Granulosa Cell Tumors of the Equine Ovary

The GCT is the most common ovarian tumor in mares. A clinical diagnosis can be made based on the presence of a unilaterally enlarged ovary and a small inactive contralateral ovary. Endocrine testing may be beneficial to confirm a diagnosis. Plasma inhibin and testosterone concentrations are elevated in approximately 90% and 50% to 60%, respectively, of mares with GCTs. Testosterone is elevated if the tumor has a significant thecal cell component, and affected mares exhibit stallion-like behavior. Hormones produced by the tumor result in suppression of pituitary FSH secretion and inactivity in the opposite ovary. Surgical removal of the tumor eliminates the adverse effect on pituitary function and results in resumption of follicular development and ovulation over time.