What It Reveals About Cushing's & Addison's Disease
Learn how the ACTH stimulation test works in dogs, what cortisol levels mean, costs, and how vets diagnose Cushing's disease and Addison's disease.
Quick Answer: What Is the ACTH Test in Dogs?
The ACTH stimulation test (also called the cosyntropin stimulation test) is a veterinary blood test that measures how well your dog's adrenal glands respond to adrenocorticotropic hormone (ACTH). A baseline blood sample is drawn, synthetic ACTH is injected, and a second sample is taken 60–90 minutes later. The test measures cortisol levels before and after stimulation to diagnose Cushing's disease (hyperadrenocorticism) or Addison's disease (hypoadrenocorticism). Results typically return within 1–3 business days. Expect to pay $150–$350 for the test itself, plus exam and blood-draw fees.
If your veterinarian has recommended an ACTH test for your dog, you likely have questions about what it involves, what the numbers mean, and what happens next. This guide breaks down the science behind the test, the physiological mechanism it evaluates, and what you need to know as an owner navigating an endocrine workup.
The Science Behind the ACTH Stimulation Test
The Hypothalamic-Pituitary-Adrenal (HPA) Axis
The ACTH test evaluates a feedback loop called the HPA axis — a communication chain between three structures:
- Hypothalamus — detects stress or low cortisol and releases corticotropin-releasing hormone (CRH)
- Pituitary gland — receives CRH and secretes ACTH into the bloodstream
- Adrenal glands — sit atop each kidney; ACTH stimulates the adrenal cortex to produce and release cortisol
Cortisol then feeds back to the hypothalamus and pituitary to suppress further CRH and ACTH release. This negative feedback loop keeps cortisol levels within a narrow, healthy range. When any part of this chain malfunctions, cortisol production goes too high (Cushing's) or too low (Addison's).
The synthetic ACTH used in the test is typically cosyntropin (brand name Cortrosyn), a 25-amino-acoid peptide identical to the first 24 amino acids of natural human ACTH. Research published in the Journal of Veterinary Internal Medicine has confirmed that cosyntropin reliably stimulates cortisol release in dogs, making it the gold standard for assessing adrenal reserve (Merck Veterinary Manual).
How the ACTH Test Works: Step by Step
Here is exactly what happens during an ACTH stimulation test at your veterinary clinic:
| Step | What Happens | Timing |
|---|---|---|
| 1. Baseline blood draw | Vet draws 1–3 mL of blood to measure pre-stimulation (resting) cortisol level | Time 0 |
| 2. ACTH injection | Cosyntropin (5 µg/kg IV or IM, max 250 µg) is administered | Immediately after baseline draw |
| 3. Waiting period | Dog rests quietly in clinic; minimal stress to avoid natural cortisol spikes | 60–90 minutes |
| 4. Post-stimulation draw | Second blood sample taken to measure peak cortisol response | 60–90 min post-injection |
| 5. Lab analysis | Samples sent to reference lab (IDEXX, Antech) or analyzed in-house | 1–3 business days |
Interpreting the Numbers
Cortisol is measured in µg/dL (micrograms per deciliter). While reference ranges vary slightly by laboratory, typical interpretation guidelines are:
| Result Pattern | Pre-ACTH Cortisol | Post-ACTH Cortisol | Likely Diagnosis |
|---|---|---|---|
| Normal response | 1–5 µg/dL | 6–18 µg/dL | Healthy adrenal function |
| Exaggerated response | 4–10 µg/dL | >20–22 µg/dL | Consistent with Cushing's disease (hyperadrenocorticism) |
| Blunted/flat response | <1–2 µg/dL | <2–3 µg/dL | Consistent with Addison's disease (hypoadrenocorticism) |
| Equivocal | Variable | Borderline | Further testing needed (LDDST, endogenous ACTH, imaging) |
Important: No single test is 100% definitive. The ACTH stimulation test has a sensitivity of approximately 80–85% for pituitary-dependent Cushing's and about 60% for adrenal tumor Cushing's, according to the Merck Veterinary Manual. Your vet will combine these results with clinical signs, a low-dose dexamethasone suppression test (LDDST), and possibly abdominal ultrasound to reach a final diagnosis.
