ACTH Stimulation Test: What It Is, How It Works, & What...
Learn how the ACTH stimulation test works in dogs, what cortisol results reveal about Cushing's and Addison's disease, costs, and what to expect.
What Is the ACTH Stimulation Test in Dogs?
The ACTH stimulation test is a diagnostic blood test that measures how well your dog's adrenal glands respond to synthetic adrenocorticotropic hormone (ACTH). A baseline blood sample is drawn, then an injection of synthetic ACTH (cosyntropin) is given, and a second blood sample is taken 1–2 hours later. The test evaluates whether the adrenal glands produce an appropriate amount of cortisol in response. It is the primary screening tool for hyperadrenocorticism (Cushing's disease) and can also help confirm hypoadrenocorticism (Addison's disease).
Typical cost: $150–$350 for the test itself, plus $80–$200 for the office visit and baseline bloodwork, bringing the total to roughly $230–$550 depending on your region and clinic.
The Endocrine Science Behind the ACTH Stimulation Test
To understand why veterinarians rely on this test, you need to follow the hormonal cascade known as the hypothalamic-pituitary-adrenal (HPA) axis. Here is how it works step by step:
- Hypothalamus releases corticotropin-releasing hormone (CRH) in response to stress, low blood sugar, or circadian signals.
- Pituitary gland (specifically the anterior pituitary) detects CRH and secretes adrenocorticotropic hormone (ACTH) into the bloodstream.
- Adrenal glands (the cortex layer, specifically the zona fasciculata) detect ACTH and respond by synthesizing and releasing cortisol, the primary stress glucocorticoid.
- Cortisol then exerts negative feedback on both the hypothalamus and pituitary, reducing further CRH and ACTH release — a self-regulating loop.
The ACTH stimulation test essentially bypasses steps 1 and 2. By injecting synthetic ACTH directly, your veterinarian can isolate and evaluate step 3: Are the adrenal glands capable of producing cortisol when told to do so?
In a healthy dog, baseline cortisol typically ranges from 1–5 µg/dL, and post-ACTH cortisol rises to 6–18 µg/dL (reference ranges vary slightly by laboratory). A blunted or exaggerated response signals disease somewhere along the HPA axis.
Why the HPA Axis Evolved This Way in Canids
The HPA axis is an ancient vertebrate system. In ancestral wolves and early canids, cortisol surges served immediate survival functions: mobilizing glucose for fight-or-flight responses, suppressing non-essential immune activity during acute injury, and sharpening alertness during territorial conflicts or hunts.
Wolves living in hierarchical pack structures experienced frequent social stressors — submissive individuals often showed chronically elevated cortisol compared to dominant pack members. Research on captive wolf endocrinology (PubMed, various studies on canid glucocorticoids) has shown that social rank directly modulates baseline cortisol output.
Domestic dogs inherited this same HPA architecture, but selective breeding has altered its regulation. Brachycephalic breeds, for example, may experience chronic low-grade HPA activation due to breathing difficulty. The ACTH stimulation test lets veterinarians determine whether a modern dog's adrenal system is functioning within normal parameters — or whether dysregulation has developed into clinical disease.
When and Why Your Veterinarian Orders This Test
The ACTH stimulation test is not a routine screening. It is ordered when clinical signs suggest adrenal dysfunction. Here are the two primary scenarios:
Suspected Cushing's Disease (Hyperadrenocorticism)
Cushing's disease occurs when the body produces too much cortisol over a prolonged period. Approximately 80–85% of canine Cushing's cases are pituitary-dependent (a benign pituitary tumor overproduces ACTH), while 15–20% are adrenal-dependent (an adrenal tumor autonomously secretes cortisol). The ACTH stim test is the most commonly used first-line screening tool, though it has limitations:
- Sensitivity for pituitary-dependent Cushing's: approximately 80–85%
- Sensitivity for adrenal-dependent Cushing's: approximately 50–60%
- Specificity: approximately 90–95%
Because sensitivity is imperfect, a normal ACTH stim result does not completely rule out Cushing's. Your veterinarian may follow up with a low-dose dexamethasone suppression test (LDDST), which has higher sensitivity (~95%) but lower specificity. According to the Merck Veterinary Manual, combining both tests provides the most reliable diagnosis.
Suspected Addison's Disease (Hypoadrenocorticism)
Addison's disease is the opposite problem: the adrenal glands fail to produce sufficient cortisol (and often aldosterone). The ACTH stimulation test is considered the gold standard for diagnosing Addison's. In affected dogs, baseline cortisol is typically very low (<1 µg/dL), and post-ACTH cortisol fails to rise above 2 µg/dL. This flat-line response is highly diagnostic.
