The myth of equine hypothyroidism

Evidence-based medicine has become the new standard. Making diagnostic and treatment decisions based on cold, hard scientific evidence puts the clinician on much firmer footing both scientifically and ethically, so the argument goes. But it can be disturbing when scientific evidence challenges the way things have historically been done.

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Traditionally, when faced with an obese horse with dry and scaly skin and a long coat that sheds slowly, a slower heart rate, poor performance or a history of infertility and a dull lethargic attitude, most clinicians would turn to blood work analysis to help with a diagnosis. All of these clinical signs lead one to consider a metabolic problem such as Cushing's disease, metabolic syndrome or hypothyroidism. The results of a simple blood chemistry profile might, in fact, confirm a lower than normal concentration of the two major thyroid hormones in horses—triiodothyronine (T3) and thyroxine (T4).

This would have qualified as evidence-based medicine since the clinical signs and blood results support the diagnosis, and horses such as these would be treated with supplemental thyroid hormone and periodic blood monitoring. Often horses treated in this manner respond with more energy, better attitudes and improved body condition, and subsequent testing often shows increased T3 and T4 concentrations—a triumph for science, a healthier patient and another positive case outcome for the esteemed clinician.

The problem with the way that thyroid function has typically been evaluated and the way these horses have been treated is that it does not match what we know and can prove regarding thyroid disorders in horses.

The implications of low thyroid hormone concentrations

"There are no scientific studies to support the assertion that low thyroid hormone concentrations negatively impact the health of horses or that supplementation is necessary," says Nicholas Frank, DVM, PhD, an associate professor at the University of Tennessee College of Veterinary Medicine.

And Franks is not alone in this view. Betta Breuhaus, MS, DVM, PhD, an associate professor at the College of Veterinary Medicine at North Carolina State University, addressed the American Association of Equine Practitioners and explained, "Horses with clinical signs or syndromes traditionally associated with hypothyroidism often have normal thyroid function.

"Thyroid hormone status," adds Breuhaus, "should be carefully evaluated, and other potential endocrine or metabolic problems should be ruled out before a horse is placed on long-term thyroid hormone supplementation."

But what about those low thyroid hormone concentrations often found on blood tests? This is part of the hypothyroidism myth, according to Frank. "The detection of low levels of thyroid hormones in the blood is not the same as hypothyroidism," he says. "The assays used to measure T3 and T4 concentrations are well established and accurate, and the hormones are relatively stable in serum. However, the major issue with measuring thyroid hormones is that concentrations vary over time and within the same individual horse and respond to many different environmental and medical conditions."

The "normal" concentrations for T3 and T4 in horses represent such a wide range that it can be difficult for clinicians to interpret results. For example, a horse may have a T3 concentration of 12 but, because of the wide normal range for T3 concentrations, we don't know if this horse always has a concentration of 12 or if this horse normally has a concentration of 24 and is depleted by 50 percent and possibly hypothyroid.

Additionally, most laboratories test for T3 and T4 concentrations that reflect the amount of hormone bound to carrier proteins such as albumin, as well as the biologically active "free" hormone. Carrier proteins are also affected by numerous medical conditions, creating another source of variability. Many laboratories now offer an evaluation of free T3 and T4 concentrations, and, while better than total hormone concentrations, various diseases and conditions outside of the thyroid gland can also affect these values.

"Common diseases such as viral respiratory infections, digestive disturbances and colic can all affect thyroid hormone concentration," says Frank, "as can different feeding patterns, transport and training stress and changes in body composition such as weight loss or obesity."

This presents an enormous challenge to practitioners. A low thyroid concentration in an individual horse is most likely a reflection of normal day-to-day variability. "Only the detection of persistently low free T3 and T4 concentrations over several days to weeks should cause concern," says Frank, "and only if the horse is free of other medical problems."

Can TSH help?

An additional test available in human medicine is a measure of thyroid-stimulating hormone (TSH). This hormone is produced by the pituitary gland to stimulate the thyroid gland. Elevated TSH concentrations in people with clinical signs of hypothyroidism provide a much more accurate and scientifically valid diagnosis. Response to TSH is also used as an indicator of thyroid function.

