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Illustration by Kip Carter
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Located in a horse’s throatlatch area below the larynx, the thyroid’s job is to produce thyroid hormone, which is needed by every cell in the body and regulates everything from metabolic processes to organ function.
It’s essentially what controls the pace at which the body operates.
When the thyroid is underactive (hypothyroid), the body slows down; when the thyroid becomes overactive (hyperthyroid), processes speed up. The complex nature of the equine endocrine system isn’t completely understood or fully documented, and thyroid symptoms in horses are different from those in people or dogs. Clinical signs on the outside are often the key to what’s going on with a horse’s internal metabolic processes.
MY, WHAT LONG HAIR YOU HAVE!
The symptoms typically thought of as hypothyroid (lethargic, overweight, non-shedding) are more often symptoms of other diseases or conditions, not an underactive thyroid. Running the proper tests and identifying the correct issues is essential.
* Thyroid Dysfunction is apparently rare in horses as a primary health issue. Some of the signs of a hypothyroid horse, such as coarse hair coat, can also be caused by Equine Cushing’s or similar conditions. A truly hypothyroid horse might also be abnormally sensitive to cold, have a decreased appetite, be intolerant to exercise, and exhibit swelling (edema) in his hind legs.
* Pituitary Pars Intermedia Dysfunction (PPID) has long been referred to as Equine Cushing’s disease. However, in humans with Cushing’s, a different part of the pituitary gland is affected, so many veterinarians now refer to the equine disease as PPID. In PPID, the middle lobe of the gland, or the pars intermedia, can become enlarged due to a benign tumor, over-producing cortisol and compressing adjacent pituitary and hypothalamus structures until they cease functioning.
Clinical signs of PPID in horses include a long hair coat that simply refuses to shed, laminitis, muscle mass loss, pocketed fat deposits in areas such as the neck crest and over the tail head, and excessive sweating, drinking, and urination. PPID also affects glucose and cortisol metabolism. Some horses have been incorrectly diagnosed with PPID when there is no actual tumor; instead, cortisol is being over-produced by the adrenal glands.
* Peripheral Cushing’s Syndrome (PCS), while it has many of the same clinical signs as PPID, has a different cause; it is believed that PCS occurs due to high amounts of circulating cortisol that originate in the liver and/or fat cells of the intestine rather than the pituitary gland.
* Insulin Resistance (IR) symptoms can mimic many of those of PPID and the two conditions can exist concurrently. However this disease, while also hormonally related, is due to the body’s failure to respond properly to insulin. The pancreas releases insulin, which in a normal horse causes the tissues to absorb glucose. In the IR horse, the resting serum insulin levels are elevated, because the pancreas continues to pump out insulin to stimulate glucose uptake. In addition to PPID-like symptoms, an IR horse can be more susceptible to disease.
* Equine Metabolic Syndrome (EMS) includes insulin resistance within its diagnostic set, but it also features the typical fatty deposits on the body mentioned above, as well as the onset of laminitis. EMS is typically managed through a low sugar/starch diet combined with exercise.
Whether your horse’s symptoms are due to thyroid or other metabolic issues, one thing is certain; the thyroid depends on iodine as a key component in its role in the body.
YOU ARE WHAT YOU EAT (AND DRINK)
Iodine is a chemical element, familiar to many horse owners as the ‘-dine’ component in disinfectant shampoos. But, dietary iodine is crucial; a lack of iodine, or the inability to absorb it properly, can be behind a horse’s thyroid slow-down.
A normal horse’s maximum daily iodine intake, as recommended by the National Research Council, should be no more than 3.2 mg per day. Combined iodine intake from ‘dry matter’ such as feed, hay, and supplements should range between 0.1 to 0.6 mg; you can get a good idea of how much your horse is taking in by totaling up the iodine listed on the feedbag and supplement container.
THE LOWDOWN ON IODINE UPTAKE
More is not better when it comes to iodine, since toxicity can result. If your horse is taking in excess iodine through the cumulative effect of multiple oral and external iodine-containing products, including supplements, he can develop iodine toxicity, signs of which include a loss of appetite, lacrimation (weeping from the eyes), scurfing of the skin, and a decreased immune response. Iodine is also transported across the placenta, and can be transferred from mare to foal via nursing, so a pregnant mare who’s received too much iodine can pass it along to her foal, resulting in developmental defects.
