ACTH Stim Test in Dogs: What It Is, Costs & Results Explained
Learn how the ACTH stim test works in dogs, what it diagnoses (Addison's & Cushing's), costs, procedure steps, and how to interpret results.
Quick Answer: What Is the ACTH Stim Test in Dogs?
The ACTH stimulation test (also called the cosyntropin stimulation test) is a 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 30–60 minutes later to measure cortisol levels. It is the gold-standard diagnostic tool for Addison's disease (hypoadrenocorticism) and a supporting test for Cushing's disease (hyperadrenocorticism). The test typically costs between $150–$350 at a general practice and $250–$500 at a specialty clinic, depending on your region.
If your veterinarian has recommended an ACTH stim test for your dog, you likely have questions about what the procedure involves, what the results mean, and whether your dog's symptoms point to a serious adrenal condition. This guide breaks down the science behind the test, the physiology it measures, and what you need to know as an owner navigating a potential endocrine diagnosis.
The Endocrine Science Behind the ACTH Stim Test
The Hypothalamic-Pituitary-Adrenal (HPA) Axis
The ACTH stim test evaluates a hormonal communication pathway called the HPA axis — a feedback loop between three structures:
- Hypothalamus (brain): Releases corticotropin-releasing hormone (CRH) in response to stress, illness, or low blood sugar.
- Pituitary gland (base of brain): Receives CRH and secretes adrenocorticotropic hormone (ACTH) into the bloodstream.
- Adrenal glands (above each kidney): Detect ACTH and respond by producing cortisol, the primary stress hormone.
In a healthy dog, this system is self-regulating: rising cortisol signals the pituitary to reduce ACTH output (negative feedback). When any part of this chain fails, cortisol production becomes abnormal — either too low (Addison's) or too high (Cushing's). The ACTH stim test directly probes the adrenal glands' ability to respond to stimulation, which is why it's so diagnostically powerful.
Why Dogs Need This Test: Addison's and Cushing's Disease
The ACTH stim test is primarily ordered when a veterinarian suspects one of two adrenal disorders:
Primary Hypoadrenocorticism (Addison's Disease)
Addison's disease occurs when the adrenal cortex is destroyed — typically by an immune-mediated process — leaving the glands unable to produce cortisol and often aldosterone. According to the Merck Veterinary Manual, Addison's is most common in young to middle-aged female dogs, with breeds like Standard Poodles, Portuguese Water Dogs, Bearded Collies, and Nova Scotia Duck Tolling Retrievers showing genetic predisposition. Prevalence estimates suggest roughly 0.28–0.6% of the general dog population may be affected, though subclinical cases likely go undiagnosed.
Classic symptoms that prompt an ACTH stim test include:
- Intermittent vomiting and diarrhea (often waxing and waning)
- Lethargy and weakness, especially after stress
- Shaking or trembling episodes
- Increased thirst and urination (PU/PD)
- Hypoglycemia (low blood sugar) on routine bloodwork
- Electrolyte abnormalities: low sodium, high potassium (Na:K ratio below 27:1)
- Acute "Addisonian crisis" — collapse, shock, bradycardia (emergency)
Hyperadrenocorticism (Cushing's Disease)
Cushing's disease involves excess cortisol production, most commonly driven by a pituitary tumor (pituitary-dependent hyperadrenocorticism, PDH — about 80–85% of cases) or an adrenal tumor (ADH — about 15–20%). While the low-dose dexamethasone suppression test (LDDST) is more sensitive for diagnosing Cushing's, the ACTH stim test helps differentiate pituitary from adrenal causes and is required before starting trilostane therapy to establish a baseline cortisol value.
How the ACTH Stim Test Procedure Works
Step-by-Step Procedure
- Preparation: Your dog should be fasted (no food after midnight) unless your vet advises otherwise. Bring a list of all current medications, especially any steroids (prednisone, dexamethasone, hydrocortisone), as these must be discontinued — typically for 24–72 hours or longer — before testing. Your vet will give you a specific washout period.
- Baseline blood draw (T=0): A veterinary technician draws approximately 2–4 mL of blood from a peripheral vein (usually the cephalic or jugular). This sample measures resting cortisol.
- ACTH injection: Synthetic ACTH (cosyntropin, brand name Cortrosyn) is administered. The standard dose is 5 µg/kg IV or 2.2 µg/kg IV depending on the protocol your clinic follows. Some clinics use a fixed dose of 250 µg per dog regardless of size. The injection is given intravenously or intramuscularly.
