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Equine Cushing's Disease: Part II
Fred Nostrant, DVM • April 26, 2024
The TRH stimulation test is one of the most valuable tools veterinarians use to diagnose Equine Cushing’s Disease, also known as Pituitary Pars Intermedia Dysfunction (PPID). This test is especially helpful for identifying early or subtle cases of PPID—sometimes before obvious clinical signs appear.
If you haven’t already, we recommend reading Part I: Equine Cushing’s Disease (PPID): A Complete Guide for Horse Owners for an overview of the condition.
What Is the TRH Stimulation Test?
The TRH stimulation test evaluates how well the pituitary gland is functioning. In horses with PPID, the pituitary gland overreacts to stimulation and releases excessive amounts of ACTH, the hormone most closely associated with Equine Cushing’s Disease.
Because early PPID cases may have normal baseline ACTH levels, this test allows veterinarians to detect disease earlier and more accurately than a single blood test alone.
How the Equine TRH Stimulation Test Works
1. Test Preparation
- Horses are typically fasted from grain before testing
- Hay and forage are allowed
- This helps ensure more accurate hormone measurements
2. TRH Administration
- TRH (thyrotropin-releasing hormone) is a naturally occurring hormone produced by the hypothalamus in the brain
- A veterinarian administers a measured dose of synthetic TRH, usually by intravenous injection
- TRH signals the pituitary gland to release hormones, including ACTH
3. Blood Sample Collection
- A blood sample is taken before TRH is given to establish a baseline
- Additional blood samples are collected after TRH administration, commonly at:
- 10 minutes
- 20 minutes
- 30 minutes
4. Measuring ACTH Levels
- The blood samples are analyzed to measure ACTH (adrenocorticotropic hormone)
- ACTH levels are the key indicator used to assess pituitary function in horses
5. Interpreting the Results
- In healthy horses, ACTH levels rise only slightly after TRH
- In horses with PPID, the pituitary gland releases an exaggerated or prolonged surge of ACTH
- Elevated ACTH levels following TRH stimulation strongly support a diagnosis of Equine Cushing’s Disease
Why the TRH Stimulation Test Is So Important
The TRH stimulation test is especially valuable because it:
- Detects early or subclinical PPID
- Helps diagnose horses with mild or vague symptoms
- Allows treatment to begin before serious complications develop
- Improves long-term outcomes by reducing the risk of laminitis, muscle loss, and recurrent infections
TRH Testing Is Only One Piece of the Puzzle
While the TRH stimulation test is highly informative, veterinarians also consider:
- Clinical signs such as:
- Long or abnormal hair coat
- Increased drinking and urination
- Lethargy
- Muscle wasting
- Recurrent infections
- Laminitis
- Baseline ACTH testing
- The horse’s age, breed, and overall health
All of this information is used together to determine the most appropriate diagnosis and treatment plan.
Early Diagnosis Makes a Big Difference
Equine Cushing’s Disease is a progressive condition, but early detection changes everything. The TRH stimulation test allows veterinarians to identify PPID before significant damage occurs, helping to:
- Start medication sooner
- Adjust diet and management early
- Prevent painful and life-altering complications
- Improve comfort, longevity, and quality of life
If you suspect your horse may be developing PPID—or if subtle changes just “don’t seem right”—talk with your veterinarian about whether the
TRH stimulation test is appropriate.
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