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Hypothyroidism in Horses

By: Dr. Melissa Mazan

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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.

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