Thyroid Dysfunction

Most people are familiar with thyroid malfunctions, most commonly either thyroid cancer or inadequate thyroid gland function, as either hypothyroid (under activity) or hyperthyroid (excess function).

Many obese people have their thyroid gland removed and spend their lives taking exogenous thyroid medication to regulate their metabolism. Sometimes obese horses (the ones that “gain weight on air” or exhibit a general poor hair coat and low energy level, causing us to suspect the thyroid gland, based on human hypothyroid symptoms. But is the equine thyroid really the culprit? It’s not always clear.

Unfortunately, equine medicine still has relatively little information about the thyroid, although We’ve come a long way in the last three decades. So, if you’re concerned about thyroid (dys)function in your horse, read on and we’ll get you headed in the right direction.

A LITTLE GLAND, WITH BIG JOBS. The thyroid gland’s purpose is to capture iodine from the blood stream and use it to produce thyroid hormone, which plays an important role in the body.

During fetal development, thyroid hormone governs organ development. In young animals, it heavily influences growth and joint/ bone development. In addition, thyroid hormone is instrumental in the body’s ability to generate heat, also termed thermogenesis. Perhaps most familiar is that thyroid hormones influence energy metabolism, the rate at which the body burns calories so that cells can function too slow or too fast can cause serious physical problems.

A CLOSER LOOK. Back in the 1980s and ’90s, hypothyroidism was often diagnosed in horses. These diagnoses, unfortunately, were most likely inaccurate, but don’t blame the veterinarians. Back then, we didn’t fully understand many of the details of equine metabolism. To make matters worse, tests for thyroid function were few and produced varying, somewhat unreliable results. So, in some cases, vets just examined horses and made a clinical diagnosis, based on physical observations, that the horse had a thyroid disorder. Today, we have a better understanding of other factors that can be in play when a horse presents with an endocrine or metabolic issue. SeeThyroid Look-Alike Conditions.

We know that several different metabolic diseases present with the same physical signs, including Cushing’s disease, equine metabolic syndrome and poor management. We also understand more about the nuances of testing for various diseases, so our interpretation of test results is much more intricate than in previous years.

TESTING. Thyroid tests still are laden with caveats, all of which limit test result accuracy. We all place trust in blood-test results, perhaps because they come out of a machine. But the fact is, machines can make a lot of mistakes, especially when many variables can influence the outcome of their analysis. With thyroid function tests, the following factors can heavily skew results to paint an inaccurate picture of thyroid health in a horse:

1. Age: Young horses have significantly higher circulating thyroid hormone than adult horses.

2. Season: Cold temperatures stimulate the thyroid gland while warm temperatures inhibit it.

3. Time of day the sample was drawn: Thyroid hormone levels generally spike in the late afternoon and are their lowest between midnight and 4 a.m.

4. Use of anti-inflammatories at the time of testing: Horses receiving corticosteroids will have suppressed thyroid function, and horses receiving NSAIDs, such as firocoxib (Previcox, Equioxx) or bute will have skewed test results due to NSAID interference with thyroid hormone binding to carrier proteins in the blood.

5. Concurrent disease: Stress, systemic inflammation (such as infection), concurrent chronic disease, and starvation all suppress thyroid function.

6. Activity level: Horses in training have more circulating thyroid hormone.

7. Sampling time in relation to feeding: Animals eating while being sampled tend to have higher thyroid hormone levels.

8. Exogenous compounds: Certain plants contain the chemicals called goitrogens that inhibit thyroid function by altering iodine metabolism. These plants include:




Greens, such as cabbage, mustard greens and kale.

Although feeds such as soybean and linseed meal can decrease thyroid function, most equine diets of hay and pasture grass (other than in the Pacific Northwest) contain far more than the daily required level of iodine, so there’s little to worry about if you’re feeding a product containing one of these plants.

In addition, the administration of certain medications such as bute and acepromazine can also lower thyroid hormone levels.

Finally, dietary imbalances in daily iodine intake (too much or too little) will cause imbalances in thyroid function.

Each of these factors must be considered when your horse is tested for thyroid function. If your horse exhibits signs of thyroid problems or routine test results come back abnormal, these eight factors should be considered as culprits. See Thyroid sidebars.

THE PREVALENCE OF THYROID PROBLEMS. The truth is, bone fide thyroid problems in horses are rare. If a horse does have a thyroid problem, hypothyroidism is the most prevalent form of dysfunction. This can occur if the horse has an iodine imbalance, which can be brought on by low or high iodine intake or by the ingestion of plants that prevent the body from using iodine.

Hyperthyroidism has only been documented in a few scattered instances. Actually most veterinarians will go through their entire career and never encounter one. Those few cases were ones in which the horses displayed clinical signs consistent with the condition, which resolved once the thyroid gland was surgically removed.

Finally, in older horses, thyroid adenomas can occur. These are benign tumors that grow usually on one of the two lobes of the thyroid gland. The average onset is between 16 and 18 years old with increasing prevalence as the horse ages. While they do enlarge the thyroid, they don’t disrupt its function. So, as alarming as they may be to owners, thyroid adenomas seen in geriatric horses are noted as an incidental finding by veterinarians since they don’t harm the horse.

BOTTOM LINE. Fortunately, equine thyroid problems are rare and of greater concern in growing animals than they are in adults.

However, if you suspect a thyroid issue, don’t just grab an over-the-counter thyroid supplement. Your horse’s problem could be coming from many different steps in the thyroid’s activity process or, most often, he’s suffering from another problem. Not only does the empirical administration of a thyroid supplement not treat those diseases, but it can make them worse.

As much as we try to creatively educate readers about how they can manage their horses effectively on their own, this is one article where the only sound advice we can offer is: Call your veterinarian if you’re worried about thyroid issues in your horse.

Article by Contributing Veterinary Editor Dr. Grant Miller.

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