"Function testing is necessary to determine whether the thyroid gland is able to respond to stimulation," says Mark Crisman, DVM, a professor at the Virginia-Maryland Regional College of Veterinary Medicine at Virginia Technical Institute. "But TSH for horses is difficult and expensive to produce, making this type of thyroid testing more problematic in the horse."

When horses are injected with TSH and thyroid hormone concentrations are later measured, clinicians can evaluate whether the T3 and T4 levels went up (euthyroid) or remained unchanged (abnormal thyroid response and possibly hypothyroid). Based on the many horses that Frank and others have worked with and tested, it seems that few fail to respond to TSH. "When this more advanced diagnostic test is used," says Frank, "it becomes clear that true hypothyroidism is a very rare endocrine disorder in the horse."

The role of thyroid supplementation

Still, some obese horses with poor coats and lethargic attitudes have low T3 and T4 concentrations and actually seem to do better on thyroid hormone supplementation. How does science account for these animals?

Frank and other researchers acknowledge that these overweight, likely insulin-resistant horses that possibly have metabolic syndrome and those with Cushing's disease and other conditions can show decreased thyroid hormone concentrations. But these animals are adding to the myth of hypothyroidism in the horse. According to Frank's research, "We have never identified a horse with persistently low thyroid hormone concentrations—the same horse will have a normal concentration the next time it is tested." Both Frank and Breuhaus agree that many horses with significant endocrine disorders will have normal thyroid hormone concentrations.

Perhaps the most persuasive piece of scientific evidence showing that hypothyroidism may not even exist as a clinical entity in horses is Frank's work with horses that have undergone a thyroidectomy. These horses have had their thyroid glands surgically removed, so they have no thyroid hormones in their systems. Frank's research showed that even profound hypothyroidism caused by the absence of thyroid glands did not induce obesity, the regional deposition of fat (crest neck or rump), laminitis or other signs typically associated with equine hypothyroidism.

Thyroid hormone supplementation has been commonly used in equine veterinary medicine. It has been advocated for anhidrotic (non-sweating) horses and as a means of increasing fertility in broodmares and stallions. In many cases, both veterinarians and owners would agree that positive test results are seen with the addition of thyroid hormones. Breuhaus designed a study to test the thyroid function in anhidrotic horses, and she concluded that "resting concentrations of thyroid hormone and thyroid stimulating hormone were not different between anhidrotic and control (normal sweating) horses."

In a study, Travis Meredith, DVM, Dipl. ACT, and Ina Dobrinski, Dr.med.vet., MVSc, PhD, Dipl. ACT, looked at 79 Thoroughbred and Standardbred broodmares between the ages of 2 and 22 years of age. T3 and T4 concentrations were measured initially and then four hours after administration of TSH. These mares were bred and pregnancy rates were evaluated.

Overall, 50 of 79 mares had low thyroid hormone concentrations. All mares had elevated T3 concentrations after TSH, and T4 concentrations increased after TSH in all horses except for two mares. The 79 mares had a pregnancy rate of 76 percent, which approaches that for all normal mares, and baseline and stimulated T3 and T4 concentrations were not significantly different between mares that were pregnant and those that were "open."

"These results suggest that decreased thyroid function is not a common cause of infertility," Meredith and Dobrinski say. "The practice of indiscriminately treating broodmares with thyroid hormone to enhance fertility appears questionable at this time."

Frank feels that thyroid hormone supplementation does have some pharmacologic effect on horses, and his research shows that levothyroxine induces weight loss and increases insulin sensitivity. It may be these actions that slim down broodmares and make them healthier and, therefore, more fertile. In fact, broodmares that underwent a thyroidectomy continued to cycle despite the absence of as thyroid gland. These mares had abnormal cycle length but still had follicular activity and ovulated without a thyroid.

Conclusion

These studies seem to indicate that it is highly unlikely that true hypothyroidism exists in horses despite many past years of inaccurate diagnosis and inappropriate treatment. Continued research and advanced diagnostics may further clarify this debate, but for now equine practitioners may have to change their mindsets and cease to see hypothyroidism as a disease requiring intervention.

Dr. Marcella is an equine practitioner in Canton, Ga.