ACTH Test in Dogs: What It Is, How It Works, and What Results Mean
Learn how the ACTH stimulation test in dogs diagnoses Cushing's disease and Addison's disease, including costs, procedure steps, and result interpretation.
Quick Answer: What Is the ACTH Test in Dogs?
The ACTH stimulation test (also called the cosyntropin stimulation test) is a diagnostic blood test that evaluates 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 1–2 hours later. The test measures cortisol levels before and after stimulation to help diagnose Cushing's disease (hyperadrenocorticism) or Addison's disease (hypoadrenocorticism). Results typically take 1–3 days, and the total cost ranges from $150 to $350 depending on your region and clinic.
If your veterinarian has recommended an ACTH test for your dog, you likely have questions about what the procedure involves, what the results mean, and whether your dog is at risk for an adrenal disorder. This guide breaks down the science, the procedure, and the practical details you need to make informed decisions about your dog's endocrine health.
The Endocrine Science Behind the ACTH Test
The Hypothalamic-Pituitary-Adrenal (HPA) Axis
The ACTH test evaluates a feedback loop that is central to your dog's stress response, metabolism, and immune regulation. Here is how the HPA axis works in a healthy dog:
- The hypothalamus (a region in the brain) detects stress, low blood sugar, or inflammation and releases corticotropin-releasing hormone (CRH).
- CRH travels to the pituitary gland, which responds by secreting adrenocorticotropic hormone (ACTH) into the bloodstream.
- ACTH reaches the adrenal glands (small organs sitting on top of each kidney) and stimulates them to produce cortisol, the primary stress hormone.
- Rising cortisol levels signal the hypothalamus and pituitary to reduce CRH and ACTH output — a negative feedback loop that keeps cortisol in a healthy range.
When any component of this axis malfunctions, cortisol production becomes dysregulated. The ACTH stimulation test directly challenges step 3 of this process by flooding the adrenal glands with synthetic ACTH (cosyntropin) and measuring how much cortisol they produce in response.
According to the Merck Veterinary Manual, approximately 80–85% of canine Cushing's cases are pituitary-dependent (a benign pituitary tumor overproduces ACTH), while 15–20% are adrenal-dependent (a tumor on the adrenal gland itself). A small percentage are iatrogenic, caused by prolonged corticosteroid medication use. The ACTH test helps differentiate these causes and confirms whether the adrenal glands are functioning normally.
Evolutionary Context: Why the HPA Axis Matters
From Wolves to Pugs: The Stress Response Legacy
The HPA axis evolved in ancestral canids as a survival mechanism. In wild wolves, acute cortisol spikes provided the energy burst needed to hunt, fight, or flee. Cortisol mobilizes glucose from liver stores, suppresses non-essential functions like digestion and reproduction, and sharpens alertness. Once the threat passed, the negative feedback loop returned cortisol to baseline.
Modern domestic dogs retain this identical physiological system, but their stressors have shifted from predator avoidance to thunderstorms, veterinary visits, and separation anxiety. The ACTH test essentially probes this ancient survival system to determine whether it is functioning within normal parameters or has become pathologically dysregulated.
Notably, research published in the Journal of Veterinary Internal Medicine has found that certain breeds — particularly Poodles, Dachshunds, and small Terriers — show higher prevalence of pituitary-dependent Cushing's disease, suggesting genetic predispositions layered on top of this ancestral endocrine architecture.
The ACTH Test Procedure: Step by Step
Understanding the logistics of the test helps you prepare your dog and set realistic expectations for the clinic visit.
Pre-Test Preparation
- Fasting: Most veterinarians recommend withholding food for 8–12 hours before the test. Water is typically allowed.
- Medication review: Tell your vet about all medications, especially corticosteroids (prednisone, dexamethasone), as these suppress the HPA axis and can produce falsely low results. Your vet may taper steroids 2–4 weeks before testing.
- Timing: Cortisol follows a diurnal rhythm (highest in the morning), so many clinics schedule the test between 8:00–10:00 AM. However, the ACTH stimulation test is less timing-sensitive than a baseline cortisol draw alone.