Hypothyroid-like symptoms can occur if a horse’s ability to absorb iodine is compromised. There’s some evidence that chlorine and fluoride (part of the ‘halogen’ family of elements, along with iodine) found in urban water sources is able to displace iodine through the effects of ‘halogen displacement,’ where the element with the lower weight will displace another halogen whose weight is greater. Iodine is the heaviest of the three elements, so it’s possible (although not documented) that excessive intake of fluoride and chlorine might inhibit a horse’s iodine uptake.
THE Ts HAVE IT
The thyroid is the only organ in the body with cells able to absorb iodine. When iodine is combined with the amino acid tyrosine, it forms the thyroid hormone thyroxine. Referred to as T4 because one molecule has four iodine atoms, thyroxine is the major hormone secreted by the thyroid.
The amount of T4 secreted is regulated by the pituitary gland’s thyroid stimulating hormone, or TSH. If the pituitary gland detects low levels of T4, it produces more TSH, telling the thyroid to produce more T4. Also part of the complex hormonal feedback loop is the hypothalamus, which releases thyrotropin releasing hormone (TRH) and also has an effect on the pituitary’s release of TSH.
The thyroid and pituitary gland work in tandem, and can turn their functioning on or off depending on the balance of the hormones each is producing. Information from the American Thyroid Association Web site (www.thyroid.org) likens the process to that of a heater (thyroid) and thermostat (pituitary); when the thermostat evaluates temperature, it can turn the heater on or off, depending on whether the temperature is high or low.
T4 is a precursor to T3, or triiodothyronine, which has three iodine atoms and is the more biologically active of the two hormones. Although the thyroid produces some T3, most is manufactured by the body in a ‘T4 to T3 conversion’ that occurs in other organs, such as the kidney, brain, and liver.
NOT A TEST YOU CAN STUDY FOR
Currently, there isn’t a good equine test for TSH, and the ‘free’ levels of T3 and T4 are possibly a better indicator than the baseline levels of the hormones. Testing is conducted via bloodwork; iodine can also be examined this way. Equine thyroid tests may be suggestive but not definitive, and the results can be misleading unless combined with a thorough look at a horse’s clinical symptoms and complete history.
If you do choose to have your horse’s thyroid function tested, keep in mind there are many variables that can affect a test’s outcome, including how recently a horse was exercised, how recently he was fed, and whether he’s recovering from an illness. For example, if a horse has had a debilitating illness, with a lowered basal metabolic rate as an effect of the illness, the thyroid hormone levels can appear low even if the thyroid is actually normal; this is referred to as euthyroid sick syndrome. Bute (phenylbutazone, used as an anti-inflammatory and for pain) has been shown to cause a drop in T3 and T4 within a few days of starting therapy, and for a period after the medication ceases. For all of these reasons, it’s important to provide your veterinarian with a complete and detailed horse care history if you’re having a thyroid test run.
ON THE METABOLIC TRAIL
If you suspect a thyroid or other endocrine or metabolic issue, begin with your veterinarian for an accurate diagnosis. Then, review your hay, feed, water, and supplements to see if there are impurities, deficiencies, or excesses that might contribute to your horse’s symptoms, including iodine levels. You can also evaluate your horse’s training, travel, and competition schedules to determine if stress could be a contributing factor. Taking the time to get to the bottom of thyroid and metabolic disorders will pay off in a clearer picture of the dynamics involved, and a healthier, better balanced horse.
Lisa Kemp is an award-winning writer and marketing consultant for the equine industry. Her definition of a good day is one filled with any combination of horse people, horse images, horse stories, and yes, actual horses. For additional information, visit www.KempEquine.com
American Thyroid Association (www.thyroid.org)
The Horse magazine (www.thehorse.com)
American Association of Equine Practitioners (www.aaep.org)
www.endocrineweb.com for human patient education
The Environmental Working Group (www.ewg.org)
Wedgewood Pharmacy, www.wedgewoodpharmacy.com