Last week, we talked about ACTH and PPID (available here). A question was asked, “Does an elevated ACTH mean that my horse has PPID?”
The short answer is “No’”.
Although elevated ACTH makes a person think that the horse has PPID, that’s not always the case. ACTH is the body’s primary stress hormone. This newsletter explores other causes for an elevated ACTH.
◆︎ Normal Seasonal Rise
In the autumn, horses have a natural rise in ACTH. Charts are available which gives a reference range for time of year, such as this one. If you are measuring ACTH during the seasonal rise and use the springtime reference range, the spike will look like a false positive.
◆︎ Pain
A horse that is in pain will have an elevated ACTH. If blood is drawn when a horse has an abscess, has colicked, has acute laminitis, or other pain episode, the ACTH will be elevated.
◆︎ Illness
Horses with systemic inflammation from an illness will have elevated ACTH. Illnesses like Strangles, West Nile, Potomac Horse Fever, will often have an elevated ACTH as the body mounts an immune response.
◆︎ Stress
The stress from trailering a horse to the vet for a blood draw will result in an elevated ACTH.
Taking a blood draw from the horse when there are other stressors can also result in high ACTH levels. For instance, drawing blood after a dental procedure, or after placing a nasogastric tube for treating a colic are stressful enough to elevate ACTH.
A spike in ACTH can also occur if the technician has difficulty accessing the vein and has to try multiple times.
Another stressor that can result in an elevated ACTH is intense exercise 12 to 24 hours before the blood draw.
◆︎ Equine Metabolic Syndrome (EMS)
Although PPID and EMS are different diseases, at times, they appear to walk hand-in-hand. Low grade foot pain from elevated insulin levels can actually fuel an elevated ACTH. And an elevated ACTH can drive high insulin values.
◆︎ Testing limitations
We know that there are limitations to ACTH testing. The accuracy of the testing matters. For the ACTH test:
☛ Sensitivity (70-80%): Out of 100 horses that truly have PPID, the ACTH test will correctly identify 70-80 of them. This means that 20-30 horses will be reported as negative when they are not. (false negatives)
☛ Specificity (80-90%): Out of 100 horses that are normal and healthy, the ACTH test will correctly identify 80-90 of them. This means that 10-20 horses will be reported as having PPID when they do not. (false positives)
◆︎ Precaution
While your horse may have elevated ACTH at this time of year, it may be a false positive. However, the danger of assuming that it is a false positive when it is not, is that your horse could go on to have a laminitic episode in another month or so.
◆︎ How to proceed:
➡ Work with your veterinarian for local advice.
➡ Consider trialing pergolide or chasteberry, discussed last week. Pergolide is the gold standard in treatment for PPID. However, some people choose to use chasteberry, despite it not lowering ACTH levels.
➡ Get through this seasonal rise, then work with your veterinarian to reassess the status in December or January, after the seasonal rise has passed.
Always work with your veterinarian to diagnose the condition and to establish the appropriate treatment plan.
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©Joan Kulifay, MSc. 2026