- Sedation: Generally not required. The test involves two blood draws and one injection, which most dogs tolerate with minimal restraint.
The Testing Protocol
| Step | Action | Timeframe | Details |
|---|---|---|---|
| 1 | Baseline blood draw | T = 0 minutes | Venipuncture (usually cephalic or jugular vein) to measure pre-stimulation cortisol |
| 2 | ACTH injection | T = 0–5 minutes | Synthetic cosyntropin (Cortrosyn) administered — dose is 5 µg/kg IV or 250 µg per dog IM, depending on protocol |
| 3 | Waiting period | 60–90 minutes | Dog rests in clinic; adrenal glands respond to ACTH |
| 4 | Post-stimulation blood draw | T = 60–90 minutes | Second venipuncture to measure stimulated cortisol level |
| 5 | Sample processing | 1–3 days | Serum separated and sent to reference lab (IDEXX, Antech) or analyzed in-house |
The entire clinic visit typically takes 2–3 hours. Some practices use a 1-hour post-stimulation draw, while others use a 2-hour protocol. Both are clinically validated, though your veterinarian will follow whichever protocol their reference laboratory recommends.
Cost Breakdown
| Line Item | Low Range | Average | High Range |
|---|---|---|---|
| Office exam fee | $50 | $75 | $120 |
| Blood draws (×2) | $30 | $50 | $80 |
| Cosyntropin (Cortrosyn) vial | $40 | $65 | $95 |
| Cortisol assay (lab fee) | $50 | $80 | $130 |
| Total | $170 | $270 | $425 |
Specialty or emergency hospitals in major metropolitan areas may charge toward the higher end. University teaching hospitals sometimes offer slightly lower lab fees but may add consultation charges.
Interpreting ACTH Test Results
Results are reported as two cortisol values: pre-ACTH (baseline) and post-ACTH (stimulated), measured in µg/dL (micrograms per deciliter). Reference ranges vary slightly by laboratory, but the following framework applies broadly:
Normal Response
- Baseline cortisol: 1–5 µg/dL
- Post-ACTH cortisol: 5–18 µg/dL (typically a 2–6× increase from baseline)
- Interpretation: Adrenal glands are functioning normally and respond appropriately to ACTH stimulation.
Results Suggesting Cushing's Disease (Hyperadrenocorticism)
- Baseline cortisol: Often elevated (>4 µg/dL)
- Post-ACTH cortisol: Exaggerated response (>18–20 µg/dL, sometimes >30 µg/dL)
- Interpretation: Adrenal glands are overproducing cortisol. In pituitary-dependent Cushing's, both glands are hypertrophied and respond excessively. In adrenal-dependent Cushing's, the tumor-bearing gland may produce cortisol autonomously while the contralateral gland is atrophied — sometimes resulting in a blunted response that mimics Addison's, which is why the Veterinary Information Network recommends combining the ACTH test with a low-dose dexamethasone suppression test (LDDST) and abdominal ultrasound for definitive diagnosis.
Results Suggesting Addison's Disease (Hypoadrenocorticism)
- Baseline cortisol: Low (<1 µg/dL)
- Post-ACTH cortisol: Minimal or no increase (stays <2 µg/dL)
- Interpretation: Adrenal glands are damaged or destroyed and cannot produce cortisol even when stimulated. This is the gold-standard diagnostic finding for Addison's disease.
Breed Predispositions to Adrenal Disorders
Not all dogs face equal risk for the conditions the ACTH test helps diagnose. Research has identified clear breed associations:
| Condition | High-Risk Breeds | Typical Age of Onset | Estimated Prevalence |
|---|---|---|---|
| Pituitary-dependent Cushing's | Poodle (Miniature/Toy), Dachshund, Yorkshire Terrier, Boston Terrier, Beagle | 7–12 years | 1–2 per 1,000 dogs/year |
| Adrenal-dependent Cushing's | Poodle, Dachshund, German Shepherd; more common in females | 8–14 years | 15–20% of all Cushing's cases |
| Addison's disease | Standard Poodle, Portuguese Water Dog, Bearded Collie, Great Dane, West Highland White Terrier | 4–7 years | 0.1–0.3% of general dog population |
The Standard Poodle is particularly notable: a study published in PLOS Genetics identified a specific genetic variant associated with Addison's disease in the breed, confirming a hereditary autoimmune basis. If you own a Standard Poodle or Portuguese Water Dog, discussing baseline endocrine screening with your veterinarian during middle-age wellness visits (around 5–6 years) is a reasonable proactive step.