Iatrogenic Cushing's Monitoring
Dogs on long-term corticosteroid therapy (e.g., prednisone for autoimmune disease or allergies) can develop adrenal suppression. The ACTH stim test may be used to assess whether the adrenal glands have atrophied from exogenous steroid exposure and to guide safe tapering schedules.
| Result Pattern | Baseline Cortisol | Post-ACTH Cortisol | Likely Interpretation |
|---|---|---|---|
| Normal response | 1–5 µg/dL | 6–18 µg/dL | Healthy adrenal function |
| Exaggerated response | Normal to high | >18–22 µg/dL | Consistent with Cushing's (pituitary-dependent) |
| Blunted/flat response | <1 µg/dL | <2 µg/dL | Consistent with Addison's disease |
| Moderate elevation | Normal | High-normal to mildly elevated | Stress-related; may need LDDST to clarify |
What Happens During the Test: Step-by-Step Protocol
Understanding the procedure helps you prepare your dog and set expectations. Here is what a standard ACTH stimulation test looks like in practice:
- Pre-test preparation: Your dog should be fasted for 8–12 hours (water is fine). Inform your vet of all medications — especially any steroids, as these must typically be discontinued weeks in advance. Even topical ear or eye medications containing steroids can interfere.
- Arrival and settling (15–30 minutes): Stress elevates cortisol, so clinics try to let your dog acclimate to the environment before drawing blood. Some vets recommend bringing your dog's favorite blanket or toy.
- Baseline blood draw (T=0): A veterinary technician draws 1–3 mL of blood from the jugular or cephalic vein. This sample measures resting cortisol.
- ACTH injection: Synthetic cosyntropin (Cortrosyn or equivalent) is administered. The standard dose is 5 µg/kg IV or 250 µg total dose IM for most dogs (protocols vary by clinic and whether the IV or IM route is chosen). Intravenous administration is considered more reliable.
- Post-injection blood draw (T=60 or T=90 minutes): A second blood sample is drawn exactly 1 hour (IV protocol) or 1–2 hours (IM protocol) after the injection.
- Sample processing: Both serum samples are sent to a reference laboratory (e.g., IDEXX, Antech) or run in-house if the clinic has a cortisol assay analyzer. Results typically return within 24–48 hours for reference labs, or same-day for in-house testing.
The entire appointment usually takes 2–3 hours. Most dogs tolerate the procedure well — the injection is a quick pinch, and the blood draws are routine.
Practical Takeaways: What Results Mean for Your Dog's Daily Life
If Your Dog Is Diagnosed with Cushing's Disease
- Medication: Trilostane (Vetoryl) is the first-line treatment. It inhibits cortisol synthesis. Typical starting dose: 1–2 mg/kg once daily with food. Monthly monitoring is required for the first 3–6 months, then every 3–6 months.
- Cost: Vetoryl runs $40–$120/month depending on dose size. Add $80–$200 per monitoring visit with repeat ACTH stim tests.
- Prognosis: With proper management, most dogs live 2–4+ years post-diagnosis with good quality of life.
- Daily life changes: Expect reduced water intake and urination within 2–4 weeks of starting treatment. Monitor for lethargy, vomiting, or collapse — these can signal overtreatment (Addisonian crisis) and require emergency vet care.
If Your Dog Is Diagnosed with Addison's Disease
- Medication: Lifelong mineralocorticoid replacement is needed. DOCP (Percorten-V, Zycortal) injections every 25–30 days ($60–$150 per dose) plus low-dose prednisone ($5–$15/month).
- Emergency risk: Addisonian crisis (acute collapse, vomiting, weakness, shock) is life-threatening. Keep an emergency injectable steroid kit at home if your vet recommends one.
- Prognosis: Excellent with consistent treatment — many Addison's dogs live a normal lifespan.
- Daily life: Stress management is critical. Any illness, surgery, boarding, or travel may require a temporary prednisone dose increase. Always inform any veterinarian treating your dog about the Addison's diagnosis.
Common Misconceptions About the ACTH Stimulation Test
| Myth | Fact |
|---|---|
| "A normal ACTH stim result means my dog definitely doesn't have Cushing's." | False. The test misses ~15–20% of pituitary-dependent cases and up to 50% of adrenal-dependent cases. A low-dose dexamethasone suppression test or abdominal ultrasound may still be needed. |
| "The test is painful and dangerous." | The test involves two routine blood draws and one small injection. Side effects are extremely rare. The synthetic ACTH has an excellent safety profile. |
| "My dog's high cortisol just means they were stressed at the vet." | Stress can mildly elevate cortisol, but post-ACTH levels above 22 µg/dL are strongly suggestive of pathology, not just white-coat syndrome. Your vet considers clinical signs alongside lab values. |
| "Cushing's only affects old dogs, so my 5-year-old can't have it." | While Cushing's is most common in dogs over 7, it can appear in middle-aged dogs (6+). Adrenal tumors can occur even earlier. Breed predisposition also matters — Poodles, Dachshunds, and small Terriers are at higher risk. |
| "If my dog is on steroids, the test won't work." | Exogenous steroids do interfere with results, which is why your vet will typically taper and discontinue them 4–8 weeks before testing. Never stop steroids abruptly without veterinary guidance — this can cause an Addisonian crisis. |
Breed Variations and Predispositions to Adrenal Disease
Not all dogs face equal risk. Certain breeds show significantly higher rates of adrenal dysfunction, which influences when and how aggressively veterinarians pursue ACTH stimulation testing:
- Poodle (all sizes): Among the highest-risk breeds for pituitary-dependent Cushing's. Miniature and Toy Poodles are especially overrepresented.