Evolutionary Context: Why Dogs Have This HPA Axis
From Wolves to Pugs: The Stress-Response Legacy
The HPA axis evolved in ancestral canids as a survival mechanism. In wild wolves, cortisol surges served critical functions:
- Mobilizing energy — cortisol breaks down fat and protein into glucose for fight-or-flight responses during hunts or territorial confrontations
- Suppressing non-essential systems — digestion, reproduction, and immune function are temporarily down-regulated so energy redirects to muscles and brain
- Circadian regulation — cortisol peaks at dawn to prepare the wolf for activity, then declines through the day
Domestication has not eliminated this system. Modern dogs retain the same HPA axis architecture as Canis lupus. However, selective breeding has altered baseline cortisol patterns in some breeds. A 2019 study in Hormones and Behavior found that companion breeds selected for tameness (e.g., Cavalier King Charles Spaniels) showed lower baseline cortisol compared to working breeds (e.g., German Shepherds), suggesting that domestication pressure has partially dampened the stress response in certain lineages.
This evolutionary backdrop matters clinically. Because cortisol is a stress hormone, a dog who is anxious at the veterinary clinic may show mildly elevated baseline cortisol — which is why vets try to minimize stress during the ACTH test waiting period and interpret results alongside clinical signs rather than in isolation.
Cushing's vs. Addison's: What the ACTH Test Detects
Cushing's Disease (Hyperadrenocorticism)
Cushing's occurs when the adrenal glands chronically overproduce cortisol. There are two forms:
- Pituitary-dependent hyperadrenocorticism (PDH) — accounts for 80–85% of cases. A benign pituitary tumor secretes excess ACTH, overstimulating both adrenal glands.
- Adrenal-dependent hyperadrenocorticism (ADH) — accounts for 15–20% of cases. A tumor on one adrenal gland autonomously produces cortisol.
Common clinical signs include increased thirst and urination (polydipsia/polyuria), pot-bellied appearance, bilateral hair loss (alopecia), increased appetite, panting, and thin skin. PDH is most common in Poodles, Dachshunds, Boston Terriers, and small terrier breeds, typically diagnosed in dogs aged 7–12 years.
Addison's Disease (Hypoadrenocorticism)
Addison's occurs when the adrenal cortex is destroyed (usually by immune-mediated adrenalitis) and cannot produce adequate cortisol or aldosterone. Without aldosterone, the body cannot retain sodium or excrete potassium, leading to dangerous electrolyte imbalances.
Signs are often vague and intermittent: lethargy, vomiting, diarrhea, weakness, shaking, collapse, and dehydration. An Addisonian crisis is a life-threatening emergency requiring immediate IV fluids and glucocorticoids. Breeds with higher predisposition include Standard Poodles, Portuguese Water Dogs, Bearded Collies, and Nova Scotia Duck Tolling Retrievers.
Common Misconceptions About the ACTH Test
| Myth | Fact |
|---|---|
| "One ACTH test result is always definitive" | Sensitivity is 80–85% for PDH and ~60% for adrenal tumors. Vets combine ACTH results with LDDST, urine cortisol:creatinine ratio, and imaging for a full picture. |
| "The ACTH test is painful for my dog" | The test requires two small blood draws and one injection — comparable to a routine wellness blood panel. Most dogs tolerate it with minimal discomfort. |
| "My dog can eat and drink normally before the test" | Most vets recommend fasting for 8–12 hours before the test. Certain medications (especially steroids) must be discontinued weeks in advance — always follow your vet's specific instructions. |
| "Cushing's only affects old dogs" | While most cases are diagnosed in dogs over 7, iatrogenic Cushing's (caused by long-term steroid medication) can occur at any age. Adrenal tumors have also been reported in dogs as young as 5–6. |
| "If the ACTH test is normal, my dog definitely doesn't have Cushing's" | False negatives occur in 15–20% of PDH cases. If clinical signs strongly suggest Cushing's, your vet may recommend an LDDST test or repeat the ACTH test at a later date. |
Practical Implications: What This Means for You and Your Dog
Preparing for the ACTH Test
- Medication review: Tell your vet about every medication, supplement, and topical product your dog uses. Corticosteroids (prednisone, dexamethasone, hydrocortisone creams, ear drops with steroids) can suppress the HPA axis and skew results. Your vet may ask you to taper off steroids 4–8 weeks before testing.
