What is an ACTH stimulation test in dogs?

The ACTH stimulation test measures how the adrenal glands respond to synthetic ACTH (cosyntropin), which normally triggers cortisol release. A blood sample is taken before and 1 hour after the ACTH injection. The post-stimulation cortisol value shows whether the adrenal glands can produce a normal cortisol response.

What does the ACTH stimulation test diagnose in dogs?

The ACTH stimulation test is the definitive test for Addison's disease (hypoadrenocorticism) in dogs, and is used to monitor treatment of Cushing's disease (hyperadrenocorticism) with trilostane or mitotane. It can also screen for iatrogenic Cushing's from steroid therapy.

What is the normal post-ACTH cortisol range in dogs?

Normal post-ACTH stimulation cortisol in dogs is typically 6–18 µg/dL (165–500 nmol/L). Values below 2 µg/dL confirm Addison's disease. Values above 18–20 µg/dL may indicate Cushing's disease, though the LDDS test is preferred for initial Cushing's diagnosis.

How is the ACTH stimulation test different from the low-dose dexamethasone suppression test?

The ACTH stimulation test measures adrenal reserve — can the glands produce cortisol when stimulated? It's the gold standard for Addison's and the preferred test for monitoring Cushing's treatment. The LDDS (low-dose dexamethasone suppression test) evaluates whether the HPA axis can be suppressed, making it better for initial Cushing's diagnosis.

Can the ACTH stimulation test diagnose atypical Addison's?

Atypical Addison's refers to cases where only glucocorticoid function is impaired (cortisol) while mineralocorticoids (aldosterone) remain normal. The ACTH stimulation test can confirm low cortisol response in these cases. Electrolytes may appear normal, making the ACTH test especially important for diagnosis.

ACTH Stimulation Test in Dogs: What It Tests & How to Read Results

Normal Post-ACTH Cortisol: 6–18 µg/dL

Post-stimulation cortisol is the key number. Pre-stimulation (resting) cortisol is also measured but is less diagnostic on its own. Always interpret in context of clinical signs.

< 2 µg/dL

Addison’s disease confirmed

6–18 µg/dL

Normal adrenal response

> 18–20 µg/dL

Possible Cushing’s — see notes

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Understand Your Dog’s Results

How the ACTH Stimulation Test Works

ACTH (adrenocorticotropic hormone) is produced by the pituitary gland and signals the adrenal glands to produce cortisol. In the ACTH stimulation test, your vet injects a synthetic form of ACTH (cosyntropin) and measures how the adrenal glands respond:

• Pre-injection sample — baseline (resting) cortisol drawn first
• Cosyntropin injection — given IV or IM
• Post-injection sample — cortisol drawn exactly 1 hour later

A normal adrenal gland responds by roughly doubling cortisol output. Failing to do so (flat or negligible response) confirms that the adrenal glands can’t mount a cortisol response — the hallmark of Addison’s disease. Excessive response can suggest Cushing’s disease or iatrogenic hypercortisolism from steroid use.

When the ACTH Stimulation Test Is Used

Diagnosing Addison’s Disease

The definitive test for hypoadrenocorticism (Addison’s). A post-ACTH cortisol below 2 µg/dL confirms the diagnosis. Addisonian dogs often present collapsed, vomiting, weak, and hyponatremic (low sodium).

Monitoring Cushing’s Treatment

After starting trilostane (Vetoryl) or mitotane (Lysodren), the ACTH stim test monitors whether cortisol is being adequately controlled. The post-ACTH target on treatment is typically 2–9 µg/dL depending on clinical signs and the treating vet’s protocol.

Screening for Iatrogenic Cushing’s

Prolonged steroid therapy (prednisone, dexamethasone) suppresses the HPA axis and can cause adrenal atrophy. A blunted ACTH response after steroid use confirms iatrogenic adrenal suppression — important before abruptly stopping steroids.

Screening for Cushing’s (Initial Diagnosis)

Less sensitive than the LDDS test for initial Cushing’s diagnosis — only about 80% of Cushing’s dogs have an abnormal ACTH stim. Most internists prefer the LDDS for initial diagnosis, reserving the ACTH stim for monitoring.

Atypical Addison’s

Atypical Addison’s involves only glucocorticoid insufficiency — electrolytes (Na/K ratio) remain normal. These dogs are diagnosed by low post-ACTH cortisol with normal electrolytes. Without the ACTH stim test, atypical Addison’s is easily missed.

Interpreting Results: Key Numbers

Post-ACTH < 2 µg/dL

Addison’s disease confirmed. Start glucocorticoid ± mineralocorticoid replacement immediately.

Post-ACTH 2–5 µg/dL

Borderline low — possible early Addison’s or over-treated Cushing’s. Retest in context of signs.

Post-ACTH 6–18 µg/dL

Normal. Addison’s ruled out. Appropriate target range for dogs on Cushing’s treatment.

Post-ACTH > 18–20 µg/dL

High response — possible Cushing’s or under-treated Cushing’s. LDDS preferred for initial diagnosis.

Post-ACTH Cortisol	Interpretation	Next Step
< 2 µg/dL	Addison’s confirmed	Start hydrocortisone/fludrocortisone; check electrolytes
2–5 µg/dL	Borderline low	Retest; evaluate for early Addison’s or steroid suppression
6–18 µg/dL	Normal	Addison’s ruled out; good Cushing’s treatment range
> 18–20 µg/dL	High — possible Cushing’s	LDDS test for initial diagnosis; increase treatment dose if on trilostane

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What Happens After the Test

Addison’s confirmed (post-ACTH < 2 µg/dL): Immediate treatment with prednisone or hydrocortisone (glucocorticoid) plus fludrocortisone or DOCP/Percorten (mineralocorticoid for most cases). Electrolytes are corrected with IV fluids if the dog is in Addisonian crisis.

On Cushing’s treatment — post-ACTH in range (2–9 µg/dL): Treatment is working well. Continue current dose and recheck in 3–6 months or sooner if signs recur.

On Cushing’s treatment — post-ACTH too high (> 9 µg/dL): Under-controlled. Your vet may increase the trilostane or mitotane dose, then recheck in 4 weeks.

On Cushing’s treatment — post-ACTH too low (< 2 µg/dL) with signs: Over-treated — adrenal crisis risk. Stop trilostane immediately, give prednisone, and recheck in 2–4 weeks before restarting at a lower dose.

Normal result — Addison’s ruled out: If signs persist, other diagnoses are investigated. Consider hypoglycemia, GI disease, or other causes of episodic weakness and GI signs.

Key Takeaway

The ACTH stimulation test is the definitive test for Addison’s disease in dogs and the standard monitoring tool for Cushing’s treatment. Post-ACTH cortisol below 2 µg/dL confirms Addison’s. The test is also essential for safely managing the transition off long-term steroid therapy. Understanding these results helps you work with your vet to protect your dog’s adrenal health.

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Thyroid and adrenal conditions often require lifelong medication and regular monitoring bloodwork. Pet insurance can help cover these ongoing costs.

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