The Hormonal Mare – Granulosa Cell Tumors of the Equine Ovary

Have you ever looked at your overly grouchy mare and wondered if there is a physiological explanation for her sub-par demeanor? It’s possible there is. Granulosa-Theca Cell tumors are one of the most common types of tumors identified in the equine reproductive tract.  They specifically involve the equine ovary and are responsible for a variety of clinical signs including prolonged, dramatic estrus events, a lack of estrus, infertility, and stallion-like behavior.  Mares demonstrating stallion-like behavior may also have an enlarged clitoris and overly muscled neck due to increased serum testosterone produced from the tumor.  These tumors may also result in poor performance either due to behavioral issues or discomfort resulting from an enlarged ovary.  GTCT usually only affect one ovary, although bilateral tumors are possible.  These tumors are specific to the ovarian tissue and do not spread (metastasize) throughout the rest of the body.


Diagnosis of Granulosa-Theca Cell Tumors (GTCT) requires bloodwork.  Inhibin is a hormone produced by the diseased ovary and is elevated in 80-85% of mares with a GTCT.  Inhibin suppresses the production of follicles on the non-diseased ovary, thus causing the normal ovary to regress into a dormant state.  Upon removal of the ovary with the GTCT, the remaining normal ovary can often reclaim functionality and return the mare to a fertile state.  Testosterone is elevated in about 50% of mares with a GTCT, and Anti-Mullerian Hormone is elevated in about 95% of mares with a GTCT.  By assessing these three values from bloodwork, we can usually attain a high degree of confidence in the presence of a GTCT, further warranting referral to a local surgical center for the removal of one or both ovaries.


Once at the referral center, the mare’s ovaries will be palpated transrectally and imaged with an ultrasound to determine size and position to formulate an appropriate surgical approach for the ovariectomy.  A routine surgical procedure is done with the horse sedated and standing.  Incision(s) to facilitate removal of the ovary are made in the flank and are closed with sutures upon the conclusion of the procedure. 

Recovery and Prognosis

Post-surgical care involves a short course of anti-microbial and anti-inflammatory therapies, with stall rest and hand walking only during the first 14 days after surgery.  At 14 days post-surgery, the sutures are removed, and the horse is cleared to return to an easy regimen of work.  Since GTCT do not metastasize, removal of the effected ovary is curative.  Should the normal ovary remain intact, normal reproductive function of the mare should return in 6-8 months post-surgery. 

If you have concerns regarding your mare’s behavior, please feel free to reach out your veterinarian to discuss an appropriate diagnostic plan.  As always, we look forward to helping you and your horse attain an optimal level of health.

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