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ACTH stimulation test in dogs: how it works and how to read the results

The ACTH stimulation test checks how well your dog’s adrenal glands make cortisol. Here is what happens on the day, how the two cortisol values are read, and why the result is always interpreted with your dog’s signs and other tests.

Written by the AI DiagAnimals editorial teamReviewed by the AI DiagAnimals TeamUpdated Our editorial policy

The essentials

The ACTH stimulation test measures cortisol twice: before and about one hour after an injection of synthetic ACTH, the hormone that tells the adrenal glands to release cortisol. A very low response points towards Addison’s disease; an exaggerated response can support Cushing’s syndrome. It is also used to monitor dogs treated for Cushing’s. No single result is a diagnosis on its own.

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What the ACTH stimulation test checks

Cortisol is a hormone made by the two adrenal glands, next to the kidneys. Its production is controlled by ACTH, a hormone released by the pituitary gland at the base of the brain. The test gives a standard dose of synthetic ACTH (cosyntropin) and measures how much cortisol the adrenal glands can produce in response [1].

Vets use it in three situations: to look for Addison’s disease (hypoadrenocorticism), when the adrenal glands do not make enough cortisol; to help investigate Cushing’s syndrome (hypercortisolism), when they make too much; and to monitor dogs treated for Cushing’s [1, 2].

It is the reference test for Addison’s disease [3, 6]. For Cushing’s syndrome, the 2012 ACVIM consensus and the 2023 AAHA guidelines describe it as one of several screening tests, none of which is perfect, and recommend testing only dogs with compatible signs [2, 3].

What happens on the day

Ask your vet about food, water and medicines on the day. If your dog takes trilostane for Cushing’s, the timing of the test relative to the morning dose matters for monitoring: follow your vet’s instructions exactly [3].

  • First blood sample: basal (resting) cortisol.
  • Injection of synthetic ACTH, usually into a vein. In a study of dogs suspected of Addison’s disease, a dose of 1 µg/kg gave results equivalent to the standard 5 µg/kg dose [5].
  • Second blood sample about one hour later: post-ACTH (stimulated) cortisol [5].
  • Results: the two serum samples are measured in the clinic or sent to a laboratory; a university laboratory, for example, reports results within one business day [1].

How to read the results

The report shows two cortisol lines, before and after ACTH. Here is how to read the second one.

An annotated line from a lab report · Illustrative example
AbbreviationResultUnitLaboratory intervalFlag
CORT post Cortisol 1 h post-ACTH1.4µg/dLLab-specificL
  1. Abbreviation Post-ACTH or stimulated cortisol. The first line (pre or basal) is the resting value.
  2. Result The cortisol produced in response to ACTH. In a published study, a stimulated cortisol below 2 µg/dL was the criterion used to diagnose Addison’s disease [4].
  3. Unit µg/dL in the United States, nmol/L in Europe (× 27.59). Use the converter below.
  4. Laboratory interval Cut-offs depend on the laboratory, the method and the question asked (Addison’s, Cushing’s or monitoring) [2]. Use the comments on your report.
  5. Flag A low post-ACTH value is the pattern of an adrenal gland that cannot respond. A high value can support Cushing’s, but only with compatible signs [2].

Unit converter

Equivalent values

  • Cortisol, nmol/L–

Conversion only changes the unit. Compare the result with the interval printed by the laboratory that measured it.

The main result patterns

Simplified patterns for orientation only. The interpretation belongs to your vet, with the history, the examination and the other results.
PatternWhat it can suggestWhat usually follows
Both cortisol values very lowAn adrenal gland that cannot produce cortisol: the pattern of Addison’s disease [3, 4].Electrolytes (sodium, potassium) and sometimes endogenous ACTH or aldosterone, to classify the type [6].
Normal rise after ACTHNormal adrenal response. Addison’s disease is unlikely [3].Other causes of the signs are investigated.
Exaggerated rise after ACTHCan support Cushing’s syndrome in a dog with compatible signs; stress from another illness can also raise it [2].Other tests, such as the low-dose dexamethasone suppression test, and abdominal ultrasound [2, 3].
Low or suppressed result in a dog on trilostane or steroidsEffect of the medicine on cortisol production [3].Dose adjustment decided by your vet.

Basal cortisol: the screening step before the full test

Some vets first measure a single resting cortisol. In a multicentre study of 151 dogs with chronic digestive signs, an ACTH stimulation test was done when basal cortisol was below 3 µg/dL; 4% of these dogs turned out to have Addison’s disease, a higher rate than in the general population [4].

