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Innovative Solutions to Animal Health Problems


 Reprinted from Horse Illustrated

Is Your Horse Hypothyroid?

This hormone disturbance can wreak havoc with your horse’s body.

"I think my mare is just showing her age," the woman said to her veterinarian. "She’s been slowing down, gaining weight, and she didn’t shed out as usual this year. But since she needs her vaccinations anyway, could you take a look at her?"

The mare had indeed gained weight. Although the horse was middle-aged, there was no sign of arthritis and no obvious explanation for her lethargic attitude. Because of her cresty neck, rough hair coat and lack of vitality, the vet suspected that the mare could be suffering form hypothyroidism.

The thyroid gland is located in the throat latch area of the horse’s neck. Small though it is, this gland is responsible for regulating the body’s metabolism. Every organ in the body is affected by the thyroid hormones.

Skin and hair are also affected by thyroid hormones. The body’s normal response to changing daylight is to shed hair each spring. Low thyroid hormone levels reduce this response. The hypothyroid horse or pony has a rough coat that sheds late, if at all.

Thyroid hormones also regulate nerve function. Normal levels allow for usual brain activity. When levels are decreased, slower reflexes and lowered sensitivity results. Thus, the hypothyroid horse may appear dull or lazy.

Regulation of body temperature is an essential function of thyroid hormones. Normal horses have an increase in thyroid activity during the winter to maintain their normal body temperature. If the gland can’t produce enough hormones, the horse becomes more sensitive to cold.

Some muscle disorders may be traced to low thyroid hormone levels. Muscular weakness or lack of muscle tone may be seen. Certain cases of tying up may be due to hypothyroidism, although other causes of this syndrome are much more common.

The reproductive system also relies on thyroid hormone for normal function. Reproductive failure is a sign of hypothyroidism in many species. Lowered milk production may occur in hypothyroid mares after foaling.

Normal growth is dependent on thyroid hormones as well. Hypothyroid foals may be work weak, with bone abnormalities or limb deformities. Common problems include jaw malformations and deformities in the hock joints. The foal may fail to thrive or may have stunted growth.

Founder is another disorder believed to be related to the thyroid gland. Founder is a painful condition that affects the tissues between the coffin bone and the hoof wall. In severe cases, the coffin bone loses its attachments and rotates of sinks out of position.

Researchers have not been able to definitely link hypothyroidism with founder. Not all hypothyroid horses will founder, and not all foundered horses are hypothyroid. Yet the body type that veterinarians associate with both diseases is similar.

Many veterinarians suspect that horses and ponies with a "hypothyroid body type" may be predisposed to founder. The horse or pony that is overweight and has a cresty neck (one that has an upward curve rather than a straight line) is one that may be predisposed to founder. Often thyroid hormone is prescribed for the horses with favorable results.

Signs of Hypothyroidism


*Intolerance to cold

*Slow shedding or lack of shedding coat in spring

*Long, rough coat

*Overweight, "easy keeper" with "cresty neck," or sometimes underweight with poor body tone

*Lethargic, dull attitude

* Muscle disorders: weakness, lack of muscle tone, or tying up


*Lack of or irregular heat cycles in mares

*Lack of milk production in foaling mares

*Foals: prematurity, weakness, growth abnormalities

Note: The presence of some or all of these signs may suggest hypothyroidism. The signs could also be present with other disorders. Consult your veterinarian.


Hypothyroid horses won’t show all of the above signs. Seeing two or three signs in combination, however, may lead you and your veterinarian to suspect a problem.

Blood tests are needed to diagnose hypothyroidism, and several different tests are suggested. A complete blood count (CBC) and serum chemistry profile must be done. These tests evaluate the horse’s overall health and the condition of its internal organs. The first way to measure thyroid function is to test the levels of thyroid hormone in the horse’s blood. If the levels are low, veterinarians often suspect that the horse is hypothyroid.

It seems, at first, that finding a low hormone level would give a definite diagnosis. Unfortunately, that’s not the case. Many different diseases besides hypothyroidism can cause a lowering of thyroid hormone levels. Only when that specific disease is treated will the thyroid hormone level return to normal.

What’s more, a high protein level in the diet can result in decreased thyroid hormone levels. And many drugs can cause the level of thyroid hormone to drop. For instance, giving a horse phenylbutazone ("bute") for one week is enough to cause its thyroid hormone level to drop into the abnormally low range.

Since measuring thyroid hormone levels alone won’t give a definite diagnosis, further testing must be done. The CBC and chemistry profile help show whether another disease is  present. The vet should take note of any medications that the horse has received recently, and it may be necessary to wait to measure the thyroid hormone level until medication is discontinued for at least 10 days.

Another, more specific test to measure thyroid hormone levels, known as the TSH stimulation test, can be done. This test measures the horse’s thyroid hormone levels before and after a drug "Thyroid stimulation hormone" is given. This tests the function of the gland itself, rather than a simple measure of the hormone. The before-and-after blood samples are taken three to six hours apart, so you will either need to drop your horse off at the vet’s for the day or have your vet make two barn visits in one day.

TSH will cause the normal thyroid gland to produce more thyroid hormone so that the "after" level is higher that the "before" level. In the true hypothyroid horse, the thyroid hormone levels will be similar both before and after TSH is administered because the gland is not functioning normally.

Sometimes veterinarians find a normal response to the TSH stimulation test in spite of low baseline hormone levels. In those cases, the vet will look for another cause of the low levels. If no other treatable cause is found, thyroid treatment may still be given. This treatment often yields favorable results in spite of the fact that the thyroid gland itself seems to be in working order.

Back to Normal

Fortunately, treatment of hypothyroidism is simple and inexpensive. Thyroid powder is added to the horse’s feed every day to supplement what is lacking in its body. Sometimes the dose has to be adjusted to see the most benefit, since hypothyroid horses have varying degrees of the own hormone present in the body. Horses with very low inherent levels will need a higher dose.

Many horses do very well with daily treatments, while others require twice-daily medication. Treatment is lifelong, but the medication is easy to add to the grain and doesn’t pose a big problem. The horse usually responds within three to six weeks with an improved attitude and appearance.

Does your horse show some of the signs mentioned here? Whether your horse has true hypothyroidism or another problem, it’s worthwhile to pursue a little diagnostic work to find the cause.


The author has written numerous articles on equine health care. She practices in the state of Washington.

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