Understanding the Thyroid–Tying-Up Connection in Mares

Introduction

When a performance mare suddenly develops stiffness, reluctance to move, or muscle cramping after exercise, we often think of classic muscle disorders like Polysaccharide Storage Myopathy (PSSM) or Recurrent Exertional Rhabdomyolysis (RER). But another less obvious contributor might be endocrine dysfunction — namely, the role of the thyroid gland. In this article we’ll explore what the thyroid does in the horse, how “tying-up” (exertional rhabdomyolysis) manifests in mares, what evidence exists for a thyroid-tying up correlation, and what practical management strategies you can consider.

---

What the Thyroid Does in the Horse

The thyroid glands produce hormones (primarily thyroxine [T4] and triiodothyronine [T3]) which regulate metabolism, energy use, thermoregulation, growth and muscle function.

According to the article “The Thyroid Gland: The Horse’s Powerhouse,” the thyroid has been implicated in problems such as infertility in mares, anhidrosis and — importantly — exertional rhabdomyolysis (“tying-up”).

However, true primary hypothyroidism (i.e., thyroid gland fails on its own) is reported to be quite rare in horses.

For broodmares, a study found that baseline and stimulated T3 and T4 concentrations did not significantly differ between mares that became pregnant versus those that didn’t — suggesting that poor thyroid function is uncommon in adult broodmares.

In short: the thyroid is biologically plausible as a contributor to muscle and metabolic issues, but overt thyroid disease in adult horses appears to be relatively rare.

---

What is “Tying-Up” (Exertional Rhabdomyolysis) in Mares?

The term “tying up” refers to a group of muscle problems in horses, often after exercise, where muscle fibres break down (rhabdomyolysis) leading to stiffness, reluctance to move, sweat, muscle spasms, sometimes dark urine. Key points:

One form, RER, often occurs in fit, nervous horses (especially females/mares) when they are excited or stressed.

Another form, PSSM, is primarily a metabolic/glycogen storage disorder (genetic) and causes tying-up in many breeds.

Management factors matter: diet (high starch, sugar), irregular exercise, stall rest, electrolyte imbalance all increase risk.

So when a mare ties-up, it's often a metabolic/muscular issue — but could endocrine factors (such as thyroid dysfunction) increase risk or aggravate the condition?

---

What Evidence is There for a Thyroid–Tying-Up Link?

Let’s summarise what we do know — and what remains speculative — about the thyroid’s role in tying-up in mares.

Supporting observations:

An older article noted that thyroid problems could be a cause of chronic tying-up. For example, “Problems of the equine thyroid” listed “muscle problems, including tying up” as a sign of hypothyroidism in adult horses.

The “The Thyroid Gland: The Horse’s Powerhouse” article explicitly mentions exertional rhabdomyolysis (tying-up) among problems blamed on the thyroid in horses.

The fact that thyroid hormones influence metabolism, muscle activity, energy usage gives a plausible mechanism: if thyroid hormone is low, muscles may not receive enough metabolic drive or may not recover effectively.

Counterbalancing/limiting evidence:

The broodmare thyroid-pregnancy study found no significant difference between pregnant vs non-pregnant mares in stimulated T3/T4. That suggests that significant thyroid dysfunction in broodmares is uncommon.

True hypothyroidism appears rare in adult horses.

Many tying-up cases have well-documented causes such as PSSM, RER, nutrition/exercise mismanagement, which do not require thyroid dysfunction to explain them.

What this means: There is an association and biological plausibility for a thyroid-tying-up link — especially as part of multifactorial risk. But thyroid dysfunction alone is unlikely to be the primary cause of tying-up in most mares.

---

Why Mares in Particular?

You asked about mares — here are some thoughts on why mares might feature in the thyroid–tying-up conversation:

Many tying-up resources note that females (mares) are more commonly affected with RER than males.

Mares may have additional hormonal fluctuations (estrus cycle, pregnancy, lactation) which could stress metabolic systems, making them more vulnerable to muscle breakdown when other risk factors (diet/exercise) present.

If a mare has sub-clinical thyroid insufficiency (low metabolism, slower clearance of metabolites, muscle fatigue) then when stressed (training, pregnancy, turnout changes) the risk of tying-up may increase.

---

Practical Signs to Watch For in Mares

If you're managing a mare and suspect a possible thyroid-muscle link, look for the following signs:

Recurrent episodes of stiffness, muscle cramping, reluctance to move after exercise, especially hindquarters/back. (Classic tying-up signs.)

Additional signs of low thyroid/metabolic rate: dull coat, poor shedding, cold intolerance, slow heart rate, exercise intolerance.

Inconsistent tying-up with no obvious cause (diet/exercise look correct) — could prompt evaluation of endocrine/metabolic status.

Mares with multiple metabolic/endocrine challenges (e.g., history of insulin dysregulation, obesity, repeated pregnancy/lactation stress) may deserve extra attention.

---

Management Strategy: What You Can Do

Given the multifactorial nature of tying-up and the possibility of thyroid involvement, here is a management blueprint:

1. Veterinary evaluation:

If tying-up is recurrent, have your veterinarian check muscle enzyme levels (CK, AST) during/after an episode.

Evaluate thyroid status: T4, T3 (and possibly TRH/TSH responses) though interpret with caution (as test variability exists in horses).

Rule out other primary causes: PSSM, RER, electrolyte imbalance, diet/starch overload.

2. Optimize diet & exercise (many tying-up cases are diet/exercise driven):

Reduce starch/sugar intake and increase forage/fat calories (especially helpful in PSSM-type cases).

Ensure consistent turnout and avoid unpredictable rest–work cycles (which predispose to “Monday morning” tying-up).

Warm-up and cool-down properly. Slow transitions between work and rest.

Monitor electrolytes (sodium, potassium, magnesium) especially if heavy sweating occurs.

3. Consider thyroid support (with vet guidance):

While true hypothyroidism is rare, if your mare shows signs of low thyroid function + tying-up risk, your vet may recommend trial treatment or thyroid monitoring.

Note: Supplements/thyroid hormone therapies should only be used under veterinary supervision; misuse can cause harm.

Address dietary goitrogens (plants that interfere with iodine uptake) especially if the mare is pregnant/lactating.

4. Monitor & adjust:

Keep records of tying-up episodes, diet changes, exercise/rest cycles, and any endocrine/metabolic work-ups.

Review changes in coat, body condition, energy levels — sometimes improvements in thyroid/metabolic health show up gradually.

Maintain regular veterinary checks, particularly for performance mares under stress or breeding/lactation regimen.

---

Key Take-aways for Mare Owners

The thyroid can play a role in tying-up, but it is seldom the sole cause. Think of it as one piece of the puzzle, not the only puzzle piece.

In performance mares, recurrent muscle issues warrant looking at the big picture: diet, exercise, muscle metabolism (PSSM/RER) and endocrine/metabolic health.

Good management (consistent exercise, low-starch diet, electrolyte support) remains foundational. Evaluating thyroid status is an added layer when tying-up is recurrent and unexplained.

Always work with your veterinarian or equine nutritionist for diagnosis and treatment—avoid self-treating with thyroid hormones or unverified supplements.

---

Final Thoughts

For a mare experiencing tying-up, it’s tempting to look for a “silver bullet” explanation. While thyroid dysfunction is a plausible contributor — especially in mares with other metabolic/hormonal stresses — the evidence suggests it is rarely the primary driver. A holistic approach combining muscle metabolism management, nutrition, exercise, and endocrine evaluation offers the best chance of reducing episodes and maintaining performance and health.

ClickHere: Buy Thyroid