Here are the basic facts about pituitary pars intermedia dysfunction (PPID), a common condition in older horses.

What is it?

Also known as Cushing’s disease, PPID occurs when an enlarged and overactive pituitary gland producing excess levels of adrenocorticotropic hormone (ACTH). These elevated ACTH levels, in turn, lead to overproduction of the steroid hormone cortisol. What causes PPID to occur is still unclear. But it’s extremely common in older horses: One study found that 85 percent of veterinarians care for at least one horse with PPID.

What does it do to a horse’s health?

Excess hormones cause a slew of systemic signs, including a long hair coat that is slow to shed, muscle-wasting, lowered immune function and susceptibility to infection.  Some horses with PPID are also prone to slow-onset chronic laminitis. In many cases, the chronic laminitis goes undetected until internal hoof structures are damaged.

How is it diagnosed?

In addition to clinical signs, veterinarians utilize two laboratory tests to diagnose PPID. The first is a blood test to measure resting levels of ACTH. The second is the thyrotropin-releasing hormone stimulation test (TRH stimulation test), which involves taking a blood sample, giving an injection of TRH, and then collecting a blood sample again 10 minutes later. ACTH increases in all horses between the first and second blood test. But horses with PPID will show a dramatically higher increase. Because of natural seasonal and geographic fluctuations in ACTH, veterinarians will use published reference ranges to interpret laboratory results.

How is it treated?

Pergolide, a medication sold under the brand name Prascend, is effective in reducing the signs of PPID. Given daily, Pergolide works by binding with receptors in the brain that normally respond to dopamine, reducing the blood levels of ACTH.

For a more comprehensive article about pituitary pars intermedia dysfunction in horses, check out Your PPID Questions Answered.