ACTH Stimulation Test
An endocrine blood test that evaluates adrenal gland reserve capacity by measuring blood cortisol before and after injecting synthetic adrenocorticotropic hormone.
💬 In Plain English
The ACTH stimulation test is the gold standard for testing adrenal gland function. The adrenal glands produce cortisol when prompted by ACTH (a hormone normally made by the brain’s pituitary gland). During the test, a baseline blood sample is drawn, synthetic ACTH (cosyntropin) is injected, and a second blood sample is drawn 1 hour later. In Addison’s disease, failing adrenal glands produce a flat zero response. In Cushing’s disease, hyperactive glands produce a massive surge of cortisol. It is also the only reliable test to monitor pets on trilostane therapy.
🔬 Clinical & Technical Depth
For Vets & TechniciansFormal Clinical Definition
Dynamic endocrine provocation test assessing functional adrenocortical reserve. Serum cortisol is measured at baseline (T0) and 1 hour post-administration of synthetic ACTH (cosyntropin at 5 µg/kg IV). Flat post-ACTH cortisol (<1 µg/dL) confirms hypoadrenocorticism; exaggerated response (>22 µg/dL in dogs) supports hyperadrenocorticism.
Why Veterinarians Use It
Veterinarians use the ACTH stim test as the sole definitive test to diagnose Addison’s disease, monitor medical therapy for Cushing’s (trilostane), and identify iatrogenic steroid-induced Cushing’s.
Clinical Context Encountered
Performed as an in-clinic morning procedure requiring roughly 1 to 2 hours of patient stay.
Clinical Reference Guide (Diagnostic Test)
- •When monitoring trilostane therapy for Cushing’s disease, a post-ACTH cortisol < 1.5 µg/dL indicates dangerous oversuppression requiring medication suspension to prevent an induced Addisonian crisis.
❓ Questions to Ask Your Veterinarian
Take these actionable questions with you to your next clinic visit or appointment:
- 1Did the test confirm a flat cortisol response (<1 µg/dL) diagnostic for Addison’s disease?
- 2If monitoring trilostane (Vetoryl) therapy for Cushing’s, was the post-ACTH cortisol within our target therapeutic window (typically 1.5 to 5.5 µg/dL)?
- 3Exactly how many hours after my pet’s morning trilostane capsule was the test started (typically 4 to 6 hours)?
- 4Are concurrent electrolyte levels (sodium and potassium) being checked?
Academic & Clinical References
- ACVIM Consensus on Diagnostic Testing for Adrenal Disorders in DogsOrganization: American College of Veterinary Internal Medicine (ACVIM) • Accessed: 2026-01-10https://www.acvim.org