- Dachshund: Elevated Cushing's risk, particularly in middle-aged to older dogs.
- Yorkshire Terrier, Boston Terrier, and other small terriers: Higher prevalence of both Cushing's and atypical presentations.
- Great Dane, Portuguese Water Dog, and Standard Poodle: Overrepresented in Addison's disease cases. The Standard Poodle has a known heritable component — some lines carry an elevated risk of immune-mediated adrenal destruction.
- Beagle: Moderately increased Addison's risk, likely due to autoimmune predisposition.
If you own one of these breeds and notice symptoms like excessive thirst and urination (polydipsia/polyuria), a pot-bellied abdomen, symmetrical hair loss, or unexplained lethargy and weight loss, ask your veterinarian whether an ACTH stimulation test is warranted.
When Adrenal Symptoms Signal Something More Serious
Emergency Red Flags — Seek Veterinary Care Immediately
- Acute collapse or inability to stand — possible Addisonian crisis or adrenal tumor rupture
- Severe vomiting and diarrhea with weakness — electrolyte imbalances from adrenal failure can cause cardiac arrhythmia
- Sudden blindness or neurological signs (head pressing, seizures, disorientation) — may indicate a large pituitary macrotumor (a complication in ~10–15% of Cushing's cases)
- Rapidly worsening abdominal distension with labored breathing — could signal internal bleeding from an adrenal mass
These scenarios require emergency veterinary intervention. Do not wait for scheduled test results. Always consult your veterinarian promptly when you observe acute changes in your dog's condition.
Frequently Asked Questions
Can the ACTH stimulation test give false results?
Yes. False negatives occur in roughly 15–20% of Cushing's cases (especially adrenal-dependent forms). False positives can occur in dogs with severe non-adrenal illness, chronic stress, or those recently on steroid medications. This is why the ACTH stim test is interpreted alongside clinical signs, a complete blood count, chemistry panel, urinalysis, and sometimes imaging (abdominal ultrasound, CT of the pituitary).
How is this different from the low-dose dexamethasone suppression test (LDDST)?
The LDDST is more sensitive for detecting Cushing's (~95% vs. ~80–85% for ACTH stim) but less specific — it can produce false positives in dogs with non-adrenal illness. The LDDST also takes 8 hours (blood draws at 0, 4, and 8 hours after dexamethasone injection). Many veterinarians start with the ACTH stim test and follow up with the LDDST if results are equivocal but clinical suspicion remains high.
Does my dog need to be sedated for the ACTH stim test?
No. Sedation is generally unnecessary and can actually interfere with cortisol measurements. The test is well tolerated by the vast majority of dogs. If your dog is extremely anxious at the vet, discuss options like a pre-visit trazodone dose (typically 5–10 mg/kg, given 2 hours before the appointment) with your veterinarian.
Can I do an at-home cortisol test instead?
At-home urine cortisol:creatinine ratio tests exist and can serve as a preliminary screening tool — a normal result helps rule out Cushing's. However, an abnormal result requires follow-up with a proper ACTH stimulation test or LDDST at your veterinary clinic. Urine tests cannot diagnose Addison's disease or distinguish between pituitary-dependent and adrenal-dependent Cushing's.
How often should my dog be retested after a Cushing's or Addison's diagnosis?
For Cushing's on trilostane: ACTH stim testing is repeated at 10–14 days after starting treatment, then at 30 days, 90 days, and every 3–6 months thereafter. For Addison's on DOCP: electrolyte panels are checked at 14 days, 30 days, and then every 3–6 months. The ACTH stim test itself is not routinely repeated once Addison's is confirmed — monitoring focuses on sodium:potassium ratios and clinical signs.
What does it mean when my dog has Cushing's symptoms but normal test results?
This scenario is called atypical Cushing's. Some dogs produce excess sex hormones (androstenedione, estradiol, progesterone) rather than cortisol, so standard cortisol-based tests appear normal. Your vet may order an extended adrenal panel measuring these intermediate hormones. The AKC's guide on Cushing's disease notes that atypical cases require specialized endocrine testing, often through university veterinary teaching hospitals.
This article is intended for educational purposes and does not replace professional veterinary advice. Always consult your veterinarian for diagnosis and treatment of adrenal disorders in your dog.
Anouk Beaumont
All our authors care for dogs every day — read more of their work on the authors page.