Common Misconceptions About the ACTH Test
| Myth | Fact |
|---|---|
| "A normal ACTH test rules out Cushing's disease completely." | False. The ACTH test has ~80–85% sensitivity for pituitary-dependent Cushing's. A normal result does not exclude the diagnosis if clinical signs are strong. An LDDST or urine cortisol:creatinine ratio may be needed. |
| "The ACTH test is painful for my dog." | The test requires two routine venipuncture blood draws and one injection — comparable to a vaccination visit. Most dogs show minimal discomfort. |
| "My dog's elevated cortisol means they definitely have Cushing's." | Stress, illness, and non-adrenal conditions (e.g., liver disease, uncontrolled diabetes) can elevate cortisol. This is why the ACTH test must be interpreted alongside clinical signs, urinalysis, and imaging. |
| "Only old dogs get adrenal disease." | While Cushing's is most common in dogs over 7, Addison's disease frequently appears in dogs aged 4–7, and juvenile cases have been reported in predisposed breeds. |
| "The test requires general anesthesia." | No anesthesia is needed. The test is performed with the dog awake, often resting quietly in a kennel between blood draws. |
Practical Takeaways: What This Means for Your Dog
Clinical Signs That May Prompt an ACTH Test
Your veterinarian may recommend this test if your dog exhibits any combination of the following:
- For Cushing's suspicion: Increased thirst and urination (polydipsia/polyuria), pot-bellied appearance, hair loss (symmetric, bilateral, sparing the head and legs), thin skin, recurrent skin infections, increased appetite, muscle wasting, panting
- For Addison's suspicion: Lethargy, vomiting, diarrhea, weight loss, weakness, collapse (especially under stress), waxing-and-waning appetite, shaking/trembling, dark or tarry stools
After the Test: What to Expect
- Same day: Your dog can go home after the second blood draw. No special post-test care is needed. Mild bruising at the venipuncture site is normal and resolves in 24–48 hours.
- Results timeline: Most reference labs report cortisol results within 24–72 hours. Your vet will call or schedule a follow-up to discuss findings.
- If Cushing's is confirmed: Treatment typically involves trilostane (Vetoryl), dosed at 1–2 mg/kg once or twice daily. Medication costs range from $40–$120/month depending on dog size. Monitoring ACTH tests are repeated at 10–14 days, then 30 days, then every 3–6 months for life.
- If Addison's is confirmed: Treatment involves mineralocorticoid replacement (DOCP/Percorten-V injections every 25–30 days at ~$50–$90 per injection, or daily fludrocortisone tablets) plus low-dose prednisone. Most Addisonian dogs live normal lifespans with consistent treatment.
How the ACTH Test Differs From Other Adrenal Tests
The ACTH stimulation test is one of several tools veterinarians use to evaluate adrenal function. Understanding how it compares helps you follow your vet's diagnostic reasoning:
| Test | What It Measures | Best For Diagnosing | Sensitivity | Cost Range |
|---|---|---|---|---|
| ACTH stimulation test | Adrenal reserve (cortisol response to ACTH) | Addison's (gold standard); Cushing's (moderate sensitivity) | Addison's: ~99%; Cushing's: ~80–85% | $150–$350 |
| Low-dose dexamethasone suppression test (LDDST) | Negative feedback integrity of HPA axis | Cushing's (high sensitivity) | ~95% for PDH; ~100% for ADH | $120–$300 |
| Urine cortisol:creatinine ratio (UCCR) | Integrated cortisol output over hours | Screening test to rule out Cushing's | ~95–100% (high sensitivity, low specificity) | $40–$80 |
| High-dose dexamethasone suppression test (HDDST) | Differentiates pituitary vs. adrenal Cushing's | Subtype differentiation | Variable (~70% for PDH) | $120–$280 |
| Endogenous ACTH level | Circulating ACTH concentration | Differentiates PDH vs. ADH | High when properly handled | $80–$150 |
In practice, many veterinarians begin with the ACTH stimulation test and LDDST together when Cushing's is suspected, as the combination provides the most complete diagnostic picture. For Addison's suspicion, the ACTH test alone is usually sufficient.
