Dog Cortisol Tests: Cushing's, Addison's and Dynamic Testing
· By Dr. Tang
TL;DR — A single resting cortisol can’t diagnose Cushing’s; you need dynamic testing. The ACTH stimulation test (post-ACTH >22 µg/dL = Cushing’s; <2 µg/dL = Addison’s) and the low-dose dexamethasone suppression test (8-hour cortisol >1.4 µg/dL = Cushing’s) are the workhorses. Addison’s is screened with a basal cortisol <2 µg/dL. Quantitative immunoassay returns these values in ~15 minutes, so the whole workup can happen in a day.
In Plain Terms
Judging adrenal function from a single random cortisol is like deciding whether someone is “an anxious person” from one photo of them at the dentist. Cortisol jumps with stress and time of day, so one snapshot is meaningless. The ACTH stimulation test and LDDST fix this by giving the adrenal gland a challenge and watching how it responds — like asking a stressful question and measuring the reaction. That response, not the resting state, is what diagnoses the disease.
Why Cortisol Testing Confuses People
Cortisol is the most misunderstood analyte in small-animal endocrinology — and for a good reason: a single cortisol value is almost useless for diagnosing Cushing’s.
Here’s why. Cortisol secretion is pulsatile and stress-responsive. A dog that just had blood drawn in a clinic has a higher cortisol than it did at home. A dog with early Cushing’s may have a perfectly normal random cortisol on any given day. The ranges of “normal,” “stressed,” and “hyperadrenocorticoid” overlap so heavily that one resting value tells you nothing.
And yet cortisol testing is essential, because it’s how you diagnose the two big adrenal diseases:
- Cushing’s syndrome (hyperadrenocorticism) — too much cortisol.
- Addison’s disease (hypoadrenocorticism) — too little cortisol.
The key is knowing which cortisol test to run. They’re not interchangeable.
The Two (Actually Three) Cortisol Tests
1. Basal (Resting) Cortisol
| Species | Typical resting cortisol |
|---|---|
| Dog | ~2–6 µg/dL (some labs 1–5) |
| Cat | <5.5–6 µg/dL |
What it’s good for: screening for Addison’s. A basal cortisol <2 µg/dL is highly suggestive of hypoadrenocorticism and warrants confirmatory ACTH stimulation testing. A basal cortisol >2 µg/dL makes Addison’s unlikely.
What it’s useless for: diagnosing Cushing’s. Don’t try.
2. ACTH Stimulation Test
This is the workhorse for both diseases. The protocol:
- Collect a baseline serum cortisol.
- Inject synthetic ACTH (cosyntropin/tetracosactide), typically 5 µg/kg IV.
- Collect a second sample at 1 hour (30–60 min).
Dog interpretation:
| Post-ACTH cortisol | Interpretation |
|---|---|
| >22 µg/dL | Consistent with Cushing’s (with compatible signs) |
| 17–22 µg/dL | Equivocal — repeat or add another test |
| <17 µg/dL | Normal response; Cushing’s less likely |
| <2 µg/dL | Confirms Addison’s (no adrenal response) |
The ACTH stimulation test is also the standard for monitoring a Cushing’s patient on trilostane or mitotane — you track the post-ACTH value to titrate the dose.
Caveat for cats: the ACTH stimulation test is less sensitive in cats; LDDST or high-dose dexamethasone suppression is often preferred.
3. Low-Dose Dexamethasone Suppression Test (LDDST)
The LDDST is the most sensitive screening test for Cushing’s and can sometimes even localize the disease. The protocol:
- Inject 0.01 mg/kg dexamethasone IV.
- Measure cortisol at 0, 4, and 8 hours.
| 8-hour cortisol | Interpretation |
|---|---|
| <1.4 µg/dL | Normal suppression |
| >1.4 µg/dL | Failure to suppress → consistent with Cushing’s |
A useful discriminator: if the dog suppresses at 4 hours but “escapes” at 8 hours, that pattern suggests pituitary-dependent Cushing’s. If there’s no suppression at either time point, the test can’t distinguish pituitary from adrenal tumor — you’d need further workup.
Which Test When?
| Clinical Question | Test |
|---|---|
| Rule out Addison’s (collapse, hyperkalemia, “ADR”) | Basal cortisol first |
| Screen for Cushing’s (PU/PD, pot belly, muscle wasting) | LDDST (most sensitive) |
| Confirm + monitor therapy for Cushing’s | ACTH stimulation |
| Monitor trilostane/mitotane dose | ACTH stimulation (post-ACTH value) |
| Cat with suspected Cushing’s | LDDST preferred |
A practical note: because the LDDST requires three timed samples and the ACTH stimulation test requires two, quantitative in-clinic testing changes the economics. Running cortisol on an immunofluorescence analyzer returns each value in ~15 minutes, so the full dynamic test is done the same day — no shipping samples to a reference lab and waiting.
