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I recently uploaded a new lecture to our online education courses; Equine Metabolic Syndrome.  I worry a little that the while the subject matter is one that most of my readers and students should know, the name may be confusing. Do you know what Equine Metabolic Syndrome means? Do you know about Insulin Resistance and what it means for your horses and their health?  It's when an overweight or obese horse develops a condition related to the obesity. They have abnormal fat deposition, exercise intolerance (lethargic and out of shape) and can develop serious medical conditions like laminitis. It's a little bit like Type II Diabetes in humans, but not close enough to use the same nomenclature (cause that would make it too easy for all of us).  While its something that has been going on in horses for as long as we’ve been putting them in stables and supplementing their feed but its something the scientific community has only been investigating for the last handful of years. And because the syndrome is old but the research is new, there is a lot of misinformation out there about why horses are fat and what we can do to fix it. One of the myths I have the hardest time debunking is that of Hypothyroidism. When people tell me their horse is hypothyroid I think of that line from A Princess Bride, “You keep using that word. I do not think it means what you think it means.” So let me tell you the schpeal that I give clients and other vets who call me for advice.

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Hypothyroidism in a dog.
Back in the day, well not too long ago, people would notice that they had a horse that had become overweight and lethargic and they wanted to know why. In people and in dogs hypothyroidism is a real, documented disease and is somewhat common. Humans with thyroid dysfunction are fatigued, weak, irritable and gain weight, or have trouble losing weight. Dogs will become dull and listless, reluctant to engage in their normal activities and will gain weight despite no change in appetite or feedings. So it would only make sense that if your horse was now dull and pudgy that hypothyroidism would be the answer. So we started testing horses thyroid hormone levels and guess what? Sometimes they were low. ">Problem solved!!!  And thus hypothyroidism was blamed for a variety of equine problems such as obesity, laminitis, anhidrosis (inability to sweat), recurrent rhabdomyolysis (tying-up) and poor fertility.  But here’s the problem with that theory…. Thyroid hormone levels that we measure in the blood may not tell us about true thyroid function. And who cares what the numbers from the lab say if the thyroid gland itself is functioning properly and doing its job to aid in growth and regulation of metabolism? We also know that certain medications (Bute & dexamethasone), strenuous exercise and diets high in energy, protein, zinc and copper will falsely alter the levels of circulating hormones in the bloodstream. The only way to know about how the thyroid functions is to perform thyroid function tests, which are rarely done in the field because the medications needed to stimulate the thyroid (to prove to us that its working) are not readily available. But when horses that are showing the “classic” signs of hypothyroidism, and have low thyroid hormone levels are tested for true thyroid function, they are found to be normal. Researchers have even tried to prove this association by removing horse’s thyroid glands to see what happens. Well what doesn’t happen is they do not become obese and they do not develop laminitis.

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“But I don’t understand!” you say. “I had a hypothyroid horse and we treated her and she got better! Was my vet wrong?” Well, yes and no. For all those years, we have been treating your horses correctly, using all of the knowledge that we knew at the time, but we were doing it for the wrong reasons. It has only been within the last few years that the veterinary community has come to define Equine Metabolic Syndrome. EMS is an endocrine disease of horses that is characterized by insulin resistance, meaning your horse’s cells don’t respond to insulin they way they should and the energy (glucose) they consume can’t be metabolized properly. It’s similar to Type II Diabetes in humans, in that it is often a result of obesity, rather than a cause. Some horses are more predisposed to being overweight, “easy keepers.” Obesity leads to insulin resistance, which in turn makes weight loss even harder to achieve. Horses with EMS are more prone to episodes of laminitis because of alterations in glucose and insulin in their blood streams. Many “hypothyroid” horses that have normal thyroid function will test positive for EMS and insulin resistance.

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“So why does treating with thyroid medication make these horses better if they don’t have a thyroid problem?” you say. Excellent question! Well thyroid supplementation causes an increase in metabolism. By increasing the horse’s metabolism they are able to loose weight and correct their insulin resistance. It works so well that researchers are now recommending treating these types of horses with a much higher dose of thyroid supplementation, a “supraphysiologic dose.” So while we have been doing the right thing all along, we only now understand why it was working. It is important to understand one other big difference. True hypothyroidism, in dogs and people, will require life long treatment. Horses who have been successfully treated for EMS do not, and unnecessary treatment with thyroid supplementation over extended periods of time is not only a waste of money, but also a potential health risk.

   So if you have a horse that is slow, sluggish and fat, or one that has been previously diagnosed as hypothyroid, learn more about Equine Metabolic Syndrome by taking our online course and talk to your vet about what you can do to test and treat this very common condition.

 

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