🩺 Vet’s 2025 Insight: Safe Use of Thyroid Supplements in Horses 🐎

Thyroid supplementation in horses is a widely used strategy, especially in cases involving equine metabolic syndrome (EMS) or Cushing’s Disease (PPID). But as with any medical intervention, there are potential risks that need to be carefully weighed. In this comprehensive 2025 guide, Dr. Duncan Houston outlines when thyroid medications can be beneficial, what dangers may arise—especially in performance horses—and how to approach supplementation safely. 💊🐴

🧠 Why Consider Thyroid Supplements for Horses?

Unlike dogs or cats, horses rarely develop true hypothyroidism. However, thyroid hormone supplementation—typically with levothyroxine sodium—is often prescribed for other reasons, particularly to assist with weight loss and manage insulin resistance. 📉🍎

💡 Common Use Cases:

• 🐎 Obese horses with difficulty losing weight
• ⚠️ Horses suffering from laminitis linked to EMS
• 📉 To decrease insulin resistance and improve metabolic function

In these cases, levothyroxine acts by boosting the horse’s basal metabolic rate, helping them burn calories more efficiently and improving their responsiveness to insulin. This can be crucial for horses at risk of founder (laminitis). 🔥🦶

📊 Metabolic Syndrome and Cushing’s Disease

Two primary endocrine disorders where thyroid supplementation might be considered are:

1. 🍩 Equine Metabolic Syndrome (EMS)

• 🚫 Characterized by insulin resistance, regional fat deposits, and a tendency to become overweight
• 🧬 Has a genetic component, more common in certain breeds like Morgans and Paso Finos
• 🥬 Managed through diet, exercise, and sometimes levothyroxine

2. 🧠 Pituitary Pars Intermedia Dysfunction (PPID or Cushing’s Disease)

• 👴 Common in older horses
• 🧪 Often causes elevated cortisol and insulin resistance
• 💊 Treated with pergolide, but some horses also benefit from thyroid support

In both syndromes, metabolic regulation is impaired, making these horses more prone to laminitis. The goal is to improve metabolic health while minimizing the risks of supplementation. 📈⚖️

⚠️ The Risks: Thyroid Medication and Atrial Fibrillation

While levothyroxine can provide real benefits, new 2025 veterinary research is reshaping how we use it—especially in performance horses. A recent Journal of Veterinary Cardiology study linked thyroid supplementation with atrial fibrillation (AFib), a serious heart rhythm abnormality. 🫀🚨

📚 Key Findings from the Study:

• 🐴 In a group of 23 horses with AFib, 60% had elevated thyroid hormone levels
• 💓 These horses also had higher resting heart rates
• 📈 Elevated systolic blood pressure and thickening of the heart’s left ventricular wall were noted
• 🧬 Hyperthyroidism may be a predisposing factor for arrhythmias in horses—just as it is in humans

This data suggests that over-supplementation—or unnecessary supplementation—could increase the risk of cardiovascular issues in otherwise healthy horses, especially those in training or competition. 🏇💔

🏁 What Horses Should Not Receive Thyroid Medication?

Thyroid supplementation should not be given to:

• 🏇 Actively competing or high-performance horses
• ❤️ Horses with pre-existing cardiac concerns
• 🧪 Horses with normal insulin levels and no signs of EMS or PPID

In these horses, the risks of arrhythmia outweigh any metabolic benefits. A “boost” in metabolism is not a valid reason for medication unless clear endocrine pathology is present. ❌⚠️

📋 Safe Guidelines for Use in 2025

🔬 Step 1: Diagnose First

Only consider levothyroxine supplementation after diagnosing EMS or PPID via blood tests:

• 🩸 Fasting insulin and glucose
• 📊 ACTH testing for Cushing’s Disease
• 🏋️ Evaluation of body condition and fat deposits

💊 Step 2: Dose Carefully

• 🧮 Start with the lowest effective dose (typically 48 mg/day for a 1,000 lb horse)
• 📉 Gradually taper off after weight loss goals are met
• 📅 Limit duration of use—commonly 3 to 6 months

🧪 Step 3: Monitor Frequently

• 📈 Recheck insulin and glucose every 4–8 weeks
• 🩺 Monitor heart rate and blood pressure regularly
• 🧠 Evaluate temperament and energy levels for signs of over-supplementation

💡 Alternatives to Thyroid Supplementation

In many cases, lifestyle changes alone can be just as effective—without the risks. Consider:

🥕 Nutritional Adjustments:

• 🌿 Feed low-NSC hay (<10%)
• 💧 Soak hay to reduce sugar content
• 🥬 Provide a low-carb ration balancer with vitamin E and selenium

🏃 Exercise and Turnout:

• 🚶 Encourage regular movement (even hand-walking)
• 🐴 Use controlled turnout with a grazing muzzle

📱 Digital Monitoring with Ask A Vet

• 📝 Track weight, supplements, and energy levels
• 📸 Upload photos of body condition for virtual vet review
• 💬 Message a vet instantly for dosage guidance

🟢 Download the Ask A Vet app to make managing EMS and PPID easier and safer than ever.

🧠 Conclusion: Use Caution, Not Fear

Thyroid supplementation can be a helpful tool in specific situations, but it should never be used casually or as a performance enhancer. In 2025, we know more than ever about the cardiac risks involved—particularly atrial fibrillation—and it’s clear that careful screening and oversight are essential. 🫀📉

Every horse is unique. Work closely with your veterinarian or connect through AskAVet.com to decide if thyroid support is right for your horse. 🐎🩺