Granulosa-Theca Cell Tumor in Horses: Mare Behavior Changes and Infertility

Granulosa-theca cell tumor, often shortened to GTCT, is the most common ovarian tumor seen in mares. It develops from the hormone-producing tissues of the ovary and is usually benign and limited to one ovary, but it can still cause major changes in behavior and fertility because it often produces abnormal amounts of reproductive hormones.

Many mares with this condition do not look sick in a general sense. Instead, pet parents may notice a mare acting unusually aggressive, showing stallion-like behavior, cycling irregularly, or failing to conceive. In some mares, the opposite pattern happens: they stop showing normal heat behavior altogether. These mixed signs can be confusing, especially when the mare otherwise appears healthy.

A key feature of GTCT is that the affected ovary often becomes enlarged while the other ovary becomes small and inactive. That happens because tumor-related hormones can suppress normal ovarian function. This is one reason mares with GTCT commonly have infertility until the problem ovary is removed and the remaining ovary has time to recover.

The good news is that many mares do well after treatment. If your vet confirms a GTCT and removes the affected ovary, behavior often improves and future fertility may return, although recovery of normal cycles can take several months.

GTCT develops when cells in the ovary called granulosa and theca cells begin growing abnormally and form a tumor. In mares, this tumor is usually unilateral and usually does not spread widely, but it can be hormonally active. Those hormone shifts are what drive many of the behavior and fertility changes pet parents notice.

The exact reason one mare develops a GTCT and another does not is not well defined. There is no clearly proven prevention plan, feed issue, or management mistake known to cause it. These tumors have been reported in mares of different breeds and ages, although they are often recognized in adult mares during breeding or performance years.

Hormone production varies from tumor to tumor. Some produce more inhibin, some more testosterone, and some alter anti-Müllerian hormone levels. That helps explain why one mare may act persistently in heat, another may stop cycling, and another may become aggressive or stallion-like. Because the signs depend on hormone output, two mares with the same diagnosis may look quite different clinically.

For pet parents, the most important takeaway is that this is not something you caused. If your mare develops a GTCT, the next step is not blame. It is a careful workup with your vet to confirm what is happening and discuss which treatment path fits her health, use, and breeding goals.

Diagnosis usually starts with your vet reviewing the mare’s behavior history, breeding history, and cycle pattern. A reproductive exam often includes rectal palpation and ultrasound to compare both ovaries. A GTCT-affected ovary may look enlarged, multicystic, or have a characteristic honeycomb appearance, while the opposite ovary is often small and inactive.

Bloodwork is an important next step. Many vets now use anti-Müllerian hormone (AMH) as a key test because it is highly useful for identifying these tumors in mares. Inhibin and testosterone may also be measured, especially when the clinical picture is less straightforward. No single test should be interpreted in isolation, so your vet will combine hormone results with the physical exam and ultrasound findings.

Your vet may also consider other causes of enlarged ovaries or abnormal cycling, such as seasonal transitional ovaries, hematoma, or other reproductive disorders. That matters because some non-tumor conditions can mimic GTCT on exam. In breeding mares, your vet may also assess the uterus and overall reproductive health to understand whether there are additional reasons for infertility.

A definitive diagnosis is confirmed after the affected ovary is removed and examined by a pathologist. Even so, many mares are diagnosed strongly enough before surgery to allow practical treatment planning. If surgery is being considered, your vet may refer you to an equine hospital or board-certified surgeon to discuss the safest approach.

There is no proven way to prevent granulosa-theca cell tumor in mares. Current veterinary evidence does not support a specific feed, supplement, exercise plan, or breeding strategy that reliably stops these tumors from forming.

What you can do is improve the chance of catching the problem earlier. Keep notes on your mare’s cycle pattern, behavior changes, breeding history, and performance. A mare that suddenly becomes aggressive, persistently in heat, unusually quiet reproductively, or unexpectedly infertile should have a reproductive exam rather than being labeled difficult.

Routine breeding soundness exams are especially helpful for broodmares and mares with unexplained infertility. For performance mares, early evaluation matters when behavior changes seem out of character or interfere with handling and riding safety. Earlier diagnosis can make treatment planning more straightforward and may reduce the chance of living with a large ovarian mass for months.

If your mare has already had one GTCT removed, your vet can guide follow-up based on her age, use, and reproductive goals. Most cases involve only one ovary, and many mares go on to have normal function from the remaining ovary after recovery.