A low basal cortisol alone does not diagnose Addison’s disease: in that study, more than half of the dogs had a basal value below 3 µg/dL, and only six had the disease [4]. The full ACTH stimulation test confirms or rules it out.

What can affect the result

  • Steroid medicines (tablets, injections, and sometimes creams, ear or eye drops) can suppress the adrenal response and mimic Addison’s disease. Tell your vet about every product your dog receives [3].
  • Sedation: in healthy beagles, butorphanol increased both basal and post-ACTH cortisol, whereas dexmedetomidine and trazodone did not change the results significantly [7].
  • Other illnesses and stress: any significant illness can raise cortisol, which is why Cushing’s tests are reserved for dogs with compatible signs [2].
  • Laboratory and method: cut-offs differ between laboratories, so results are read against the laboratory’s own comments [2].

Results often read with the ACTH stimulation test

Electrolytes (sodium, potassium)

Often abnormal in Addison’s disease, although some dogs have normal electrolytes [4, 6].

White cell pattern

Sick dogs usually show a “stress” pattern; its absence in an unwell dog can be a clue to low cortisol [6, 8].

Read the article

ALP (alkaline phosphatase)

Often raised in dogs with excess cortisol, because cortisol induces a specific form of ALP in dogs [8].

Read the article

En français : test de stimulation à l’ACTH du chien.

Questions to ask your veterinarian

  1. Is the test looking for Addison’s disease, Cushing’s syndrome, or monitoring a treatment?
  2. Should my dog eat, drink or receive its usual medicines on the morning of the test?
  3. Could any medicine, cream or drops my dog receives affect the result?
  4. Which cut-offs does your laboratory use, and where does my dog’s result fall?
  5. What are the next steps if the result is borderline?
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Frequently asked questions

What is a stim test for dogs?

It is the ACTH stimulation test: cortisol is measured before and about one hour after an injection of synthetic ACTH, to see how well the adrenal glands respond [1, 5].

Do you need to fast a dog for an ACTH stimulation test?

Follow your vet’s instructions, which depend on the laboratory and on whether the dog takes trilostane. Ask too about water and the morning medicines [3].

What is the normal range for an ACTH stimulation test?

There is no universal range: cut-offs depend on the laboratory, the method and the question asked [2]. For Addison’s disease, a stimulated cortisol below 2 µg/dL (about 55 nmol/L) is a commonly used diagnostic criterion [4].

Can the ACTH stimulation test diagnose Cushing’s disease?

It can support the diagnosis in a dog with compatible signs, but no Cushing’s test is perfect and other tests are often combined [2, 3].

Can sedation affect the ACTH stimulation test?

Some sedatives can. Butorphanol raised cortisol in a study of healthy dogs, while dexmedetomidine and trazodone did not significantly change the results [7].

Sources

  1. University of Missouri Veterinary Medical Diagnostic Laboratory. ACTH stimulation. vmdl.missouri.edu/clinical-pathology/acth-stimulation
  2. Behrend EN et al. Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM consensus statement (small animal). J Vet Intern Med. 2013;27(6):1292-1304. doi.org/10.1111/jvim.12192
  3. Bugbee A et al. 2023 AAHA Selected Endocrinopathies of Dogs and Cats Guidelines. J Am Anim Hosp Assoc. 2023;59(3):113-135. doi.org/10.5326/JAAHA-MS-7368
  4. Hauck C et al. Prevalence and characterization of hypoadrenocorticism in dogs with signs of chronic gastrointestinal disease: a multicenter study. J Vet Intern Med. 2020;34(4):1399-1405. doi.org/10.1111/jvim.15752
  5. Botsford A et al. Low-dose ACTH stimulation testing in dogs suspected of hypoadrenocorticism. J Vet Intern Med. 2018;32(6):1886-1890. doi.org/10.1111/jvim.15256
  6. Kalenyak K, Heilmann RM. Canine hypoadrenocorticism: an update on pathogenesis, diagnosis and treatment. Tierarztl Prax Ausg K Kleintiere Heimtiere. 2018;46(3):163-175 (in German). doi.org/10.15654/TPK-180351
  7. Hunt A et al. The effects of selected sedatives on basal and stimulated serum cortisol concentrations in healthy dogs. PeerJ. 2024;12:e16955. doi.org/10.7717/peerj.16955
  8. eClinPath, Cornell University College of Veterinary Medicine. Leukogram patterns (stress leukogram). eclinpath.com/hematology/leukogram-changes/leukogram-patterns

Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.

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