PPID ACTH Testing Chart (Cushing's Reference Ranges)
A reference chart for PPID testing in horses: resting ACTH ranges, the seasonal fall rise, the TRH stimulation test, clinical signs, and retesting for Cushing's.
Quick answer: PPID (equine Cushing's disease) is most commonly screened by measuring resting blood ACTH, interpreted against season-specific reference ranges. Outside the fall months, a resting ACTH below roughly 30 pg/mL is generally considered normal at most labs, with higher values suggesting PPID. ACTH naturally rises in late summer and autumn (about August to October), so labs apply much higher fall reference ranges. The TRH stimulation test helps catch early or borderline cases. Always interpret results with your veterinarian using your lab's seasonal ranges.
PPID ACTH Reference Chart
The values below are general guides to how resting ACTH is interpreted. Exact cutoffs vary by laboratory and by year, and your lab will provide its own season-specific reference ranges, which are what your veterinarian uses to make a diagnosis. This chart is for orientation, not for diagnosing your horse at home.
| Season | General Interpretation of Resting ACTH | Notes |
|---|---|---|
| Non-fall (roughly Nov to July) | Below ~30 pg/mL often considered normal; higher raises suspicion of PPID | Borderline values may warrant a TRH stimulation test |
| Seasonal rise (Aug to Oct) | Normal horses have much higher ACTH; labs apply elevated fall ranges | A sensitive window for detecting early PPID |
| Equivocal / borderline | Between the normal cutoff and clearly elevated | Consider TRH stimulation, repeat testing, or correlating with signs |
| Clearly elevated | Well above the season-specific range | Supports PPID, especially alongside clinical signs |
The single most important point is that ACTH cannot be read without knowing the season. A value that is perfectly normal in October could be distinctly abnormal in February, which is why the calendar date of the blood draw is recorded and matched to the right reference range.
Why Season Matters: The Fall Rise
Healthy horses naturally produce more ACTH in late summer and autumn as part of normal seasonal physiology. In PPID horses this rise is exaggerated. That makes fall a useful, sensitive time to catch early disease, because affected horses spike far above normal. But it also means a fall result has to be compared against elevated fall reference ranges, or a perfectly healthy horse could be misdiagnosed. Many vets test suspect horses in the fall for sensitivity, while using non-fall ranges the rest of the year.
The TRH Stimulation Test
Resting ACTH can miss early or mild PPID. The TRH stimulation test improves detection of these cases. The vet draws a baseline ACTH sample, injects thyrotropin-releasing hormone, and draws another sample at 10 minutes, sometimes also at 30. In PPID horses the pituitary overresponds, producing a larger ACTH rise than a normal horse would. It is particularly valuable in the non-fall months for a horse with suggestive clinical signs but a borderline resting ACTH. Results are still interpreted against season-specific cutoffs.
Clinical Signs That Prompt Testing
Testing usually follows clinical suspicion. The hallmark sign is hypertrichosis, a long, curly coat that sheds late or incompletely, which is strongly associated with PPID. Other signs include:
- Loss of topline muscle and a potbellied appearance
- Regional fat deposits, including above the eyes
- Increased drinking and urination
- Lethargy and reduced performance
- Recurrent infections and slow wound healing
- Laminitis or a heightened laminitis risk, often the most dangerous consequence
Because many of these overlap with ordinary aging, ACTH testing is what separates a normally old horse from one with PPID that would benefit from treatment.
Retesting and Monitoring
PPID is managed, not cured, usually with daily medication prescribed by your vet. After diagnosis and starting treatment, ACTH is commonly rechecked to confirm the horse is responding, then monitored periodically, often once or twice a year, ideally in the same season for fair comparison. Monitoring guides dose adjustments and tracks whether clinical signs and laminitis risk are under control. Your vet sets the retesting rhythm based on the individual horse.
Related Reading
- NSC Values in Horse Feeds - Low-sugar feeding for PPID and EMS horses.
- Senior Horse Vital Signs Chart - Including the digital pulse that warns of laminitis.
- Equine Deworming Schedule - PPID horses may shed parasites more heavily.
- Henneke Body Condition Score Chart - Tracking the muscle and fat changes PPID causes.
This chart is educational and complements, but does not replace, your veterinarian. PPID diagnosis and treatment require veterinary testing and prescription medication.
Frequently Asked Questions
What is ACTH and why is it tested for PPID?
ACTH (adrenocorticotropic hormone) is a hormone produced by the pituitary gland. In PPID, formerly called equine Cushing's disease, the dysfunctional pituitary overproduces ACTH and related hormones, so a blood test measuring resting ACTH is the most common screening tool. An elevated ACTH, interpreted against season-specific reference ranges, supports a PPID diagnosis. Because ACTH naturally rises in late summer and fall, the time of year is essential to interpreting the result correctly.
What is a normal ACTH level in a horse?
There is no single number, because normal ACTH depends on the season. In the non-fall months, a resting ACTH below roughly 30 pg/mL is generally considered normal at most labs, while higher values raise suspicion of PPID. During the seasonal rise in late summer and autumn, normal horses have much higher ACTH, so labs apply elevated fall reference ranges. Always interpret ACTH using your laboratory's season-specific ranges and your veterinarian's judgment, not a fixed cutoff.
What is the seasonal rise in ACTH?
Healthy horses normally produce more ACTH in late summer and autumn, roughly August through October in the Northern Hemisphere, as part of natural seasonal physiology. This seasonal rise is exaggerated in PPID horses. It is clinically useful: fall can be a sensitive time to detect early PPID because affected horses show a much larger spike than normal horses. But it also means a result must always be read against the correct seasonal reference range, or a normal fall value could be misread as abnormal.
What is the TRH stimulation test?
The TRH (thyrotropin-releasing hormone) stimulation test is used to detect early or mild PPID that a resting ACTH might miss. The vet measures baseline ACTH, injects TRH, then measures ACTH again at 10 (and sometimes 30) minutes. In PPID horses the pituitary overresponds, producing a larger ACTH rise than normal. It is especially valuable in the non-fall months for horses with suggestive signs but a borderline resting ACTH. Like resting ACTH, results are interpreted against season-specific cutoffs.
What clinical signs suggest PPID?
The classic sign is a long, curly coat that fails to shed properly (hypertrichosis), which is highly suggestive of PPID. Other signs include regional fat deposits, muscle loss over the topline, a potbellied look, increased drinking and urination, lethargy, recurrent infections, delayed wound healing, and a heightened risk of laminitis. Many of these overlap with normal aging, which is why testing matters. A senior horse with several of these signs is a candidate for ACTH testing.
How often should a PPID horse be retested?
After a PPID diagnosis and starting treatment, vets commonly recheck ACTH to confirm the horse is responding, often a few weeks to a couple of months after beginning or adjusting medication, and then periodically thereafter, frequently once or twice a year. Retesting in the same season allows fair comparison. Monitoring tracks whether the medication dose is controlling ACTH and the clinical signs. Your vet sets the schedule based on the individual horse, its signs, and how well it is controlled.
Does feeding affect ACTH test results?
Stress, pain, and illness can transiently raise ACTH, so testing is best done in a calm horse that is not acutely sick or in pain. Severe stress, like the stress of transport or a painful condition, may elevate results. Diet does not invalidate a resting ACTH test the way it would a glucose or insulin test, but related metabolic testing for insulin should follow specific fasting or feeding protocols. Follow your vet's instructions on how to prepare your horse for whichever tests are being run.
Need more help with your senior horse?
Browse our guides by topic to find practical solutions.
Wellness Planner: $39