Understanding Cushing’s Disease (PPID)

A HØØF Knowledge Hub article

Cushing’s disease, now more accurately called Pituitary Pars Intermedia Dysfunction (PPID), is a hormonal condition commonly associated with older horses and ponies. Although the biology behind it sounds complicated, the basic process is surprisingly easy to understand.

1. It Starts in the Brain

Within the brain is an area called the hypothalamus.

Among its many jobs, the hypothalamus produces a chemical messenger called dopamine, which helps control another small but important structure — the pituitary gland.

For understanding PPID, think of dopamine as a brake.

It tells part of the pituitary:

“Don’t become too active.”

2. The Brake Begins to Fail

As some horses age, the nerve cells responsible for supplying dopamine gradually deteriorate.

This means less dopamine reaches a particular part of the pituitary called the pars intermedia.

In simple terms:

Less dopamine = less braking.

The pituitary hasn’t become disconnected from the brain. Rather, an important part of the system that normally keeps its activity under control has weakened.

3. The Pituitary Becomes Overactive

Without enough dopamine applying the brake, the pars intermedia becomes increasingly active and often enlarges.

This enlargement is sometimes described as a pituitary tumour, and benign tumour-like growths can occur. However, PPID is better understood as a disorder involving the loss of normal control of this part of the pituitary, rather than simply as “a brain tumour.”

The overactive pituitary then produces abnormal amounts of several hormones and hormone-related substances.

One particularly useful one to know about is ACTH.

4. What Is ACTH?

ACTH stands for adrenocorticotropic hormone.

You don’t need to remember the name.

What matters is that ACTH is one of the chemical messages involved in communication between the pituitary and the adrenal glands, which sit near the kidneys.

The adrenal glands produce hormones including cortisol, which has important roles in metabolism, stress responses and many other normal body functions.

PPID disrupts the normal regulation of this hormonal system.

This is why measuring ACTH in the blood can be useful when vets are investigating PPID.

5. Is PPID Just Too Much Cortisol?

No.

This is one of the reasons PPID can seem confusing. It was traditionally compared with Cushing’s disease in humans and dogs and consequently became closely associated with cortisol.

But horses with PPID don’t necessarily have abnormally high levels of cortisol circulating in their blood.

The problem is broader than one hormone. The pituitary has lost some of its normal control and several hormonal systems can be affected.

Think back to the brake:

Less dopamine → less control → an overactive pituitary → abnormal hormone production.

6. What About the Classic Curly Coat?

This gives us something we can actually see.

The classic PPID coat — unusually long, thick, sometimes curly and failing to shed normally — is called hypertrichosis.

It isn’t simply caused by too much cortisol.

PPID disrupts the normal hormonal control of hair growth and seasonal coat changes, which is why affected horses can eventually develop the characteristic long or curly coat that fails to shed normally.

The important point for owners is that the unmistakable “Cushing’s coat” is generally a later sign of the disease.

A horse can have PPID before it ever looks like the classic shaggy PPID horse.

Earlier changes can be much less obvious. These may include loss of muscle or top-line, reduced energy, increased drinking and urination, subtle changes in normal shedding, or unexplained or recurrent laminitis.

None of these signs on their own confirms PPID.

HØØF Top Tip

Don’t wait for the curly coat.

The classic coat is easy to recognise, but PPID may already be present before it develops.

Loss of muscle or top-line, reduced energy, increased drinking and urination, changes in normal shedding, or unexplained/recurrent laminitis can all provide earlier clues that something isn’t quite right.

None of these signs automatically means PPID, but particularly in an older horse or pony, they are good reasons to have a conversation with your vet.

7. What Has All This Got to Do With Laminitis?

This is where PPID becomes particularly important to us at HØØF.

Horses with PPID have an increased risk of laminitis.

That relationship isn’t simply:

PPID → too much cortisol → laminitis.

The hormonal disturbance is more complicated, and insulin dysregulation appears to be particularly important in PPID-associated laminitis.

Not every horse with PPID will develop laminitis, and PPID is not the only cause of laminitis.

But in an older horse or pony experiencing unexplained or recurrent laminitis, PPID is an important condition for your vet to consider.

The Simple Version

The basic PPID pathway can be reduced to this:

Hypothalamus → less dopamine → the pituitary loses some of its “brake” → part of the pituitary becomes overactive → abnormal hormone production → disruption of normal hormonal and metabolic regulation → increased risk of laminitis.

HØØF Take-Home

You don’t need to understand every hormone involved in PPID.

Remember the brake.

Dopamine normally helps keep part of the pituitary under control. In PPID, that brake weakens. The resulting hormonal disturbance affects the whole horse — and one of its most important consequences can be an increased risk of laminitis.

And don’t wait for the curly coat.

PPID can be present before that classic appearance develops. Changes in top-line, energy, drinking, urination and shedding — or unexplained laminitis — may give you reasons to investigate earlier.

If you suspect PPID, or your horse has experienced unexplained or recurrent laminitis, speak to your vet about appropriate investigation and testing.

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