Cortisol Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is Cortisol?

Cortisol is a steroid hormone produced by the adrenal glands — two small, triangular glands that sit on top of your kidneys. Often called the "stress hormone," cortisol is in fact far more than a marker of stress. It is one of the most essential hormones in the human body, involved in regulating metabolism, immune function, blood pressure, blood sugar, inflammation, and the sleep-wake cycle.

Cortisol production is governed by the hypothalamic-pituitary-adrenal (HPA) axis — a sophisticated feedback system involving three organs. The hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH), which in turn tells the adrenal cortex to produce cortisol. When cortisol levels rise sufficiently, they feed back to suppress CRH and ACTH, completing a negative feedback loop that maintains balance.

Cortisol follows a pronounced diurnal (daily) rhythm. Levels are highest in the early morning — typically peaking between 6:00 and 8:00 AM — helping you wake up and face the day. They then gradually decline throughout the day, reaching their lowest point around midnight. This circadian pattern is important to understand when interpreting cortisol blood test results, because the time of day the sample is taken significantly affects the result.

In the blood, approximately 80–90% of cortisol is bound to proteins (cortisol-binding globulin and albumin), with only 5–10% existing as "free" cortisol — the biologically active form. Total cortisol blood tests measure both bound and free cortisol together.

Why Is Cortisol Tested?

Cortisol testing is used to diagnose and monitor conditions of excess or deficient cortisol production:

• Diagnosing Cushing's syndrome: A condition of excess cortisol, caused by adrenal tumours, pituitary tumours (Cushing's disease), ectopic ACTH production (often from lung cancer), or prolonged use of corticosteroid medications.
• Diagnosing Addison's disease (adrenal insufficiency): A condition where the adrenal glands fail to produce adequate cortisol. Can be autoimmune, infectious (historically tuberculosis), or caused by adrenal haemorrhage or metastatic disease.
• Investigating secondary adrenal insufficiency: Caused by pituitary disease or — most commonly — sudden withdrawal of long-term corticosteroid therapy, which suppresses the HPA axis.
• Evaluating chronic fatigue: While "adrenal fatigue" is not a recognised medical diagnosis, genuine adrenal insufficiency can present with severe, debilitating fatigue, and cortisol testing helps distinguish between the two.
• Assessing stress response: Cortisol is measured in critical care settings to evaluate adrenal function during severe illness (relative adrenal insufficiency).
• Monitoring cortisol replacement therapy: Patients on hydrocortisone for adrenal insufficiency require monitoring to ensure adequate but not excessive replacement.
• Investigating virilisation or hirsutism: Cortisol testing helps exclude Cushing's syndrome as a cause of excess hair growth or menstrual irregularity in women.

Normal Cortisol Ranges

Cortisol is measured in nanomoles per litre (nmol/L) in the UK. Because of the strong diurnal rhythm, the time of blood collection is critical:

Time of Day	Normal Range (nmol/L)
Morning (7:00–9:00 AM)	170–540 nmol/L
Afternoon (3:00–5:00 PM)	70–250 nmol/L
Midnight	Below 140 nmol/L (ideally below 50 nmol/L)

Some key points about interpreting cortisol results:

• Morning cortisol below 100 nmol/L is strongly suggestive of adrenal insufficiency and warrants further investigation with a Synacthen (ACTH stimulation) test.
• Morning cortisol above 500 nmol/L generally excludes adrenal insufficiency.
• Morning cortisol between 100 and 500 nmol/L is indeterminate and may require a Synacthen test for clarification.
• Midnight cortisol above 200 nmol/L raises concern for Cushing's syndrome (loss of normal diurnal rhythm).

Factors that can affect cortisol levels include stress (physical or emotional), illness, oestrogen-containing medications (the oral contraceptive pill and HRT raise cortisol-binding globulin, increasing total cortisol), pregnancy, shift work, and recent travel across time zones.

Group	Notes
Women on combined oral contraceptive	Total cortisol may be 1.5–2× higher due to raised CBG. Free cortisol remains normal.
Pregnant women	Total cortisol rises progressively, reaching 2–3× non-pregnant levels by the third trimester.
Shift workers	Diurnal rhythm may be altered. Discuss timing with your doctor.

What Do High Cortisol Levels Mean?

Persistently elevated cortisol — particularly with loss of the normal diurnal rhythm — suggests Cushing's syndrome. However, cortisol can also be transiently elevated by many factors.

