • Mares who can't get pregnant
  • Chronically foundering horses
  • Foals with orthopedic malformations
  • Recurrent tying up syndrome
  • Exercise intolerance
  • Inability to sweat properly
  • Delayed shedding
  • Retarded growth


    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

  • Poorly formed or incompletely formed bones
  • Contracted limbs
  • Monkey jaw – the upper jaw is shorter than the lower
  • Tendon ruptures
  • Weakness and death within the first few hours or days of birth
  • Incoordination or recumbency due to abnormal nervous system development
  • Prematurity
  • Long hair coat
  • Domed head
  • Cold intolerance


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

  • Complete blood count (CBC) to look for signs of inflammation, infection, and prematurity, among other things
  • Chemistry profile, to examine the function of the internal organs
  • Blood levels of the thyroid hormones T3 and T4. Foals normally have very high levels of thyroid hormone, which can confuse the diagnosis.
  • Thyroid stimulation test to provide a definitive diagnosis. Unfortunately, this is often impossible to do, so the diagnosis may remain presumptive.


    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.

  • The body temperature and heart rate drop below normal
  • The red blood cell count is reduced (the horse becomes anemic)
  • Milk production is decreased.
  • Energy levels are greatly diminished
  • The hair coat becomes very coarse
  • The appetite is decreased
  • Stallions have decreased libido as well as decreased total sperm counts. In mares, the reproductive cycle becomes irregular, but they are still able to conceive and give birth to term foals.

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

    What to Watch For

  • Lethargy and decreased appetite
  • Rear limb edema (thickening of the lower part of the hind legs)
  • Decreased reproductive function
  • Low heart rate
  • Poor performance
  • Obesity


    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.

  • Physical examination. Your veterinarian will look for signs of hypothyroidism. He will also be on the hunt for any signs of underlying illness that could be the cause of the problem. He will carefully palpate the upper neck area, looking for any signs of a goiter. Additional tests may include:
  • Complete blood count and chemistry profile to determine if there may be any other underlying disease that could be contributing to your horse's problem. Anemia may be consistent with hypothyroidism, but low-grade anemia can be found with any chronic inflammatory or infectious condition.
  • Thyroid stimulation test or simple T3/T4 levels. It is important to remember that thyroid hormone levels tend to be highest in the early evening, and lowest in the morning.
  • Feed analysis. If your horse has signs or test results that are suggestive of hypothyroidism, your veterinarian may choose to do a feed analysis to see if your horse is receiving either too much or too little iodine. The more certain your veterinarian is that your horse actually has hypothyroidism, the more important it is to make sure that your horse's dietary intake of iodine is optimal.


  • Treatment depends on the cause of the hypothyroidism. If your horse is hypothyroid because she does not receive enough iodine in her feed, then the obvious solution is to supplement your horse with dietary iodine. It is important to do this under the auspices of either your veterinarian or a reputable feed company, because excessive iodine can have the same deleterious effects on your horse as too little iodine.
  • If your veterinarian is fairly confident of the diagnosis of hypothyroidism, then the treatment is to give thyroid hormone supplement, thyroxine. Your veterinarian will need to evaluate the effectiveness of the dose prescribed for your horse. Often, the dose must be changed multiple times until the optimum amount for your particular horse is found. Remember, however, that it can take several weeks to see a response to hormone supplementation, so you must be patient before asking your veterinarian to increase the dose.
  • Your veterinarian will need to check your horse's blood periodically to make sure that the thyroxine (T4) levels have not become too high.

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

  • Weight loss, especially muscle loss, despite a ravenous appetite
  • Nervousness
  • High heart rate
  • Cardiac arrhythmias
  • Weakness and tremors

    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.

  • If your horse has been receiving thyroxine and you need to reduce or eliminate the dose, it is important to do so slowly, because exogenous (meaning from outside the body) thyroid hormone feeds back to the thyroid gland and suppresses the normal production of thyroid hormone. If your horse were abruptly taken off thyroid supplementation, the normal thyroid hormone production would be low from suppression, and your horse would effectively become truly hypothyroid until the thyroid gland kicked back into production.

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