ACTH Blood Test: Normal Range, Results & Meaning

When a patient describes unexplained fatigue that doesn't improve with rest, combined with dramatic weight changes or persistent low blood pressure, my thoughts often turn to the adrenal axis. Evaluating adrenocorticotropic hormone (ACTH) is a fundamental step in unravelling these complex presentations.

What is Adrenocorticotropic Hormone (ACTH)?

ACTH is a peptide hormone synthesised and secreted by the anterior pituitary gland in the brain. Its primary role is to stimulate the adrenal cortex to produce and release cortisol, a vital steroid hormone that regulates metabolism, inflammation, and the body's response to stress.

Clinical Indications for ACTH Testing

I order an ACTH test when a patient's symptoms suggest a dysfunction in the hypothalamic-pituitary-adrenal (HPA) axis. This test is rarely interpreted in isolation and is almost always paired with a cortisol measurement.

Symptoms Suggesting HPA Axis Dysfunction

Patients with potential cortisol excess (Cushing's syndrome) may present with central obesity, a round 'moon' face, purple striae, and hypertension. Those with cortisol deficiency (adrenal insufficiency) often report severe fatigue, weight loss, salt craving, and hyperpigmentation in Addison's disease.

The ACTH Blood Test Procedure

The test involves a standard venous blood draw. Due to the diurnal rhythm of ACTH—peaking in the early morning and reaching a nadir around midnight—samples are routinely collected between 7:00 and 9:00 AM after the patient has been awake and relaxed for at least 30 minutes.

Interpreting ACTH Test Results

The interpretation hinges on the relationship between ACTH and cortisol levels. In my practice, I use a simple framework: if cortisol is abnormal, is the ACTH level appropriately high or low?

What Do High ACTH Levels Indicate?

Elevated ACTH can signal that the pituitary is working overtime, often due to a pituitary tumour (Cushing's disease). It can also indicate that the adrenal glands are failing to produce enough cortisol, as seen in primary adrenal insufficiency, prompting the pituitary to release more ACTH in response.

What Do Low ACTH Levels Indicate?

A low ACTH level in the context of low cortisol points to a problem originating in the pituitary or hypothalamus. This is termed secondary adrenal insufficiency and is frequently seen in patients on long-term exogenous steroid therapy, which suppresses the HPA axis.

ACTH Reference Ranges

Normal values can vary between laboratories. The following table provides general reference intervals. The internationally recognised LOINC code for this test is 2157-6.

These ranges are for adults. Paediatric references differ and require specialist interpretation.

ACTH in Special Populations and Conditions

Is a High ACTH Level Dangerous?

While a high ACTH level itself isn't immediately harmful, it is a marker for underlying conditions that, if left untreated, can be. For instance, uncontrolled Cushing's syndrome significantly increases the risk of diabetes, fractures, and cardiovascular events.

How Does Pregnancy Affect ACTH Levels?

ACTH and cortisol levels rise progressively during a normal pregnancy. Interpreting results in pregnant patients requires expertise, as normal gestational ranges differ, and conditions like adrenal insufficiency must not be missed.

Can Medications Influence My ACTH Test?

Absolutely. Corticosteroids (e.g., prednisolone), estrogen-containing drugs, and certain anticonvulsants can affect results. Always inform your doctor of all medications and supplements you are taking prior to testing.

Next Steps After an Abnormal ACTH Result

An abnormal result is a starting point, not a diagnosis. Further dynamic testing, such as the dexamethasone suppression test or ACTH stimulation test, is usually necessary. Imaging with an MRI of the pituitary or CT of the adrenals may also be recommended to identify structural causes.