We all know that in the autumn, a horse's ACTH levels naturally rise. We also know that uncontrolled Equine Pituitary Pars Intermedia Dysfunction (PPID) can result in laminitis. But what good is a seasonal rise in these hormones? Why do healthy horses have it in the first place?
Autumn and the Dopamine "Brake Pedal"
Dopamine acts as a biological brake pedal for many physiological systems. In the horse's brain, dopamine keeps the pituitary gland quiet. In the fall, as daylight decreases, the horse's body naturally produces less dopamine. This means the "brake pedal" on the pituitary gland is let up, triggering a cascade of survival activity to prepare for winter.
This decrease in dopamine allows the pituitary gland to ramp up production of a master protein called POMC. Special "scissor enzymes" then cut this POMC protein into different hormonal parts:
◆︎ Increased ACTH (Energy Storage)
➡ Normal Horses: ACTH stimulates the release of cortisol. A mild, seasonal bump in cortisol alters glucose and fat metabolism, telling the body to store additional energy before winter. This is why healthy horses naturally get a bit "cresty" and gain weight in the fall.
➡ PPID Horses: Because their pituitary gland is unregulated, the increase in cortisol is extreme and catabolic (tissue-destroying). We see a loss of topline muscleand a pot-bellied appearance. Chronically high cortisol also suppresses the immune system, resulting in skin infections, sinus issues, and recurrent hoof abscesses.
◆︎ Increased α-MSH (Coat & Inflammation)
➡ Normal Horses: Some of the ACTH gets cut into α-MSH. This is highly protective, as it turns down inflammation so the horse can withstand harsh winter conditions. It also signals the hair follicles to grow a winter coat.
➡ PPID Horses: The massive overproduction of α-MSH sends the hair follicles into overdrive. Instead of a normal winter coat, it causes the hair to grow excessively long and curly, and prevents it from shedding. It also alters fat metabolism, leading to abnormal, bulging fat pads.
◆︎ Increased 𝛽-endorphin (Pain Control)
➡ Normal Horses:𝛽-endorphin is a natural painkiller. A seasonal increase helps the wild horse withstand the physical pain of freezing temperatures and walking on hard, frozen ground.
➡ PPID Horses: In advanced cases, the horse's brain is completely overwhelmed by these natural opioids. This is why unmedicated PPID horses are often lethargic, dull, and have a glazed, "spaced out" expression.
◆︎ Increased CLIP Peptide (Insulin Spikes)
➡ Normal Horses: CLIP travels to the pancreas and tells the 𝛽-cells to secrete insulin, regardless of the sugar content of what was eaten. This is helpful so that the horse gets as much energy as possible out of the last rich grasses of the year..
➡ PPID Horses: This is where the true danger lies. Extreme levels of CLIP force the pancreas to produce elevated levels of insulin. Chronically high insulin destroys the attachments inside the hoof capsule, directly triggering laminitis.
◆︎ What Does This Mean?
The overproduction of this cascade is what gives us the classic PPID symptoms:
● A long, non-shedding coat
● Muscle loss along the topline
● A pot-bellied appearance
● Inappropriate regional fat pads
● Elevated insulin levels
We see the shaggy coat, but that won't kill a horse. The reason that we treat the horse for PPID is to avoid the heartbreak of laminitis.
◆︎ Treatment Plans:
➡ Pergolide (Prascend or Zygolide):This is the gold-standard, FDA-approved treatment. It acts as an artificial dopamine "brake pedal" to control the tumor. Keep in mind that adequate treatment may require higher doses than the packaging insert suggests.
➡ Chasteberry: Although it is less effective, some people choose to use chasteberry instead.Some people find that they get symptom relief, but it fails to control ACTH levels.
Always work with your veterinarian to diagnose the condition and to establish the appropriate treatment plan.
We do the best we can for our horses. Understanding our horse's needs is a part of that.
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©Joan Kulifay, MSc. 2026