- Fasting: Withhold food for 8–12 hours before the appointment. Water is usually fine.
- Minimize stress: Bring a familiar blanket or toy. If your dog has severe vet anxiety, ask your vet about a pre-visit dose of trazodone (typically 2–5 mg/kg given 90 minutes before the appointment) to reduce stress-induced cortisol elevation.
- Plan for a 2-hour visit: The baseline draw, waiting period, and post-stimulation draw take roughly 90 minutes plus check-in time.
Cost Breakdown
| Item | Estimated Cost (2026) |
|---|---|
| Office exam fee | $60–$120 |
| ACTH stimulation test (cosyntropin + lab analysis) | $150–$350 |
| LDDST (if also ordered) | $100–$250 |
| Abdominal ultrasound (if recommended) | $350–$600 |
| Full endocrine workup (combined) | $600–$1,300+ |
If your dog is diagnosed with Cushing's, ongoing treatment with trilostane (Vetoryl) typically costs $40–$120/month depending on dog size and dose, plus monitoring ACTH tests every 3–6 months. Treatment for Addison's involves lifelong fludrocortisone (Florinef) or desoxycorticosterone pivalate (DOCP/Percorten-V) injections, generally $50–$150/month.
When to See the Vet Immediately
Emergency red flags: If your dog shows sudden collapse, severe vomiting/diarrhea, extreme weakness, or inability to stand — these may signal an Addisonian crisis. Seek emergency veterinary care immediately. For suspected toxic ingestions, contact the ASPCA Animal Poison Control Center at (888) 426-4435 (24/7, consultation fee applies).
Breed Variations in HPA Axis Function
Research and clinical data show breed-level patterns in adrenal disease prevalence:
| Condition | Higher-Risk Breeds | Typical Age at Diagnosis |
|---|---|---|
| Pituitary-dependent Cushing's | Poodles (all sizes), Dachshunds, Boston Terriers, Yorkshire Terriers, Cocker Spaniels | 7–12 years |
| Adrenal tumor Cushing's | Poodles, Dachshunds, German Shepherds (larger breeds slightly overrepresented) | 8–14 years |
| Addison's disease | Standard Poodles, Portuguese Water Dogs, Bearded Collies, Nova Scotia Duck Tolling Retrievers, West Highland White Terriers | 4–7 years (often younger than Cushing's) |
Notably, Standard Poodles appear on both lists — this breed has a documented genetic predisposition to immune-mediated adrenal disease. If you own a breed on either list, discuss baseline endocrine screening with your veterinarian during annual wellness exams, especially once your dog reaches middle age.
How Is the Canine HPA Axis Different From Humans?
While the fundamental HPA axis architecture is shared across mammals, there are clinically important differences between dogs and humans:
- Cortisol binding: Dogs have lower levels of cortisol-binding globulin (CBG) than humans, meaning a larger fraction of circulating cortisol is biologically active (free cortisol). This affects how lab reference ranges are set.
- Stress leukogram: In dogs, elevated cortisol produces a characteristic white blood cell pattern (neutrophilia, lymphopenia, eosinopenia, monocytosis) that is more pronounced and longer-lasting than in humans. Vets use this pattern as a secondary clue.
