Thyroid Supplementation in Horses: Understanding the Link Between Hormones, Fillies, and Tying Up

In the world of equine performance and care, thyroid function isn't always the first thing that comes to mind. However, for many fillies and mares, an underactive thyroid could be the silent culprit behind performance issues, behavior changes, and a frustrating condition known as tying up.

Let’s explore why thyroid health matters, why young females might be more vulnerable, and how smart supplementation could make a big difference.

---

What is Thyroid Function in Horses?

The thyroid gland produces hormones—mainly T3 (triiodothyronine) and T4 (thyroxine)—that regulate metabolism, energy production, and overall cellular function. When the thyroid isn’t producing enough hormones (hypothyroidism), a horse might appear lethargic, have trouble building muscle, gain weight easily, or have a poor coat. In performance horses, especially young, athletic females, these issues can be more than cosmetic—they can directly affect performance and recovery.

---

What is Tying Up?

Tying up, or exertional rhabdomyolysis, is a muscular disorder seen primarily in high-performance horses. Symptoms include:

Muscle stiffness or cramping

Sweating

Reluctance to move

Elevated heart rate and respiration

Dark urine (due to muscle breakdown)

This condition is especially common in fillies and mares, often between 2-4 years old, and is influenced by genetics, nutrition, and hormone balance.

---

The Link: Why Fillies and Mares?

Fillies and mares are more prone to hormonal fluctuations, especially around the estrous cycle. These fluctuations can influence not only mood and energy levels but also thyroid activity. Low thyroid function, in turn, affects how muscles metabolize energy. When energy pathways aren't functioning optimally, muscles become more prone to cramping and breakdown under stress.

In some cases, low thyroid hormones may exacerbate a horse’s genetic predisposition to tying up—especially in breeds like Thoroughbreds, Standardbreds, and Quarter Horses used for high-intensity work.

---

Signs Your Mare Might Be Low in Thyroid Function:

Prone to tying up after light work or stress

Unexplained lethargy or laziness under saddle

Difficulty maintaining muscle along topline

Seasonal or cyclic behavioral changes

Poor hair coat, slow shedding

---

Thyroid Supplementation: When and Why

Veterinarians may recommend thyroid supplementation (like levothyroxine) after blood testing (T3/T4 panels) reveals low levels or subclinical hypothyroidism. However, sometimes horses with “normal” blood work still benefit from short-term supplementation—especially when symptoms strongly suggest an imbalance.

Benefits of thyroid supplementation can include:

Improved energy metabolism

Better muscle function and recovery

Weight control

More consistent behavior and mood

It’s especially useful as part of a broader plan that includes dietary management (low-starch diets, balanced minerals), consistent exercise, and possibly hormone support if estrous cycling is intense.

---

Takeaway

If your filly or mare is tying up, don’t overlook the thyroid. While not always the root cause, low thyroid function can tip the scales toward muscle dysfunction—especially in hormonally sensitive or genetically predisposed horses. With a thoughtful approach, including testing and possible supplementation, you can help your mare feel better, move freely, and thrive in her work.

ClickHere: BUY THYROID

---

.