- Post-stimulation blood draw (T=30 or T=60): A second blood sample is collected 30 to 60 minutes after the injection (timing depends on the specific protocol — both 30-minute and 60-minute protocols are validated, though the 60-minute post-ACTH sample is most commonly used in clinical practice).
- Sample processing: Serum is separated and either analyzed in-house (if the clinic has an immunoassay analyzer like an Immulite) or sent to an external lab such as IDEXX, Antech, or Texas A&M's GI Lab.
The entire appointment typically takes 60–90 minutes. Most dogs tolerate the procedure without sedation. The two venipunctures are no more uncomfortable than a routine blood draw. Some dogs may experience mild discomfort at the injection site or transient nausea from the cosyntropin, but adverse reactions are rare.
Interpreting ACTH Stim Test Results
Understanding your dog's results requires looking at both the pre-ACTH (baseline) and post-ACTH cortisol values together. Reference ranges vary slightly by laboratory, but the following table provides widely accepted interpretive guidelines:
| Diagnosis | Baseline Cortisol (µg/dL) | Post-ACTH Cortisol (µg/dL) | Interpretation |
|---|---|---|---|
| Addison's Disease | Low (<2 µg/dL) | Low (<2 µg/dL, minimal rise) | Adrenal glands cannot respond to ACTH — diagnostic for primary hypoadrenocorticism |
| Normal | 1–5 µg/dL | 6–17 µg/dL (adequate rise) | Healthy adrenal function — Addison's effectively ruled out |
| Cushing's (PDH) | Normal to high | Exaggerated (>20–22 µg/dL) | Adrenal glands over-respond — consistent with PDH, but LDDST needed to confirm |
| Adrenal Tumor (ADH) | Variable | Minimal or no rise | Tumor produces cortisol autonomously; normal tissue is suppressed |
| Iatrogenic (steroid-induced) | Low | Low (blunted response) | Prolonged exogenous steroid use has suppressed adrenal function |
Key point: A post-ACTH cortisol below 2 µg/dL in a dog with compatible clinical signs is considered diagnostic for Addison's disease with near-100% specificity. This makes the ACTH stim test one of the most definitive endocrine tests in veterinary medicine. For Cushing's disease, however, the ACTH stim test has a sensitivity of only about 60–65% for PDH and ~60% for ADH, which is why it is typically paired with the LDDST and often abdominal ultrasound.
Evolutionary Context: Why the Adrenal System Matters
From Wolf Pack to Living Room: The Stress Response Legacy
The HPA axis evolved as a survival mechanism in ancestral canids. When a wolf encountered a threat — a rival pack, a predator, or prey that fought back — cortisol mobilized glucose for energy, suppressed non-essential functions like digestion and immune activity, and sharpened alertness. Once the threat passed, cortisol levels dropped and homeostasis resumed.
In modern domestic dogs, this same system operates, but the stressors have changed. Chronic low-grade stressors (separation anxiety, noise phobias, multi-dog household tension) can keep the HPA axis activated in ways it was never evolutionarily designed to sustain. While Addison's and Cushing's are primarily disease processes (immune-mediated destruction and neoplasia, respectively), the clinical signs we observe — lethargy, GI disturbance, altered thirst — reflect dysregulation of an ancient fight-or-flight system that domestication has not fundamentally rewired.
Interestingly, breed predispositions to Addison's disease (Standard Poodles, Portuguese Water Dogs) suggest that selective breeding may have inadvertently concentrated immune-related genetic variants that increase autoimmune risk — a trade-off of the intense breed development that occurred primarily in the 19th and 20th centuries.
Cost Breakdown: What to Budget
One of the most common questions owners ask is how much an ACTH stim test will cost. The price varies depending on your location, whether you visit a general practitioner or a board-certified veterinary internist, and whether the cortisol assay is run in-house or sent out.
| Cost Component | General Practice | Specialty/University Hospital |
|---|---|---|
| Office exam fee | $55–$95 | $150–$250 |
| ACTH stim test (blood draws + cosyntropin + lab) | $120–$280 | $180–$350 |
| Additional bloodwork (CBC, chemistry, electrolytes) | $80–$180 | $120–$250 |
| LDDST (if also ordered for Cushing's workup) | $100–$200 | $150–$300 |
| Abdominal ultrasound (if adrenal tumor suspected) | $300–$500 | $400–$700 |
| Total estimated workup | $350–$750 | $600–$1,500+ |
Always consult your veterinarian about which tests are necessary for your dog's specific presentation. Not every dog needs a full endocrine workup — if clinical signs and electrolytes strongly suggest Addison's, your vet may recommend proceeding directly to the ACTH stim test without the LDDST.
