ACTH Stimulation Test

The ACTH stimulation test is a blood test where your dog receives an injection of a synthetic hormone – this allows the vet to check whether the adrenal cortex still responds to stimuli at all.

How does the ACTH test work?

Allow about two hours. The costs range from 150 to 250 euros depending on the practice, and your dog does not need to come fasting – which is at least a relief.

To begin with, the vet takes an initial blood sample to determine the baseline cortisol level. This is followed by an injection of synthetic ACTH – usually Cosyntropin, either directly into a vein or subcutaneously. Exactly 60 minutes later, the second blood draw takes place.

Some practices also measure after 30 minutes. It sounds like a minor detail, but it makes a real difference for diagnostic certainty in borderline cases.

What do the cortisol levels mean?

Normally, the cortisol level rises from below 5 µg/dl to 15–25 µg/dl (400–690 nmol/l) after stimulation. This shows: the adrenals are working.

If the level remains low despite ACTH administration – usually below 5 µg/dl – this points to Addison’s disease. The adrenals are then so exhausted that they simply can no longer respond.

In Cushing’s syndrome, the picture is reversed: the cortisol level shoots up to over 25 µg/dl, or the baseline value is already noticeably elevated. The overactive adrenals produce cortisol largely uncontrollably in this case.

Why is this test performed?

The ACTH test is considered the gold standard when Addison’s is suspected – and for good reason. Symptoms such as weakness, vomiting or sudden collapse can have dozens of causes. Only this test can truly prove or rule out adrenal insufficiency.

It is also standard when Cushing’s syndrome is suspected, and anyone treating their dog with Trilostane will inevitably need regular ACTH checks. Important here: a normal resting cortisol level alone does not rule out an adrenal disease.

What happens with abnormal results?

If Addison’s disease is confirmed, your dog will receive lifelong cortisol replacement therapy – usually with prednisolone. The prognosis is very good with timely diagnosis and consistent treatment; many dogs live completely normally with it.

If Cushing’s syndrome is present, further investigations follow, such as the low-dose dexamethasone test, to narrow down the exact cause. Whether a brain tumour or an adrenal tumour is behind it then determines which treatment path is taken.