By The Pawline veterinary teamVeterinary-reviewed by Alastair Greenway, BVM&S MRCVS · 14 August 2026
Cortisol is the body's main stress hormone, and a single resting measurement is almost never diagnostic: it fluctuates far too much. Adrenal disease is diagnosed with dynamic tests (ACTH stimulation, LDDST) that watch how cortisol responds to a challenge, which is what makes the result interpretable.
Cortisol is produced by the adrenal glands, which sit just in front of the kidneys, under instruction from the pituitary gland via ACTH. It is the body's principal stress hormone: mobilising glucose, modulating inflammation and immunity, and helping maintain blood pressure. Too much causes Cushing's disease (hyperadrenocorticism); too little causes Addison's disease (hypoadrenocorticism). Both are important, treatable, and diagnosed almost entirely through cortisol testing.
Cortisol comes from the adrenal glands under pituitary instruction. Because it fluctuates so much, adrenal disease is diagnosed by dynamic testing.
Why a single cortisol tells you almost nothing
This is the central point. Cortisol is released in pulses and surges with stress: a car journey, a waiting room, restraint for a blood draw. A resting ("basal") cortisol therefore overlaps enormously between healthy and diseased animals, and a high value in a frightened pet means very little. For that reason, basal cortisol is not used to diagnose Cushing's disease.
It has one genuinely useful role, and it is an exclusion: a basal cortisol comfortably above the screening cut-off — the convention is 2 µg/dL (55 nmol/L) — makes Addison's disease very unlikely, with studies putting the sensitivity of that threshold at or near 100% (Lennon et al., 2007; Bovens et al., 2014). That is the single number's real power: ruling the disease out in a suspicious case. The reverse does not hold. A basal cortisol at or below the cut-off does not confirm Addison's — the threshold's specificity is modest, and most dogs below it still prove normal on the confirmatory test — so a low value's only correct next step is an ACTH stimulation test.
The dynamic tests
Because a static number is uninformative, adrenal function is assessed by challenging the system and watching the response:
ACTH stimulation test: cortisol is measured, a dose of synthetic ACTH is given to instruct the adrenals to work, and cortisol is measured again. A flat, minimal response confirms Addison's disease (the adrenals cannot respond). An exaggerated response supports Cushing's. This is the definitive test for Addison's and the test used to monitor treatment of Cushing's.
Low-dose dexamethasone suppression test (LDDST): a small dose of steroid is given, which in a healthy animal switches cortisol production off via feedback. Failure to suppress supports Cushing's disease. The LDDST is generally the more sensitive screening test for Cushing's.
Urine cortisol:creatinine ratio (UCCR): a urine screening test, collected calmly at home, useful mainly for ruling out Cushing's.
Once Cushing's is confirmed, further testing helps distinguish the pituitary-driven form (most cases) from an adrenal tumour, because treatment differs.
Interpreting in context
Two cautions matter throughout. First, non-adrenal illness can distort these tests, so they are ideally run on a stable patient rather than during acute illness. Second, steroid medication interferes in two distinct ways: some steroids (prednisolone, hydrocortisone) cross-react with the assay and are measured as cortisol, falsely raising the number, while ongoing steroid treatment of any kind suppresses the gland's own output, falsely lowering it — and that includes skin, ear and eye preparations that are easy to forget when asked about medicines. (Dexamethasone does not cross-react, which is part of why the suppression test is built on it.) Your vet needs the full medication history, topicals included.
In chronic disease
Cortisol testing is as much about monitoring as diagnosis. Dogs treated for Cushing's are followed with ACTH stimulation tests (or equivalent monitoring) to ensure cortisol is controlled but not over-suppressed; Addisonian patients are monitored clinically and with electrolytes. The trend across those tests, alongside clinical signs, is what guides dose adjustment over years.
Cat vs dog
Both conditions occur in each species, but Cushing's and Addison's are far more commonly diagnosed in dogs. Feline Cushing's is uncommon and characteristically associated with poorly-controlled diabetes; feline adrenal testing uses different protocols and interpretation. The core principle applies to both: dynamic testing, not a single resting value.
About the reference range
This page shows no range strip. Cortisol results are meaningless without knowing which test was run and at which time point (pre- or post-ACTH, post-dexamethasone), and the interpretive cut-offs are protocol- and laboratory-specific. Pawline draws a band only where it holds a verified interval, so cortisol is presented as an explainer. Read your result against the protocol-specific interpretation on your own report.
For orientation only, one laboratory's published cut-offs (Zoetis interpretive guide, alongside the 2023 AAHA endocrinopathy guidelines; µg/dL, ×27.6 → nmol/L for UK reports): ACTH-stim (dog) resting 0.5–4.0, post 8–20 normal, 18–22 equivocal, >22 (~>600 nmol/L) consistent with Cushing's, <2 consistent with Addison's; trilostane monitoring target pre & post 1.0–5.0. LDDST (dog) 8-hour post-dexamethasone <1.1 (~30 nmol/L) not Cushing's, >1.4 (~40 nmol/L) consistent, 1.1–1.4 grey zone. UCCR rule-out >20 (see that page). Crucial caveat (2012 ACVIM consensus): these are probability thresholds that only apply when there is clinical suspicion of Cushing's/Addison's. They are not valid as screening numbers on an asymptomatic wellness panel, and the panel itself notes the LDDST cut-offs may need lowering. Read your own report's protocol-specific interpretation.
What to do next
If Cushing's or Addison's is suspected, expect your vet to run a dynamic test rather than a single cortisol, and to want your pet reasonably settled and off interfering medication where possible. A single high cortisol from a stressful visit is not a diagnosis of anything; the response to a challenge is what carries the answer.
This page is educational and is not a diagnosis. Lab results are interpreted in the full clinical context by your veterinary surgeon; a single value rarely tells the whole story.