Common Misconceptions About the ACTH Stim Test
| Myth | Fact |
|---|---|
| "The ACTH stim test diagnoses all adrenal diseases by itself." | It is definitive for Addison's but only a partial tool for Cushing's. The LDDST, high-dose dexamethasone suppression test, endogenous ACTH levels, and imaging are often needed for a complete Cushing's diagnosis. |
| "If my dog's cortisol is normal, they can't have Cushing's." | Approximately 35–40% of dogs with PDH have a normal post-ACTH cortisol. A normal result does not rule out Cushing's — it only makes Addison's unlikely. |
| "The test is painful or requires anesthesia." | The test involves two standard blood draws and one injection. No sedation or anesthesia is needed. Most dogs show no distress beyond mild restraint during venipuncture. |
| "My dog's steroids won't affect the test." | Exogenous steroids (prednisone, dexamethasone, even some ear/eye medications containing hydrocortisone) can suppress the HPA axis and produce falsely low results. Your vet must know about all medications. A washout period of 24–72+ hours is typically required. |
| "Addison's disease is a death sentence." | With lifelong hormone replacement (typically DOCP injections every 25–30 days plus low-dose prednisone), most Addisonian dogs live normal lifespans with excellent quality of life. Monthly treatment costs range from $50–$150 depending on dog size. |
Breed Variations and Risk Factors
While any dog can develop adrenal disease, certain breeds carry significantly higher risk for Addison's disease. The American Kennel Club and veterinary endocrinology literature identify the following breeds as predisposed:
- Standard Poodle: One of the most commonly affected breeds; estimated prevalence up to 8.6% in some studies — dramatically higher than the general population.
- Portuguese Water Dog: Genetic studies have identified heritable components; breed clubs actively fund Addison's research.
- Bearded Collie: Higher incidence documented in UK and US populations.
- Nova Scotia Duck Tolling Retriever: A breed-specific Addison's-associated genetic marker has been identified.
- Wheaten Terrier (Soft Coated): Overrepresented in Addison's case series.
- Great Dane, West Highland White Terrier, and Rottweiler also appear with increased frequency in some datasets.
For Cushing's disease, small-breed dogs — particularly Poodles (all sizes), Dachshunds, Boxers, Boston Terriers, and Yorkshire Terriers — are overrepresented in PDH cases. Adrenal tumors (ADH) tend to occur in larger breeds and older dogs (mean age 10–11 years).
Practical Implications: Living With the Results
If Your Dog Is Diagnosed With Addison's Disease
- Treatment is lifelong but manageable: Most dogs receive desoxycorticosterone pivalate (DOCP, brand name Percorten-V or Zycortal) injections every 25–30 days, plus a small daily dose of prednisone (typically 0.05–0.1 mg/kg). Your vet will monitor electrolytes at 2 weeks, 4 weeks, and then every 3–6 months to adjust dosing.
- Stress dosing: During periods of illness, surgery, travel, or other stress, your dog's prednisone dose may need to be doubled or tripled temporarily. Ask your vet for a written stress-dosing protocol.
- Emergency preparedness: Keep an injectable dose of dexamethasone-SP (or similar) at home and know the signs of Addisonian crisis: sudden collapse, severe vomiting/diarrhea, extreme weakness, cold extremities. This is a life-threatening emergency — go to the nearest open veterinary clinic immediately.
- Prognosis: With consistent treatment, dogs with Addison's disease have a normal life expectancy. Many owners report their dog seems "years younger" once hormone levels are corrected.
If Your Dog Is Diagnosed With Cushing's Disease
- Medical management: Trilostane (Vetoryl) is the most commonly prescribed medication, given once or twice daily. Starting doses range from 1–2 mg/kg. ACTH stim tests are repeated at 10–14 days, 30 days, and then every 3–6 months to monitor cortisol control. Monthly medication costs range from $40–$200 depending on dose and compounding availability.
- Surgical option: For adrenal tumors (ADH), adrenalectomy may be curative. This is a specialized surgery with costs of $3,000–$7,000+ and carries risks of hemorrhage and perioperative complications. Referral to a board-certified surgeon is standard.
- Monitoring at home: Track your dog's water intake, urination frequency, appetite, and panting. Improvement in these signs often precedes lab changes and is a useful clinical indicator.
How Is This Different From Humans?
The HPA axis functions similarly in dogs and humans — both species rely on the same CRH → ACTH → cortisol cascade. However, several key differences affect diagnosis and disease presentation:
- Cortisol rhythm: Humans exhibit a strong diurnal cortisol pattern (highest in the morning, lowest at night). Dogs have a less pronounced diurnal rhythm, which is why random cortisol measurements are unreliable in dogs and stimulation/suppression tests are essential.
