A question we frequently receive is, which test is more appropriate for my horse? An ACTH test to check for Cushing’s disease, or an insulin test to determine whether my horse has insulin resistance?
The answer depends on the individual horse, but many owners share the same goal: reducing the risk of a laminitis episode.
It is important to remember that these tests help us investigate two different endocrine conditions that can be associated with laminitis: pituitary pars intermedia dysfunction (PPID), commonly known as equine Cushing’s disease, and equine metabolic syndrome (EMS).
What Is Cushing’s Disease, and When Should We Test ACTH Levels?
PPID is an endocrine disorder that primarily affects older horses. It is associated with changes in pituitary function that can lead to increased ACTH production and alterations in other hormonal pathways. For this reason, we measure ACTH levels in the blood to help diagnose or rule out the disease.
Although PPID can occur in horses as young as 10 years old, it is more common in older horses, particularly those between 15 and 20 years of age.
Some clinical signs that may raise suspicion of PPID include:
Changes in the coat, with poor hair quality
A winter coat that takes longer to shed or incomplete shedding
Recurrent hoof abscesses and other hoof-related problems (persistent thrush)
Skin problems and recurrent infections
Increased susceptibility to certain respiratory conditions
Weight loss or loss of muscle mass
A sunken-back appearance, often referred to as swayback
Increased water consumption and urination
A history of laminitis
Not every horse develops the same signs, and some may show only subtle changes. This is why a veterinary examination is essential.
What Is Equine Metabolic Syndrome, and When Should We Evaluate Insulin Levels?
ACTH, insulin, and glucose testing are useful diagnostic tools, but not every horse needs all three. The appropriate tests depend on the horse’s age, body condition, medical history, clinical signs, and risk factors.
As a general guideline:
Horses younger than 10 years: If they have metabolic risk factors, localized fat deposits, or a history of laminitis, evaluating insulin levels and, depending on the testing protocol, glucose levels may be particularly useful.
Horses older than 10 years: ACTH testing may be appropriate, and insulin and glucose testing may also be recommended depending on the horse’s history and risk factors.
Ideally, establishing individual baseline values can be helpful when comparing results over time. However, the appropriate tests should always be determined in consultation with your veterinarian.
What If a Horse Has Cushing’s Disease and Elevated Insulin Levels?
This is a particularly important point.
Horses with PPID can also develop insulin dysregulation. In some cases, this metabolic abnormality contributes to an increased risk of laminitis.
After PPID is diagnosed and appropriate treatment is started, insulin regulation may improve in some horses. Depending on the horse’s response, additional insulin-specific treatment may not be necessary. However, this should never be assumed. Follow-up testing is important to determine whether insulin dysregulation persists.
Why Is Insulin Testing So Important?
In addition to helping identify metabolic abnormalities associated with laminitis, insulin testing can help investigate potential factors contributing to recurrent episodes of this condition.
It can also help us monitor treatment responses and guide clinical management decisions. In selected cases, medications such as metformin or SGLT2 inhibitors (i.e. canagliflozin), may be considered under veterinary supervision to help manage insulin dysregulation. The choice of medication depends on the individual patient, the available evidence, and the veterinarian’s assessment.
Even so, medication is only one part of the overall plan. A comprehensive approach should also include appropriate dietary management, weight control, exercise when safe, and a hoof-care plan tailored to each horse’s needs.
A Few Additional Things to Keep in Mind
It is important to emphasize that these tests can be performed at any time of the year. Some people believe that ACTH levels can only be accurately evaluated during the spring or fall. However, there are different ways to interpret and adjust ACTH results depending on the time of year, allowing your veterinarian to assess the results appropriately.
Additionally, when performing insulin and glucose testing, proper preparation is essential. Your veterinarian may recommend an appropriate fastening plan, including withholding grain and/or hay for approximately four hours before the test. Always follow your veterinarian’s specific instructions to ensure the results are as accurate and useful as possible.
A Final Recommendation
Remember that every horse is different, and there is no single combination of tests that is appropriate for every patient.
Talk to your veterinarian to determine which tests are best suited to your horse’s age, body condition, medical history, and individual risk factors.
If our goal is to identify potential metabolic problems early and reduce the risk of laminitis, establishing baseline ACTH, insulin, and glucose values can be a useful strategy when clinically indicated.
Prevention starts with a thorough evaluation, an accurate diagnosis, and an individualized management plan for every horse.