Normal Female Equine Testosterone Levels

Small amounts of testosterone are present in female equines, known as mares.

Testosterone is a hormone that we recognize as the primary male hormone, but it is also present in female mammals. For equine females, or mares, the normal testosterone level in mares who are not pregnant is less than 45 picograms per millileter of blood serum.

About Testosterone

Testosterone is a steroid hormone produced by the male testes. It and other sex hormones are derived from cholesterol. In females, ovarian cells known as thecal cells synthesize testosterone in smaller quantities. This also occurs in a part of the adrenal cortex and in the placenta.

Testosterone in Equine Females

Mares with normal testosterone levels should not exhibit aggressive behaviors.

Janet Roark, a veterinarian in Austin, Texas, says that mares with normal testosterone levels will not display signs of aggression commonly associated with some stallions or even some geldings that may exhibit "stud-like" behavior. If your mare exhibits these behaviors, you should consult with your veterinarian for possible elevated testosterone. Monitoring your mare's testosterone through blood serum draws and testing can also help determine pregnancy, fertility and breeding cycles.

Elevated Levels

Aggressive behaviors in mares may indicate elevated testosterone.

Abnormal levels of testosterone on equine females can lead to behavior changes, typically aggression. Other behavior changes include prolonged heat cycles and nymphomania. The most common reason for this elevation is an ovarian tumor known as a granulosa thecal cell tumor. In addition to increased testosterone, a blood test will show high elevations of a polypeptide hormone known as inhibin/

Tumor Treatment

Horses that have a granulosa thecal cell tumor removed can usually still get pregnant.

The granulosa thecal cell tumor is a benign tumor. Once the affected ovary with the tumor is removed, the prognosis for recovery and even breeding is generally good, although Roark recommends a uterine biopsy prior to surgery for mares that will be used as broodmares. Testosterone levels should return to normal after the tumor is removed.

References

• Janet M. Roark, DVM, Hill Country Mobile Veterinary Service, Austin, Texas
• Cambridge Specialist Laboratory Services
• Purdue University: Granulosa-Theca Cell Tumors in the Mare
• World of Molecules: Testosterone

Resources

• Horse Channel: Hormones and Horse Behavior