Granulosa-theca cell tumors (GTCTs) are usually tentatively diagnosed by rectal palpation and ultrasound examination after an owner complains of poor performance or aggressive, sexual, stallion-like behaviors. However, the diagnosis can’t be confirmed unless an exploratory laparotomy is performed and the tumor is biopsied (or removed and sectioned for histopathology). These tumors tend to make the affected ovary very large and full of cystic structures, while the opposite ovary shrinks considerably. Unfortunately, not all GTCTs follow this pattern of development, and only about 50% of affected mares have increased testosterone in their blood that brings on stallion-like behavior. In women with GTCTs, there is a blood test for two forms of the hormone inhibin, pro-inhibin and inhibin-C, which are markedly increased if a tumor is present. If mares with GTCTs have elevated concentrations of pro- and -C inhibin as well, veterinarians could possibly confirm the presence of a GTCT without exploratory surgery. Researchers from the University of Edinburgh designed a study to measure and compare concentrations of these hormones in mares with GTCTs as well as normal cycling mares.

The results confirmed that mares with GTCTs have significantly increased concentrations of pro- and -C inhibin compared to normal mares. Both forms of inhibin were indeed detected in normal mares, especially around the time of ovulation. However, the concentrations were significantly lower, as much as 10 times less. Therefore, measurement of pro- and -C inhibin in combination with rectal palpation and ultrasound examination should provide veterinarians with a more reliable means for diagnosis of GTCTs in the mare.

Watson, E.D.; Heald, M.; Leask, R.; et al. Equine Veterinary Journal, 34(2), 203-206, 2002