PPID (Cushing’s) Testing in Horses: TRH Stimulation vs. Baseline ACTH
Subtle signs of PPID (Cushing’s disease) can be easy to miss—especially in performance horses or during the early stages of disease. Identifying affected horses early and choosing the most appropriate diagnostic test is key to managing the condition effectively.
While most horses with PPID are diagnosed after age 15, clinical experience and recent research support screening horses as early as age 10. In fact, we’ve confirmed PPID in horses under 10 years old, particularly those showing vague but suspicious signs.
Early Signs: What to Watch For
In younger horses, especially those still in full work, the earliest indication of Cushing’s may be poor hoof growth. Farriers often notice subtle changes before obvious clinical signs appear. If your farrier expresses concern about hoof quality, growth rate, or sole depth—it may be worth testing.
Additionally, some performance horses with PPID may present with recurrent or slow-healing soft tissue injuries. Recent studies suggest that elevated ACTH levels and endocrine dysfunction can interfere with tissue repair and collagen turnover. If a horse shows an unusual pattern of suspensory or tendon injuries, Cushing’s should be part of the diagnostic workup.
Baseline ACTH Test
- Single blood draw
- Easy screening tool
- Affected by stress, pain, or season
- Can miss compensated or early disease
This test measures endogenous ACTH levels and is commonly used for routine screening. However, it can yield false negatives, especially during certain times of year or in horses under stress or pain.
TRH Stimulation Test
- Involves administering TRH (thyrotropin-releasing hormone)
- Blood samples taken at 0 and 10 minutes post-injection
- More sensitive for early or borderline PPID cases
- Requires precise timing and chilled sample handling
The TRH stimulation test is considered more sensitive for identifying PPID in its earliest stages, especially in horses with subtle or vague signs. However, proper sample handling and precise timing are critical to obtaining accurate results.
🗓️ Seasonal Restriction:
The TRH stimulation test should not be performed from mid-August to mid-October. This is due to a natural seasonal increase in ACTH that can make results difficult to interpret.
💊 Competition Consideration:
Pergolide, the standard treatment for PPID, requires a Therapeutic Use Exemption (TUE) under USEF regulations and is not allowed under FEI rules. This is an important factor when managing sport horses actively competing.
At Daniel Equine, we tailor diagnostic plans to your horse’s age, use, and risk factors—ensuring accurate diagnosis and proper regulatory compliance for treatment. Contact us to schedule Cushing’s testing or discuss whether your horse may benefit from screening.