MRI of the Equine Pituitary Gland: What is Normal?

Current tests for diagnosing pituitary pars intermedia dysfunction (PPID, equine Cushing’s disease) do not always provide clear answers, leaving owners scratching their heads in frustration. Identifying affected horses, especially at an early stage of disease, could help prevent important consequences of the disease, such as chronic laminitis and infections. In the future, magnetic resonance imaging (MRI) might play a vital role in definitively diagnosing PPID, but some basic background work must first be performed.

PPID is most commonly caused by an abnormal growth, an adenoma, in the pars intermedia region of the pituitary gland, which is located at the base of the horse’s brain. The growth causes damage by either compressing adjacent structures or by altering hormone production in the brain. For example, high levels of adrenocorticotropic hormone (ACTH) may be produced in the presence of adenoma which, in turn, controls the stress hormone cortisol. Overproduction of cortisol can wreak havoc on numerous body systems, resulting in classic clinical signs of PPID.

Definitively diagnosing PPID can be a maddening exercise for owners and veterinarians alike. The two most widely used tests—measuring ACTH and the thyroid-releasing hormone assay—don’t always yield definitive results. As an alternative approach, veterinarians looked at MRI to determine if it could assist in the diagnostic process.

The first step in the process was to identify the shape and size of normal equine pituitary glands on MRI. For this, researchers scanned 27 healthy horses with no evidence of PPID.

They found most normal pituitary glands to be snail-shaped (81% of horses) and the remainder oval. The glands were fairly consistent in size regardless of the physical size of the horse. In fact, no correlation was noted between the size of the pituitary gland and brain weight, other brain measurements, or body weight.

The data attained from this preliminary trial will guide further studies that evaluate the potential to diagnose PPID by MRI.

Veterinarians recommend pergolide for confirmed cases of PPID, often coupled with diet changes.

“Diets should be based on high-quality forages. Forage selection based on a conservative nonstructural carbohydrate content may also be necessary if insulin dysregulation is a factor,” explained Catherine Whitehouse, M.S., a Kentucky Equine Research nutritionist.

Whitehouse also made the following suggestions when feeding horses with PPID:

• Assess your horse’s current and ideal body condition scores as well as muscle mass to determine the type and amount of feed needed to achieve optimal health and condition.
• Consider offering a ration balancer. Such products are ideal for supplying quality protein and other nutrients to balance a forage-based diet.
• Evaluate potentially beneficial nutritional supplements, such as antioxidants, to help combat oxidative stress and low-grade inflammation.

Though these general recommendations are appropriate for many horses with PPID, affected horses should be fed based on their particular set of signs. Consultation with a nutritionist well versed in feeding horses in disease states is the best place to start, and it’s especially encouraging when the nutritionist and a veterinarian can work together to formulate the best plan for the horse.

*Hobbs, K.J., E. Porter, C. Wait, M. Dark, and R.J. MacKay. 2022. Magnetic resonance imaging of the normal equine pituitary gland. Veterinary Radiology and Ultrasound:13072.