112: When the Thyroid Tests Normal but the Metabolism Doesn't — How Stress and Toxicity Slow Your Metabolic Horse Down

ast week, we explored the liver’s four key metabolic roles in equine metabolic syndrome, including converting T4 into the active thyroid hormone, T3. Today, we’re looking at how thyroid function affects your horse’s metabolism, and why you may still struggle to understand what’s happening when the bloodwork and what you see in your horse don’t match—even when you’re doing everything right. The Thyroid The thyroid helps horses regulate their metabolism, energy, body temperature, coat quality, and hoof growth. It produces T4, which then gets converted into active T3. This conversion happens largely in the tissues, including the liver, and requires nutrients such as selenium. Normal Thyroid Tests If your horse is struggling with weight, low energy, or insulin resistance despite your best efforts, don't assume a normal thyroid test means everything is working as it should. True thyroid gland disease is rare in adult horses, and metabolic problems can exist even when thyroid blood results are normal. So consider your horse's symptoms, history, and overall health, not just the numbers. Slow Thyroid Hormone Conversion Illness, stress, and food scarcity can slow the conversion of T4 into active T3 to conserve energy. This doesn't necessarily mean the thyroid gland itself is faulty. In some horses, the underlying stress may be linked to a major health event, like an infection, or an emotionally stressful experience, such as losing a herd mate or moving to a new environment. Recognising these factors can help you better understand what may be contributing to your horse's metabolic difficulties. Stress Chronic stress can elevate cortisol, which can interfere with thyroid hormone production and the conversion of T4 into T3. Stress is not always obvious. Chronic pain, laminitis, isolation, restricted movement, and a lack of opportunities for the horse to express natural behaviours can be stressful. So, consider these factors when trying to understand why your horse's metabolism is not responding as expected. Over-restricting Feed Horses are designed to chew for much of the day. If your horse finishes its hay within a couple of hours and then goes for long periods without anything to eat, its body may respond as though food is scarce, slowing its metabolism down to conserve energy. While managing sugar intake is important for metabolic horses, further feed restriction may actually work against you. The goal is to manage your horse’s sugar intake while ensuring it has an appropriate feeding plan that allows for regular chewing. A Vicious Cycle Pain creates stress, and stress can worsen any existing problems. For example, a horse with painful feet may experience ongoing stress, and long periods without forage can cause ulcers and create additional discomfort. So, look beyond the most obvious symptom and consider anything else that might be contributing. Identifying the factors driving this cycle is an important step towards your horse’s recovery. The Liver Your horse's liver plays an important role in converting T4 into active T3. When the liver is under pressure, this process may be affected, even if the thyroid gland itself is healthy. If your horse doesn’t respond as expected to dietary changes, remember that metabolic health involves more than one organ, and the liver, thyroid, and insulin regulation are all interconnected. Total Toxic Load Chemical residues from sprayed hay and pasture, fly sprays, topical products, and drinking water can add to the toxic load the liver has to process. Even small chemical exposures can add up over time, increasing the overall burden on the horse’s liver. When the liver is already under pressure, this can affect its ability to carry out its many functions, including converting T4 into active T3. So, look at the bigger picture and consider everything your horse is exposed to, rather than focusing on just one product or source. Supplements More supplements are not always the solution. Iodine, for example, is essential for thyroid hormone production, but too much can suppress thyroid function. Focus instead on proper nutrition, clean ingredients, and nutrients your horse can actually use, rather than assuming that more supplements will produce better results. Nutrient Deficiencies Stress and toxicity can also contribute to nutrient deficiencies, so consider the whole nutritional picture rather than supplementing individual nutrients without reviewing the horse’s diet. Medications Some medications can lower measured thyroid hormone levels, and the effects can persist for several days after the last dose. If your horse is having thyroid tests, ensure that your veterinarian knows which medications it has received to provide some context for the results. Signs to Look For Persistent fat pads, difficulty losing weight, low energy, coat changes, delayed shedding, slow hoof growth, shivering, and slow healing can indicate something is not right. These signs can overlap with other conditions, however, including Cushing's disease, so look at the whole picture rather than simply assuming it's a thyroid problem. Three Drivers of EMS Stress, toxicity, and nutrient deficiencies are three key drivers of equine metabolic syndrome and insulin resistance. They often overlap, so identify which factors may be affecting your horse before deciding where to focus. The Right Order If your horse does not improve, don't restrict its feed or add more supplements. Instead, look at the bigger picture, including liver function, nutrient availability, and stress. Always focus on root causes before symptoms, and remember that no two horses are the same, so no two programs should be either. Links and resources: Connect with Elisha Edwards on her website. Your Horse's Root Cause Reset: Join the waitlist FREE TRAINING: Why Your Metabolic Horse Isn't Getting Better: The Three Drivers of Equine Metabolic Syndrome (EMS) Join my email list to be notified about new podcast releases and upcoming webinars.