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Hypothyroidism in Horses

By: Dr. Melissa Mazan

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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.

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