ACTH Test

ACTH levels change throughout the day. They’re highest in the morning around 8–9am. Your blood sample must be taken at this time.

You cannot usually have this test at your GP surgery. You’ll need to go to a hospital with a laboratory on site.

The test uses a blood sample from a vein in your arm.

Follow any specific instructions your doctor or hospital gives you. Collection methods may vary between hospitals.

What the test measures

This test measures ACTH (adrenocorticotropic hormone) in your blood. ACTH is made by your pituitary gland, which sits at the base of your brain.

ACTH’s main job is to tell your adrenal glands to make cortisol. Your adrenal glands sit on top of your kidneys.

Cortisol is vital for life. It helps regulate blood sugar, blood pressure, and your immune system. It also helps your body respond to stress.

Normally, when cortisol is low, ACTH goes up to signal your adrenals to make more. When cortisol is high, ACTH goes down. This is called a feedback loop.

By measuring both ACTH and cortisol together, your doctor can find where the problem is – in your pituitary gland or in your adrenal glands.

What your results mean

Your doctor will look at your ACTH result alongside your cortisol result. Different patterns point to different conditions:

If both cortisol and ACTH are high

This suggests:

• Cushing disease – a pituitary tumour making too much ACTH
• Ectopic ACTH – a tumour elsewhere (usually in the lungs) making ACTH

If cortisol is high but ACTH is low

This suggests an adrenal tumour making cortisol directly. The low ACTH shows your pituitary has switched off because there’s already too much cortisol.

If cortisol is low and ACTH is high

This suggests Addison disease. Your pituitary is making lots of ACTH trying to stimulate your adrenal glands, but they’re damaged and can’t respond.

If both cortisol and ACTH are low

This suggests hypopituitarism – your pituitary gland isn’t working properly, often due to a tumour pressing on it.

If your results aren’t clear

Sometimes the initial results don’t give a clear answer. Your doctor may request additional tests:

Synacthen test (ACTH stimulation test):

You’re given synthetic ACTH. If your adrenal glands are healthy, your cortisol level will rise. If they’re damaged (Addison disease), cortisol won’t increase.

Dexamethasone suppression test:

You’re given dexamethasone (a strong steroid). Normally this should stop ACTH production. Different doses help identify the cause of Cushing syndrome.

If you’re diagnosed with Cushing syndrome

Your doctor will work with you to:

• Find the cause (pituitary tumour, adrenal tumour, or ectopic source)
• Request imaging scans (MRI of pituitary, CT of adrenals)
• Plan treatment (surgery, medication, or radiotherapy)
• Monitor your recovery after treatment

If you’re diagnosed with Addison disease

You’ll need lifelong hormone replacement:

• Daily cortisol replacement (hydrocortisone tablets)
• Sometimes aldosterone replacement (fludrocortisone)
• Extra medication during illness or stress
• Regular monitoring to adjust doses

Several factors can affect your ACTH levels:

Time of day:

• ACTH is highest around 8am and lowest at midnight. This is why timing is crucial.

Stress:

• Severe stress (illness, surgery, childbirth, mental health crisis) can significantly increase ACTH.

Medications that can increase ACTH:

• Insulin
• Levodopa (for Parkinson’s)
• Metoclopramide (for nausea)
• Amphetamines

Medications that can decrease ACTH:

• Steroid medications (prednisolone, dexamethasone, hydrocortisone)
• Megestrol acetate (used in cancer treatment)

Your doctor may request these tests alongside or following your ACTH test:

• Cortisol test – always measured with ACTH
• Synacthen test – stimulation test for adrenal function
• Dexamethasone suppression test – to diagnose Cushing syndrome
• Aldosterone and renin – if Addison disease is suspected
• Glucose test – cortisol affects blood sugar
• Electrolytes – sodium and potassium levels
• Pituitary MRI scan – to look for tumours
• Adrenal CT scan – to check adrenal glands

The pituitary and adrenal glands work together to control cortisol levels. Problems can occur in either gland. Cushing syndrome happens when there’s too much cortisol. Addison disease happens when there’s too little cortisol.

Most pituitary tumours are benign. Treatment usually involves surgery or medication. Addison disease requires lifelong hormone replacement.