Cortisol
Serum Cortisol
What this test measures
Cortisol is a hormone made by your adrenal glands. It helps control your stress response, blood sugar, blood pressure, and immune system. Levels naturally vary throughout the day.
Understanding the numbers
Typical range: Morning (9am): 140-700 nmol/L | Midnight: <140 nmol/L
Cortisol is highest in the morning and lowest at night. The time of your blood test matters.
- Severely low: 0–100 nmol/L (9am)
- Low — investigate adrenal: 100–140 nmol/L (9am)
- Borderline — dynamic test needed: 140–350 nmol/L (9am)
- Adequate: 350–700 nmol/L (9am)
- Raised — consider Cushing's: 700–1500 nmol/L (9am)
Key numbers
- 140 nmol/L (9am) — Below = adrenal insufficiency likely
- 350 nmol/L (9am) — Borderline — short Synacthen test to confirm
- 450 nmol/L (9am) — Often sufficient to rule out severe adrenal insufficiency
9am serum cortisol is the starting screen. <140 nmol/L is highly suggestive of adrenal insufficiency; 140-350 is indeterminate and requires a short Synacthen test. The timing matters: samples must be 8-9am. Patients on oral steroids (including inhalers) can have suppressed results.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- Can be caused by stress, illness, or steroid medications
- May indicate Cushing's syndrome (too much cortisol)
- Symptoms can include weight gain, high blood pressure, and mood changes
- Further tests are usually needed to find the cause
Questions you could ask your GP
- What might be causing my abnormal cortisol level?
- Do I need further tests like a synacthen test?
- Could my symptoms be related to my cortisol level?
- Will I need to see an endocrinologist?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Addison's disease — severely low cortisol
Kalyani had lost 8 kg, felt exhausted, was tanned in places that shouldn't be (gums, palms). 9am cortisol 78, ACTH very high, sodium low, potassium high.
What the GP said: “This looks like Addison's disease — primary adrenal insufficiency. You need urgent endocrinology assessment; meanwhile, we arrange hydrocortisone and fludrocortisone. You'll need a steroid emergency card and an emergency injection kit.”
What happened next: Diagnosis confirmed; once on replacement she regained energy and weight. She carries a steroid card and emergency hydrocortisone injection for illness.
Addison's disease is rare but lifelong. Tanned skin in unusual places (gums, palm creases), weight loss and low sodium/high potassium can be tell-tale. The steroid card and illness-dose rules prevent a life-threatening crisis.
Cushing's syndrome
Maya had weight gain around the abdomen and face, new stretch marks, and uncontrolled hypertension. Overnight dexamethasone suppression test failed.
What the GP said: “This looks like Cushing's syndrome. Endocrine will investigate whether it's coming from a pituitary tumour, an adrenal tumour, or an external source.”
What happened next: She was found to have a pituitary adenoma. Transsphenoidal surgery resolved the Cushing's and her blood pressure.
Cushing's syndrome is over-production of cortisol. Clues: abdominal weight gain, thin limbs, purple stretch marks, new diabetes, uncontrolled hypertension. Endocrine tests localise the cause; treatment is cause-specific.
Adrenal suppression from inhaled steroids
Joe had asthma on high-dose inhaled fluticasone for years. 9am cortisol 120. He felt tired but otherwise well.
What the GP said: “High-dose inhaled steroids can suppress the adrenal axis over time. We need a Synacthen test to confirm, and you'll need a steroid emergency card and replacement advice during illness or procedures.”
What happened next: Synacthen confirmed mild secondary adrenal insufficiency. Joe was given a steroid card and replacement hydrocortisone for illnesses.
Long-term inhaled, topical, or oral steroids can suppress the adrenals. During serious illness or surgery, extra steroids must be given to prevent a crisis. Patients on long-term steroids should carry a steroid emergency card.
Reassuring morning cortisol
Ilsa had fatigue and dizziness on standing. 9am cortisol was 510, electrolytes normal.
What the GP said: “Your morning cortisol is firmly normal — adrenal insufficiency is very unlikely. Let's look elsewhere for your symptoms — thyroid, blood pressure on standing, and sleep.”
What happened next: Further testing showed postural hypotension; dietary salt and compression stockings helped.
A well-sampled 9am cortisol above about 450 nmol/L largely rules out adrenal insufficiency. That is reassuring and focuses the workup on other causes.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.