With all the horses that receive thyroid supplementation, you’d think information on thyroid function and disease in horses would be readily available. Not so. In fact, if you consult veterinary textbooks, you may find statements to the effect that horses don’t have thyroid problems.
Practitioners, however, will tell you they can spot a horse that will respond well to thyroid supplements a mile away and give them routinely, often without doing blood work. They’ll also tell you that when they do perform blood tests, odds are the tests will say the horse is normal, while the horse’s response to the hormone tells the veterinarian and the owner otherwise. Who’s right'
Part of the problem with confirming a diagnosis of hypothyroidism (an under-active thyroid) may well be in what we call “normal.” T4 is the thyroid function test most commonly used in horses. Normal range will vary by the laboratory, but can be almost a 400% difference between the upper and lower end. The body is usually a lot more precise than that.
Some of the variability is caused by the time of day a sample is taken, since T4 levels do fluctuate a bit over the course of 24 hours. There have also been reports of breed and use differences. None of this helps you much, though, when you and your veterinarian are looking at the test results for your individual horse. What should your horse’s levels be' Until someone does a systematic study of various breeds, this question will remain unanswered.
Another big problem is the way we test for thyroid function in horses. If a person, or even a dog or cat, was suspected to have hypothyroidism the treating physician or veterinarian would never base their diagnosis only on the result of a T4 test. T4 measures the level of thyroxine, the major thyroid hormone. This is the level the brain “reads” to determine if thyroid hormone level is normal or not.
In people, the TSH (thyroid stimulating hormone) is considered the most diagnostic test since you can have a normal T4 level but still have a sluggish thyroid gland that needs to be constantly prodded into action by the brain secreting TSH.
In small animals, TSH is also looked at, as are free T4--the level of T4 that is not bound to protein in the blood and is therefore available to be read by the cells of the brain--and T3, which is the form of thyroid hormone that is actually biologically active in influencing metabolism. It’s possible to have a person or human that is hypothyroid but has a normal total T4. If you look closer by doing the other tests, one or more of them will uncover the problem and confirm the diagnosis.
What Can Go Wrong
When textbooks say horses don’t have problems with hypothyroidism, what they really mean is that the most common causes of this disease in people and other animals, such as autoimmune diseases of the thyroid, haven’t been seen in horses. However, this isn’t the same thing as saying they don’t exist — it’s just that they haven’t been diagnosed yet. Horses do develop both benign and malignant tumors in the thyroid gland, which become increasingly common with advancing age.
The textbooks will also tell you that these tumors are “not clinically significant,” which means the animals don’t have any glaringly obvious classic symptoms of severe hypothyroidism. It also means, though, that there is no information available on what happened to the thyroid-function tests in animals that have thyroid tumors.
One cause of hypothyroidism that affects horses of all ages, and has been clearly documented in mares and foals born of hypothyroid mares, is iodine and selenium deficiency. Iodine is required for the synthesis of thyroxine, and iodine deficiency is the most common cause of hypothyroidism in man and every animal species worldwide. It’s also one of the most common deficiencies in equine diets across the board.
Selenium can also produce hypothyroidism. Selenium is required by the liver enzyme that converts the inactive form, T4, into the active hormone, T3. A horse with hypothyroidism related to selenium deficiency would be expected to have a normal or even a high T4 but a low T3. Since T3 is rarely tested, there is a good chance that many of the so-called normal horses (by virtue of a normal T4) that nonetheless respond well to thyroid hormone replacement would have been diagnosed if someone had checked their T3 levels.
In backyard, breeding, show and pleasure horses, the symptoms most likely to lead to a suspicion of hypothyroidism are lethargy and easy weight gain. These are fairly classic for hypothyroidism in any species. In racehorses, where low levels of thyroid hormones are known to be common, the picture is a little different. Instead of weight gain, there is often weight loss because these horses have poor appetites and their calorie needs are extremely high. Elevated muscle enzymes and even tying-up, caused primarily by poor utilization of glucose by the muscles in hypothyroidism, is also often seen.
The toughest of all to sort through have been horses and ponies that are overweight, cresty and laminitic. Veterinarians have long suspected they were hypothyroid, despite normal thyroid function tests, but have had variable results in treating them with thyroid hormone. Again, a major part of the problem here is inadequate testing. T4 levels alone just aren’t enough to prove or disprove hypothyroidism. With the recent interest in the role that insulin resistance, a diabetes-like state, may play in weight gain and laminitis, the role of hypothyroidism tends to be ignored.
However, in people and animals, hypothyroidism can actually cause insulin resistance, probably by altering the metabolism of cortisol, which in turn leads to insulin resistance. This is the same mechanism by which classic Cushing’s disease causes insulin resistance, although by different pathways. If it’s not getting confusing enough at this point, you also need to consider that elevated cortisol, as in Cushing’s, can result in hypothyroidism.
If you’re dealing with one of these complicated metabolic problems, it becomes important to sort through all of this because the results will strongly influence the appropriate therapy.
We prefer to finish our articles with a solid conclusion and information you can take home and use. This time it’s just not that simple or easy.
The bottom line is that there is a lot more to be learned about hypothyroidism in horses and how it might relate to other endocrine and metabolic problems. However, one thing’s clear: The next time someone tries to tell you horses don’t get thyroid problems, remember what this really means is that they may not have looked hard enough yet to be able to accurately make that statement.