The ACTH (Adrenocorticotropic Hormone) Stimulation Test is a crucial diagnostic tool in veterinary medicine, primarily used to evaluate the function of a dog’s adrenal glands. It measures how well the adrenal glands produce cortisol, a vital stress hormone.
What is the ACTH Stimulation Test?
The test involves administering a synthetic form of ACTH and then measuring the dog’s cortisol levels before and after the administration. The synthetic ACTH mimics the natural hormone produced by the pituitary gland, which normally stimulates the adrenal glands to produce cortisol.
The principle is simple:
If the adrenal glands are healthy, they will respond to the synthetic ACTH by producing a significant amount of cortisol.
If they are underactive (Addison’s disease), they won’t produce much cortisol, or any at all.
If they are overactive (Cushing’s disease), they will produce an excessively high amount of cortisol, even with stimulation.
Why is it Performed (Indications)?
The ACTH stimulation test is primarily used for:
Diagnosing Addison’s Disease (Hypoadrenocorticism): This is the definitive test for Addison’s disease. Dogs with Addison’s have adrenal glands that cannot produce adequate cortisol (and often aldosterone), even when stimulated.
Diagnosing Cushing’s Disease (Hyperadrenocorticism): While other tests (like the low-dose dexamethasone suppression test) can be more sensitive for diagnosing Cushing’s, the ACTH stimulation test is also commonly used. Dogs with Cushing’s will typically show an exaggerated response to ACTH stimulation.
Monitoring Treatment for Cushing’s Disease: For dogs being treated with medications like trilostane (Vetoryl) or mitotane (Lysodren), regular ACTH stimulation tests are essential to ensure the medication dose is effective and safe, preventing the adrenal glands from becoming too suppressed or not suppressed enough.
Evaluating Adrenal Function after Long-Term Steroid Use: If a dog has been on long-term corticosteroid medication, their own adrenal glands may have become suppressed. An ACTH stim test can assess if they are starting to recover normal function.
How is the Test Performed? (Procedure)
The test typically involves two or three blood samples taken over a short period:
Preparation:
Fasting: Often, an overnight fast (8-12 hours) is recommended, though it’s not strictly necessary for cortisol levels unless other blood work is being done concurrently. Water is usually allowed.
Medications: It’s critical to inform your veterinarian about all medications your dog is receiving, especially corticosteroids (oral, injectable, topical, inhaled, eye drops), as they can significantly interfere with the test results. Some other medications (e.g., phenobarbital, ketoconazole, trilostane) can also impact the results. Your vet will advise if any medications need to be temporarily stopped.
Step 1: Baseline Blood Sample (Time 0)
A blood sample is drawn from your dog. This sample measures the baseline cortisol level before any stimulation. The sample is put into a plain red-top or serum separator tube (SST).
Step 2: ACTH Administration
Immediately after the baseline sample, a synthetic ACTH analogue (most commonly cosyntropin or tetracosactide) is injected. This is typically given intravenously (IV) for a more consistent and rapid absorption, but can sometimes be given intramuscularly (IM). The dose is standardized based on the dog’s weight.
Step 3: Post-Stimulation Blood Sample (Time +60 to +90 minutes)
Approximately 60 to 90 minutes after the ACTH injection (the exact timing can vary slightly depending on the specific protocol and ACTH product used), a second blood sample is drawn. This sample measures the stimulated cortisol level, showing how much cortisol the adrenal glands produced in response to the ACTH. This sample is also put into a red-top or SST tube.
Laboratory Analysis:
Both blood samples are then processed (centrifuged to separate the serum) and sent to a veterinary diagnostic laboratory for cortisol measurement.
Interpreting the Results
Interpretation depends on the disease being investigated:
1. Diagnosing Addison’s Disease
Normal Dog: The baseline cortisol will be within the normal reference range. The post-stimulation cortisol will show a significant increase, rising well above the baseline and into the normal stimulated range.
Dog with Addison’s Disease: Both the baseline and post-stimulation cortisol levels will be very low or undetectable, showing little to no increase after ACTH administration. This indicates that the adrenal glands are unable to produce cortisol.
2. Diagnosing Cushing’s Disease
Normal Dog: As above, a normal baseline and a significant but appropriate rise in cortisol into the normal stimulated range.
Dog with Cushing’s Disease: The baseline cortisol may be normal or elevated. The post-stimulation cortisol will show an exaggerated increase, with levels significantly higher than the normal stimulated range. This indicates the adrenal glands are over-responding due to chronic overstimulation (pituitary-dependent Cushing’s) or autonomous overproduction (adrenal tumor).
Important Note: Up to 15-20% of dogs with Cushing’s may have a “normal” response to an ACTH stim test, especially in early stages or with intermittent disease. Therefore, a normal ACTH stim does not definitively rule out Cushing’s if clinical signs are strong. Other tests like the Low-Dose Dexamethasone Suppression Test might be more sensitive for diagnosis.
3. Monitoring Cushing’s Treatment (e.g., Trilostane)
The goal of trilostane treatment is to bring the post-stimulation cortisol level into a specific “therapeutic range” (e.g., typically 2-5 µg/dL or 40-140 nmol/L, though specific ranges can vary by lab and clinical signs).
The test is usually performed 4-6 hours after the morning dose of trilostane.
If the post-stim cortisol is too high: The trilostane dose may need to be increased.
If the post-stim cortisol is too low (or undetectable): The trilostane dose may need to be decreased, or the medication may need to be stopped temporarily, as this indicates over-suppression of the adrenal glands, potentially leading to iatrogenic Addison’s disease.
Clinical signs are crucial: The test results must always be interpreted in conjunction with the dog’s clinical signs (e.g., thirst, urination, appetite).
Potential Limitations and Considerations
Cost: As it involves multiple blood draws and specialized lab analysis, it can be more expensive than routine blood tests.
Stress: A stressed or anxious dog can have a slightly elevated baseline cortisol, potentially influencing interpretation, though the stimulated value is usually more diagnostic.
Medication Interference: As mentioned, exogenous steroids are the most significant confounder.
Non-Adrenal Illness: Dogs with other severe illnesses (non-adrenal illness) can sometimes have an altered ACTH response, making interpretation challenging.
ACTH Availability: Synthetic ACTH can sometimes have supply issues, making the test temporarily unavailable.
The ACTH stimulation test is a powerful and generally safe procedure that provides invaluable information for diagnosing and managing adrenal gland disorders in dogs. Your veterinarian will determine if this test is appropriate for your dog and will guide you through the process and interpretation of the results.
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