The ACTH stimulation test checks whether your adrenal glands can produce cortisol normally. You receive an injection of synthetic ACTH, the hormone that naturally signals your adrenal glands to release cortisol, and then your blood is drawn to measure how your cortisol levels respond. It’s the primary test used to diagnose adrenal insufficiency, a condition where your body doesn’t make enough cortisol on its own.
How the Test Works
Cortisol is a hormone your adrenal glands produce in response to signals from your pituitary gland. That signal comes in the form of ACTH (adrenocorticotropic hormone). In healthy adrenal glands, a burst of ACTH triggers a reliable spike in cortisol. The stimulation test recreates this process with a synthetic version of ACTH to see if your adrenals respond the way they should.
A baseline blood draw measures your cortisol before the injection. You then receive a shot, typically into the muscle of your shoulder. Blood is drawn again at 30 minutes, 60 minutes, or both, depending on the dose used. The lab compares your cortisol levels across these samples. If your adrenal glands are functioning properly, cortisol rises sharply after the injection. If it stays flat or barely increases, that points to adrenal insufficiency.
What It Helps Diagnose
The test is most commonly used to evaluate two types of adrenal insufficiency. In primary adrenal insufficiency (Addison’s disease), the adrenal glands themselves are damaged and can’t produce enough cortisol regardless of how much ACTH they receive. In secondary adrenal insufficiency, the problem is upstream: the pituitary gland isn’t sending enough ACTH to the adrenals in the first place, so the glands gradually lose their ability to respond.
Distinguishing between these two conditions matters because the causes and treatment differ. Doctors look at your ACTH blood levels alongside the stimulation test results. A high or elevated ACTH level paired with a poor cortisol response suggests primary insufficiency, meaning the adrenals are getting the signal but can’t deliver. A low or undetectable ACTH level suggests the pituitary gland is the source of the problem. Aldosterone, another hormone produced by the adrenal glands, can also help differentiate the two: in primary insufficiency, aldosterone production typically fails to rise above a key threshold after stimulation, while in secondary insufficiency it remains intact.
Beyond adrenal insufficiency, the test is also used in evaluating conditions involving excess cortisol, such as Cushing’s disease, where a pituitary tumor drives ACTH production too high and floods the body with cortisol.
Standard Dose vs. Low Dose
The standard version of the test uses 250 micrograms of synthetic ACTH. This is actually a massive dose compared to what your body produces naturally. It’s effective at catching severe adrenal insufficiency, but because it floods the adrenal glands with such an overwhelming signal, it can sometimes produce a normal-looking cortisol response even when the glands are partially impaired. In other words, it can miss milder cases.
A low-dose version uses just 1 microgram, which is closer to a natural physiological stimulus. This smaller dose is more sensitive at detecting subtle problems, particularly in people who have been on corticosteroid medications long-term. Prolonged steroid use gradually suppresses the pituitary-adrenal connection, and the low-dose test is better at catching that suppression. Research has shown that some patients on chronic steroids will pass the standard 250-microgram test but fail the 1-microgram version, revealing adrenal suppression the higher dose would have missed.
Despite this advantage, many clinics still default to the standard dose because it’s simpler to prepare and has been used for decades. Your doctor’s choice between the two depends on what they’re looking for and how subtle the suspected problem is.
Understanding Your Results
The traditional threshold for a “passing” result is a cortisol level of 18 micrograms per deciliter (500 nmol/L) or higher at the 30- or 60-minute blood draw. If your cortisol reaches that level, your adrenal glands are considered to be responding adequately.
However, this cutoff was established using older laboratory methods. Newer, more precise assays measure cortisol differently, and recent research suggests the appropriate cutoff for these modern tests is lower, closer to 14 to 15 micrograms per deciliter. Using the old 18-microgram threshold with newer assays leads to false positives, flagging people as insufficient when their adrenal function is actually fine. This is why normal reference ranges can vary between labs, and why the range printed on your specific lab report is the one that matters most for interpreting your result.
For the low-dose test, some research indicates the optimal cutoff may be even lower. One study found that dropping the threshold to roughly 14.5 micrograms per deciliter (401.5 nmol/L) maintained perfect sensitivity for catching true adrenal insufficiency while dramatically improving specificity from 67% to nearly 94%, cutting down on false alarms.
What to Expect During the Test
The test itself is straightforward and usually takes about an hour. You’ll have an initial blood draw, receive the injection, and then sit in the clinic or lab while you wait for the follow-up draws. Most people feel nothing beyond the standard discomfort of a needle stick and an intramuscular shot.
Side effects from the synthetic ACTH injection are uncommon. Rarely, people experience changes in heart rate, a temporary increase in blood pressure, mild swelling, or a rash. Allergic reactions including anaphylaxis are possible but very rare. The injection is not given to anyone with a known allergy to synthetic ACTH.
Your doctor may ask you to stop taking certain medications beforehand, particularly corticosteroids like prednisone or hydrocortisone. These drugs directly affect cortisol levels and can interfere with the accuracy of the test. The timing of when to stop depends on the specific medication and dose, so your provider will give you instructions tailored to your situation. Some facilities recommend the test be done in the morning, when cortisol levels are naturally at their highest, though practices vary.
After the Test
Results are typically available within a day or two. If your cortisol response is normal, adrenal insufficiency is unlikely and your doctor will look elsewhere for the cause of your symptoms. If your response is blunted, further testing usually follows to pinpoint whether the problem is in the adrenal glands, the pituitary gland, or related to medication use. This might include imaging of the adrenal or pituitary glands, additional hormone panels, or a repeat stimulation test with a different dose.
A failed test doesn’t automatically mean you’ll need lifelong treatment. In cases of steroid-induced adrenal suppression, for example, the glands often recover over time once the medication is gradually tapered. In Addison’s disease, the damage is typically permanent and requires daily hormone replacement. The path forward depends entirely on the underlying cause, which the stimulation test helps narrow down but rarely identifies on its own.