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Hypothyroidism in Horses

By: Dr. Melissa Mazan

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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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