Thyroid Health in Horses: Can Low Thyroid Cause Tying-Up?

Understanding Thyroid Function in Horses

The thyroid gland plays a critical role in regulating a horse’s metabolism, energy production, muscle function, and overall performance. It produces two key hormones—T4 (thyroxine) and T3 (triiodothyronine)—which influence how efficiently the body uses energy at the cellular level.

When thyroid hormone levels are too low, a horse’s entire metabolic system can slow down. While true primary hypothyroidism is relatively rare in adult horses, functional or secondary low thyroid levels are commonly observed and can contribute to performance issues, weight problems, and muscle disorders—including tying-up.

What Is Tying-Up in Horses?

Tying-up, clinically referred to as Exertional Rhabdomyolysis (ER), is a muscle condition where muscle fibers break down during or after exercise

. Horses with tying-up may show:

Muscle stiffness or cramping

Reluctance to move

Sweating excessively

Elevated heart rate

Dark or coffee-colored urine

Pain in the hindquarters or back

Tying-up can range from mild to severe and often has metabolic, nutritional, hormonal, or genetic components.

How Low Thyroid Function Can Contribute to Tying-Up

Low thyroid hormone levels can increase a horse’s risk of tying-up through several interconnected mechanisms:

1. Impaired Muscle Energy Metabolism

Thyroid hormones regulate how muscles convert glycogen (stored sugar) into usable energy. When thyroid levels are low:

Muscles may store excess glycogen

Energy release becomes inefficient

Lactic acid can accumulate faster

This creates the perfect environment for muscle fatigue and breakdown during exercise.

2. Reduced Mitochondrial Efficiency

Thyroid hormones support mitochondrial function, which is essential for muscle endurance. Low thyroid levels can reduce oxygen utilization in muscle cells, causing muscles to fatigue and cramp more easily—especially during work.

3. Altered Insulin Sensitivity

Low thyroid function can negatively affect insulin regulation, increasing the risk of abnormal glucose handling. This is particularly relevant for horses prone to:

Polysaccharide Storage Myopathy (PSSM)

Recurrent exertional rhabdomyolysis (RER)

Both conditions are strongly linked to tying-up.

4. Muscle Weakness and Poor Recovery

Horses with low thyroid levels often experience:

Muscle soreness

Slow post-exercise recovery

Decreased tolerance for regular training

Over time, this chronic muscle stress increases the likelihood of repeated tying-up episodes.

Signs of Low Thyroid Function in Horses

Symptoms of low thyroid levels can be subtle and are often mistaken for poor conditioning or training issues. Common signs include:

Recurrent tying-up

Weight gain or difficulty losing weight

Lethargy or low energy

Poor topline or muscle tone

Cold intolerance

Dull or coarse coat

Reduced performance

Diagnosing Thyroid Issues in Horses

Thyroid function is typically evaluated through blood tests measuring:

Total T4

Free T4

T3 (in some cases)

However, thyroid levels can be influenced by diet, stress, illness, and exercise, so interpretation should always be done by an equine veterinarian in context with clinical signs.

Managing Low Thyroid and Preventing Tying-Up

If low thyroid function is suspected or confirmed, management may include:

✔ Balanced Nutrition

Reduce excessive non-structural carbohydrates (sugars and starches)

Ensure adequate protein and amino acids

Provide sufficient selenium and vitamin E (as advised by a vet)

✔ Thyroid Support or Medication

Synthetic thyroid hormone (e.g., levothyroxine) may be prescribed short-term

Used carefully to avoid suppressing natural thyroid function

✔ Consistent Exercise

Regular, low-stress conditioning

Avoid long periods of stall rest

Gradual increases in workload

✔ Address Underlying

Metabolic Issues

Insulin resistance

PSSM or RER

Electrolyte imbalances

Final Thoughts: Thyroid Health Matters for Muscle Function

While low thyroid function is not the sole cause of tying-up in horses, it can be a significant contributing factor, particularly in horses with recurrent or unexplained muscle issues. Supporting thyroid health is an important part of managing equine metabolic health, muscle performance, and long-term soundness.

If your horse struggles with repeated tying-up episodes, evaluating thyroid function alongside diet, training, and genetics can provide valuable insights—and potentially life-changing relief.

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