When Adrenal Dysfunction Signals a Deeper Problem
While the ACTH test evaluates adrenal output, abnormal results can sometimes point to conditions beyond primary adrenal disease:
- Atypical Cushing's: Some dogs show clinical signs of Cushing's with normal cortisol levels on standard tests. These dogs may overproduce sex hormones (progesterone, androstenedione, estradiol) rather than cortisol. An extended adrenal panel measuring sex hormone precursors is needed.
- Pituitary macroadenoma: In ~10–20% of pituitary-dependent Cushing's cases, the pituitary tumor grows large enough (>10 mm) to cause neurological signs — head pressing, disorientation, seizures, or vision changes. If your dog shows neurological symptoms alongside Cushing's signs, your vet may recommend an MRI or CT scan (cost: $1,500–$3,000 at a specialty center).
- Adrenal carcinoma: Adrenal tumors can be malignant and may invade the caudal vena cava. Abdominal ultrasound is essential when adrenal-dependent Cushing's is suspected to evaluate tumor size, vascular invasion, and metastasis risk.
- Concurrent diabetes: Approximately 10–15% of dogs with Cushing's also develop diabetes mellitus due to cortisol-induced insulin resistance. Blood glucose and fructosamine testing should accompany adrenal workups.
Frequently Asked Questions
Can my dog eat before the ACTH test?
Most protocols require an 8–12 hour fast before the test. Food intake can influence cortisol metabolism and may interfere with result accuracy. Water is generally permitted. Confirm specific fasting instructions with your veterinarian's office before the appointment.
Is the ACTH test safe for all dogs?
Yes. Cosyntropin (synthetic ACTH) has an excellent safety profile. Adverse reactions are extremely rare. The test is safe for puppies, senior dogs, and dogs with most concurrent illnesses. The only significant contraindication is recent corticosteroid use, which can suppress the HPA axis and produce misleading results — your vet will advise on medication washout periods.
How is this different from a baseline cortisol test?
A baseline (resting) cortisol test measures a single cortisol value at one moment in time. It is highly variable and influenced by stress, time of day, and illness. The ACTH stimulation test challenges the adrenal glands directly, providing a dynamic assessment of their functional capacity. A resting cortisol alone is rarely diagnostic for adrenal disease.
What does it mean when my dog's post-ACTH cortisol is very high?
An exaggerated cortisol response (typically >20 µg/dL post-stimulation) suggests the adrenal glands are hyperresponsive, which is consistent with pituitary-dependent Cushing's disease. However, stress and non-adrenal illness can also elevate cortisol, so this result must be interpreted alongside clinical signs and additional testing like the LDDST.
Is Cushing's disease curable?
Pituitary-dependent Cushing's is manageable but not typically curable — most dogs require lifelong trilostane therapy with periodic monitoring. Adrenal-dependent Cushing's can sometimes be cured with surgical removal of the affected adrenal gland (adrenalectomy), though this is a complex procedure performed by board-certified veterinary surgeons (cost: $3,000–$7,000). Radiation therapy is an option for pituitary macroadenomas.
Can stress cause a false positive on the ACTH test?
Acute stress (car rides, veterinary anxiety, recent boarding) can elevate baseline cortisol, potentially complicating interpretation. This is one reason the ACTH stimulation test is preferred over a resting cortisol alone — the stimulated value is less affected by transient stress. However, if your dog is extremely anxious, discuss options with your vet, such as scheduling at a quieter time or using a mild anxiolytic protocol.
This article is for educational purposes and does not replace professional veterinary advice. If your dog is showing signs of adrenal disease, consult your veterinarian for proper diagnosis and treatment.
Priya Sutaria
All our authors care for dogs every day — read more of their work on the authors page.
