03/10/2026
ACTH is your stress hormone. If it’s high it indicates that your horse is in a state of stress. High ACTH does not necessarily indicate your horse has Cushings. It’s good to see a vet posting about its over use as an indicator of Cushings
🐴 A HIGH ACTH DOES NOT MEAN YOUR HORSE HAS PPID (“Cushings”)
There is an important difference between an high blood test result and a diagnosis.
The horse matters more than the number.
This is where pre-test probability becomes incredibly important.
Imagine we test two horses:
One is a 22yo pony with history of laminitis, delayed coat shedding, muscle loss and other clinical signs strongly suggestive of PPID.
The other is a 9yo horse with no characteristic clinical signs of PPID.
Even if both return an ACTH concentration above the laboratory reference interval, those results do not carry the same diagnostic meaning.
The current BEVA PPID guidelines specifically state that the pre-test probability of PPID is low in horses under 10 years of age. In this age group, other explanations for the horse’s clinical signs should be investigated, and PPID testing should generally be reserved for horses with multiple compatible clinical signs that genuinely increase suspicion of the disease.
This matters because the lower the probability that a horse actually has PPID before we run the test, the greater the potential for a positive result to be a FALSE POSITIVE.
In fact, using published diagnostic-performance data, the BEVA guidelines estimated that during the autumn seasonal rise, around 80% of positive basal ACTH results in horses with a low pre-test probability of PPID could be false positives.
That is a really important number.
Why do I find this concerning?
I am increasingly seeing horses under 10 years of age being diagnosed with PPID and started on lifelong pergolide treatment based on a single ACTH result, sometimes without convincing clinical signs or further investigation.
Starting a young horse on lifelong medication based predominantly on one abnormal laboratory value, when the horse has a very low pre-test probability of actually having the disease, is simply not good veterinary practice. More importantly, it’s often bad news for the horse.
The problem isn’t simply that the horse may receive pergolide unnecessarily.
Once we attach a diagnosis of PPID to that horse, there is a risk that we stop asking the more important question:
If it isn’t PPID, what is actually going on?
An incorrect diagnosis can mean that another disease or management problem responsible for the horse’s clinical signs is overlooked or treatment is delayed. This concern is specifically raised in the BEVA guidelines, which note that false-positive PPID results may lead to unnecessary lifelong treatment and failure to identify and treat a potentially important alternative diagnosis.
ACTH is an extremely useful test when investigating PPID, but a high ACTH concentration on its own does not confirm that a horse has PPID.
ACTH is a biologically variable hormone. Its concentration can be influenced by many factors completely separate from PPID, and this needs to be considered before interpreting a result.
ACTH is a diagnostic tool. It is not a diagnosis.
An elevated ACTH should prompt us to look at the whole horse: age, clinical signs, history, breed, season, concurrent disease and the circumstances surrounding the blood sample.
Sometimes all of those pieces fit together beautifully and PPID is the logical diagnosis.
Sometimes they don’t.
When the laboratory result and the horse in front of us don’t match, the answer shouldn’t automatically be “start pergolide.”
The answer should be “let’s investigate further.”
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