- Disease prevalence: Cushing's disease is relatively common in middle-aged to older dogs but rare in humans (estimated 0.7–2.4 per million per year in people). Conversely, Addison's disease is more common in humans relative to Cushing's, while in dogs, Cushing's vastly outnumbers Addison's.
- Immune-mediated destruction: Autoimmune adrenalitis accounts for ~80% of human Addison's cases and is also the leading cause in dogs, but the specific immune markers and genetic associations differ between species.
- ACTH stim test protocols: While the same synthetic ACTH (cosyntropin) is used in both species, dosing and sampling intervals differ. Human endocrinology also uses insulin tolerance tests and metyrapone tests that are not standard in veterinary practice.
When to Seek Veterinary Care
Emergency Red Flags — Go to the Vet Immediately
- Sudden collapse or inability to stand
- Severe, repeated vomiting or bloody diarrhea
- Heart rate below 60 bpm with weakness (possible hyperkalemia from Addisonian crisis)
- Known Addisonian dog that has missed a DOCP injection and is showing lethargy/vomiting
- Seizures in a dog with suspected or confirmed adrenal disease
An Addisonian crisis can be fatal within hours without IV fluid resuscitation, dexamethasone, and electrolyte correction. If you suspect a crisis, do not wait — seek emergency veterinary care.
Frequently Asked Questions
Does my dog need to fast before the ACTH stim test?
Most veterinarians recommend fasting your dog after midnight before the test, as food intake can cause lipemia (fatty serum) that may interfere with cortisol assay accuracy. However, if your dog has Addison's and is prone to hypoglycemia, your vet may advise a small meal. Always follow your specific clinic's instructions.
Can medications interfere with ACTH stim test results?
Yes. Any exogenous glucocorticoid — including prednisone, dexamethasone, methylprednisolone, and even hydrocortisone-containing topical/ear/eye medications — can suppress the HPA axis and produce falsely low cortisol results. Mitotane (Lysodren) and trilostane (Vetoryl) used to treat Cushing's will also affect results. Inform your veterinarian of every medication, supplement, and topical product your dog receives. Phenobarbital and some other anticonvulsants can increase cortisol metabolism and may also affect interpretation.
How long does it take to get results?
If your clinic runs cortisol in-house on an immunoassay analyzer, results may be available the same day. If samples are sent to an external reference laboratory (IDEXX, Antech Diagnostics, or a university lab), turnaround is typically 1–3 business days. Your vet will contact you once results are in and interpreted.
Is the ACTH stim test safe for all dogs?
The test is considered very safe. Cosyntropin (synthetic ACTH) has an excellent safety profile, and the procedure involves only standard blood draws. Dogs with severe anemia, clotting disorders, or extreme debilitation may require modified handling, but the test itself does not worsen their condition. There is no radiation, no anesthesia, and no lasting side effects.
Can a dog have Addison's with a "normal" ACTH stim result?
This is extremely rare. The ACTH stim test has near-100% sensitivity and specificity for primary hypoadrenocorticism when post-ACTH cortisol is below 2 µg/dL. However, "atypical Addison's" — where only the zona fasciculata (cortisol-producing layer) is destroyed while the zona glomerulosa (aldosterone-producing) remains functional — may present with normal electrolytes. The ACTH stim test still detects these cases because cortisol fails to rise. Secondary hypoadrenocorticism (pituitary failure, not adrenal failure) can produce borderline results and may require additional testing.
What is the difference between the ACTH stim test and the LDDST?
The ACTH stim test evaluates the adrenal glands' maximum capacity to produce cortisol when directly stimulated. The low-dose dexamethasone suppression test (LDDST) evaluates whether the HPA axis can be suppressed by a synthetic steroid — in a normal dog, dexamethasone shuts off ACTH production and cortisol drops; in a Cushing's dog, cortisol remains elevated. The ACTH stim is better for diagnosing Addison's; the LDDST is more sensitive for diagnosing Cushing's. They are complementary tests, not interchangeable.
References & Resources
- Merck Veterinary Manual — Hypoadrenocorticism (Addison's Disease) in Dogs
- AKC — Addison's Disease in Dogs: Symptoms, Treatment, and Prevention
- American Veterinary Medical Association (AVMA) — Find a veterinarian and understand endocrine testing protocols
Anouk Beaumont
All our authors care for dogs every day — read more of their work on the authors page.