- Steroid metabolism: Dogs metabolize exogenous corticosteroids faster than humans, which is why dogs on prednisone typically receive higher mg/kg doses than human patients.
- Seasonal variation: Unlike humans, some dogs show mild seasonal fluctuations in baseline cortisol, potentially linked to photoperiod effects inherited from their wolf ancestors.
Related Tests Your Vet May Recommend
The ACTH stimulation test is one tool in a broader endocrine toolkit. Depending on results, your veterinarian may also order:
- Low-Dose Dexamethasone Suppression Test (LDDST): More sensitive (~95%) for diagnosing Cushing's but less specific. Involves three blood draws over 8 hours after a low-dose dexamethasone injection. Helps differentiate pituitary from adrenal causes.
- Urine Cortisol:Creatinine Ratio (UCCR): A non-invasive screening test collected at home. Very sensitive but not specific — a normal result effectively rules out Cushing's, but an elevated result requires follow-up testing.
- Endogenous ACTH Level: Measures the dog's own ACTH production. High levels suggest pituitary origin; low/undetectable levels suggest an adrenal tumor suppressing pituitary ACTH via negative feedback.
- Abdominal Ultrasound: Visualizes adrenal gland size and shape. Enlarged, symmetrical glands suggest PDH; one enlarged and one atrophied gland suggests an adrenal tumor. Also screens for liver changes consistent with chronic cortisol excess.
Frequently Asked Questions
Why does my dog need an ACTH test?
Your vet recommends the ACTH test when clinical signs (excessive thirst, urination, hair loss, pot belly, or conversely, lethargy, vomiting, collapse) suggest an adrenal hormone disorder. The test objectively measures whether your dog's adrenal glands can respond appropriately to hormonal stimulation.
Can my dog be on medications during the ACTH test?
Some medications interfere with results. Corticosteroids (prednisone, dexamethasone, hydrocortisone) must typically be discontinued 2–8 weeks before testing. Mitotane and trilostane (Cushing's treatment drugs) also affect results — if your dog is already on Cushing's medication, the ACTH test is used differently, as a monitoring tool to assess whether the dose is appropriate. Always consult your veterinarian before stopping any prescribed medication.
Is the ACTH test safe for my dog?
Yes. Cosyntropin is well-tolerated in the vast majority of dogs. Rare side effects include mild allergic reaction at the injection site. The test involves minimal risk — two small blood draws and one injection. Dogs with known severe allergies should be monitored during the waiting period.
What does it mean when my dog's post-ACTH cortisol is very high?
A post-stimulation cortisol exceeding 20–22 µg/dL (lab-dependent) is consistent with hyperadrenocorticism (Cushing's disease). The adrenal glands are overresponding because they have been chronically overstimulated by excess pituitary ACTH or because an adrenal tumor is autonomously producing cortisol. Your vet will correlate this with clinical signs and may order an LDDST or ultrasound to confirm and differentiate the cause.
Is a single ACTH test enough to diagnose Addison's disease?
In most cases, yes. A blunted or flat cortisol response (pre- and post-ACTH cortisol both below 2–3 µg/dL), combined with compatible electrolyte abnormalities (low sodium, high potassium, Na:K ratio <27), is highly specific for Addison's disease. The AVMA and veterinary endocrinology guidelines consider the ACTH stimulation test the definitive diagnostic for hypoadrenocorticism.
How often should the ACTH test be repeated for monitoring?
For dogs on trilostane (Vetoryl) for Cushing's disease, ACTH stimulation tests are typically performed at 10–14 days after starting treatment, then at 30 days, then every 3–6 months long-term. For dogs on DOCP (Percorten-V) for Addison's, electrolyte panels are checked every 3–6 months, with ACTH tests performed less frequently unless clinical signs change.
This article is for informational purposes only and does not replace professional veterinary advice. Always consult your veterinarian for diagnosis, treatment planning, and medication management for your dog's specific condition.
Jonas Cole
All our authors care for dogs every day — read more of their work on the authors page.