The Addison’s Connection: A Diagnostic Emergency
Addison’s is the adrenal disease that gets missed — sometimes fatally. The classic presentation is vague: waxing-waning lethargy, vomiting, collapse, and — critically — electrolyte abnormalities (hyperkalemia, hyponatremia) that can mimic kidney disease or a dozen other things.
The screening is simple: a basal cortisol <2 µg/dL is highly suggestive. If you get that number, run the ACTH stimulation test to confirm. A post-ACTH cortisol that stays <2 µg/dL (no response) confirms hypoadrenocorticism.
This is a case where a fast, quantitative cortisol result genuinely changes the outcome — Addison’s is treatable once you know what you’re dealing with.
Sample Handling and Timing Traps
Cortisol testing lives and dies by timing and technique:
- Stress is the enemy. Cortisol rises with clinic stress, so a stressed patient’s basal value is unreliable. Minimize handling before the draw.
- Time the post-injection sample precisely. In the ACTH stimulation test, the 1-hour sample must be drawn close to 60 minutes — early or late sampling changes the value.
- Dexamethasone dosing must be exact. The LDDST depends on a precise 0.01 mg/kg dose; a dosing error invalidates the whole test.
- Sample type and stability. Use serum or plasma per the kit insert, and process promptly.
The lesson: cortisol testing is a protocol, not a single draw. Getting the protocol right matters more than the analyzer.
What a Same-Day Cortisol Workup Is Worth
Adrenal testing is one of the few diagnostic areas where the protocol — not just the result — drives the clinic’s value:
- Addison’s is a life-or-death screen. A collapsed, hyperkalemic dog with a basal cortisol <2 µg/dL is an emergency that a 15-minute number turns from a mystery into a treatable disease. Few tests pay for themselves in saved lives this directly.
- Cushing’s is a long-term monitoring relationship. Once diagnosed, a Cushing’s patient on trilostane or mitotane needs serial ACTH stimulation tests to titrate the dose — typically every few months, for the rest of the dog’s life. That’s a recurring, protocol-driven revenue stream, not a one-off test.
- Same-day dynamic testing keeps the workup in-house. The ACTH stimulation (2 samples) and LDDST (3 timed samples) would each mean multiple reference-lab submissions and a wait. On an in-clinic analyzer, each cortisol value returns in ~15 minutes, so the whole dynamic test finishes the same day — and the revenue stays in the practice.
For the owner, the framing is straightforward: a hormone test that tells us whether your dog’s drinking, pot belly and hair loss are Cushing’s — and, once we know, lets us tune the medication to the right dose over time.
How the Test Works
Cortisol is measured from serum or plasma on a quantitative immunofluorescence analyzer:
- Collect the sample(s) per the dynamic-test protocol (timed draws).
- Load into the cortisol cartridge.
- The analyzer quantifies cortisol against a calibration curve.
- Read the result in µg/dL within roughly 15 minutes.
Related product: Canine Cortisol Test
Related reading: Veterinary Biomarker Testing: The Complete Guide · Thyroid Testing in Dogs and Cats: T4 and TSH Explained
Key Takeaways
- A single resting cortisol cannot diagnose Cushing’s — 1 sample is not a diagnosis; you need dynamic testing.
- ACTH stimulation test: post-ACTH >22 µg/dL = Cushing’s; <2 µg/dL = Addison’s. It’s also the monitoring standard.
- LDDST: 8-hour cortisol >1.4 µg/dL = failure to suppress = Cushing’s (and it’s the most sensitive screen).
- Addison’s screening: basal cortisol <2 µg/dL is highly suggestive — a fast result can be life-saving.
- Quantitative in-clinic testing completes a 3-step dynamic workup in ~15 minutes per result, finishing the case in a day instead of shipping samples out.
References
- Cornell eClinPath — Adrenal function testing: https://eclinpath.com/chemistry/adrenal-gland/adrenal-function-testing/
- MSD Veterinary Manual — Hyperadrenocorticism: https://www.msdvetmanual.com/endocrine-system/pituitary-disorders/hyperadrenocorticism
This content is for educational and product-selection purposes only. It is not a substitute for veterinary diagnosis — any dog with suspected adrenal disease should be evaluated by a veterinarian. Reference ranges and cutoffs vary by assay; always use your analyzer’s validated intervals. Product specifications are as published by Migibio (Guangzhou Magic Biotech Co., Ltd.) and may change.
Sources & Verification
- Author: Dr. Tang — veterinary diagnostics specialist.
- Review: Protocols and cutoffs cross-checked against Cornell eClinPath and the MSD Veterinary Manual (References above).
- Last updated: 2026-08-29.
- Note: Equivocal cutoffs vary slightly by source (e.g., 17–22 vs 18–22 µg/dL); confirm against your analyzer’s validated intervals.