Cortisol in cats and dogs: single value, ACTH stimulation and dexamethasone tests

What it shows

The pituitary gland releases ACTH, ACTH tells the adrenal glands to make cortisol, and rising cortisol tells the pituitary to slow down. Function tests use this loop: the vet gives synthetic ACTH and looks at how strongly the adrenals answer, or gives dexamethasone, a synthetic cortisol, and looks at whether the body’s own cortisol falls. So one test produces several values — each is a sample at a set time, and the meaning lies in the comparison.

• Cortisol (baseline, resting) — a single sample, or the first sample of a test before anything is given.
• Cortisol post-ACTH stimulation — the sample after synthetic ACTH, in dogs usually one to two hours later; compared with the baseline.
• Cortisol 4 h and 8 h after dexamethasone (the test with the smaller amount of dexamethasone; on forms LDDST 4h / 8h) — the second and third of three samples.
• Cortisol after the test with the larger amount of dexamethasone (on forms HDDS) — the same principle with more dexamethasone, run on a different day.

Higher than reference — what it may be a sign of

• An exaggerated rise after ACTH — typical of Cushing’s syndrome (hyperadrenocorticism). The test confirms the syndrome but does not show whether it starts in the pituitary or in an adrenal gland.
• No fall at 8 hours after dexamethasone — supports Cushing’s syndrome. A fall at 4 hours with a rise again by 8 hours (“escape”) is a pattern that points to the pituitary form.
• No fall even in the high suppression test — suggests an adrenal origin; a fall at 4 or 8 hours points to the pituitary.
• Other illness and stress — animals with chronic disease or in stressful situations may also fail to suppress, so a “positive” result is read only when the signs fit.

Lower than reference

A vet reads a low baseline with little or no rise after ACTH as confirming Addison’s disease (hypoadrenocorticism), when the adrenals make too little hormone. A raised resting cortisol rules Addison’s disease out; a low one does not prove it. Corticosteroid medicines make the pituitary release less ACTH: in a pet that has received them they can distort the result of the ACTH test, and stopping them abruptly can cause a temporary Addison-like state. That is why the vet needs a full list of medicines before the test.

Look at together with

• Clinical signs — increased drinking and urination, appetite, coat; tests are run when the signs point to the adrenals.
• Sodium and potassium — a low sodium : potassium ratio is part of the picture in Addison’s disease.
• Urine cortisol : creatinine ratio — a home-collected screening test; a result in range makes Cushing’s syndrome unlikely.
• Glucose and ALP — diabetes often goes with Cushing’s syndrome in cats; ALP is less telling in cats than in dogs.
• Endogenous ACTH — helps tell the pituitary form from an adrenal tumour; routine blood and urine tests and abdominal ultrasound help exclude other illnesses first.

Units and conversion

European labs report cortisol in nmol/L, North American labs in µg/dL. One µg/dL equals 27.59 nmol/L. All lines of a test — baseline, post-ACTH, 4 h and 8 h — use the same unit. In the app each row keeps the unit printed on the form; the chart converts to one unit.

Reference range

The reference range is on your lab’s form, and for function tests the form often adds the lab’s interpretation bands. The lines are read as a set: the post-ACTH value against the baseline, the 4 h and 8 h values against the sample before dexamethasone. For Addison’s disease the Merck Veterinary Manual states that a baseline above 2 µg/dL (about 55 nmol/L) effectively rules it out, while a lower value needs an ACTH test.

Cats and dogs

Sources

1. VCA Animal Hospitals — Dexamethasone Suppression Tests
2. VCA Animal Hospitals — Cushing's Disease: Testing
3. VCA Animal Hospitals — Testing and Monitoring in Addison's Disease
4. Greco DS. Cushing Disease (Pituitary-Dependent Hyperadrenocorticism) in Animals. Merck Veterinary Manual
5. Van Vertloo L. Addison Disease (Hypoadrenocorticism) in Animals. Merck Veterinary Manual