Cushing's syndrome

True Cushing's syndrome is caused by:

• Cushing's disease (70% of endogenous cases): A pituitary adenoma producing excess ACTH, which drives the adrenals to overproduce cortisol.
• Adrenal tumours (15–20%): Adenomas or carcinomas of the adrenal gland producing cortisol autonomously.
• Ectopic ACTH (10–15%): Certain cancers — particularly small cell lung cancer and carcinoid tumours — produce ACTH outside the pituitary.
• Exogenous (iatrogenic) Cushing's: The most common cause overall. Prolonged use of corticosteroid medications (prednisolone, dexamethasone, inhaled steroids in high doses) suppresses the HPA axis and produces Cushing's features.

Symptoms of Cushing's syndrome

• Central obesity (weight gain around the abdomen and face — "moon face")
• Buffalo hump (fat accumulation between the shoulders)
• Purple striae (stretch marks) on the abdomen, thighs, and arms
• Thin, fragile skin that bruises easily
• Muscle weakness, particularly in the legs
• High blood pressure
• High blood sugar (diabetes or impaired glucose tolerance)
• Osteoporosis and fractures
• Menstrual irregularity in women, erectile dysfunction in men
• Depression, anxiety, insomnia, and cognitive impairment

Non-pathological causes of elevated cortisol

• Acute stress: Physical illness, surgery, trauma, and severe emotional stress trigger a cortisol surge. This is a normal, adaptive response.
• Intense exercise: Vigorous physical activity temporarily raises cortisol.
• Depression and anxiety: Chronic psychiatric conditions can cause sustained HPA axis activation.
• Alcohol excess: Heavy alcohol consumption can mimic biochemical features of Cushing's syndrome ("pseudo-Cushing's").
• Obesity: Particularly central obesity, which is associated with HPA axis overactivity.
• Oral contraceptives: Raise total cortisol via increased cortisol-binding globulin (free cortisol remains normal).

What Do Low Cortisol Levels Mean?

Low cortisol can be life-threatening if severe and unrecognised. It requires prompt diagnosis and treatment.

Primary adrenal insufficiency (Addison's disease)

The adrenal glands themselves are damaged and cannot produce adequate cortisol (or aldosterone). The most common cause in the UK is autoimmune adrenalitis (80% of cases). Other causes include tuberculosis, adrenal haemorrhage, metastatic cancer, and fungal infections.

Secondary adrenal insufficiency

The pituitary gland fails to produce adequate ACTH, so the adrenals are not stimulated. Causes include pituitary tumours, pituitary surgery or radiotherapy, and — most commonly — sudden withdrawal of long-term corticosteroid therapy.

Symptoms of low cortisol

• Severe, debilitating fatigue
• Unexplained weight loss and loss of appetite
• Low blood pressure (postural hypotension — dizziness on standing)
• Nausea, vomiting, and abdominal pain
• Muscle and joint pain
• Hyperpigmentation of the skin (darkening, particularly in skin creases, scars, and gums) — specific to primary adrenal insufficiency
• Salt cravings
• Low mood and irritability

Adrenal crisis

An adrenal crisis is a medical emergency characterised by profound hypotension, shock, severe dehydration, and altered consciousness. It can be triggered by infection, surgery, trauma, or missed cortisol replacement doses in patients with known adrenal insufficiency. Untreated, it is fatal. Emergency treatment involves immediate intravenous hydrocortisone.

How to Support Healthy Cortisol Levels

Cortisol balance is essential for health. Both chronically elevated and deficient cortisol are harmful. Here are evidence-based strategies for supporting healthy cortisol regulation:

Reducing chronically elevated cortisol

• Manage stress: Mindfulness meditation, yoga, deep breathing exercises, and cognitive behavioural therapy (CBT) have all been shown to reduce cortisol levels in clinical trials.
• Prioritise sleep: Aim for 7–9 hours of quality sleep per night. Poor sleep and sleep deprivation elevate cortisol. Maintain a consistent sleep schedule and avoid screens before bed.
• Exercise moderately: Regular moderate exercise reduces cortisol over time, though acute bouts of intense exercise temporarily raise it. Walking, swimming, cycling, and yoga are excellent choices.
• Limit caffeine: Caffeine stimulates cortisol release. If you are sensitive to its effects, limit intake to 1–2 cups of coffee per day, consumed before noon.
• Reduce alcohol: Heavy drinking activates the HPA axis and raises cortisol.
• Eat a balanced diet: Blood sugar fluctuations trigger cortisol release. Eating regular meals with a balance of protein, fat, and complex carbohydrates helps maintain stable blood sugar and cortisol levels.
• Social connection: Positive social interactions and supportive relationships are associated with lower cortisol levels.
• Spend time in nature: Research shows that even 20–30 minutes in a natural environment reduces salivary cortisol significantly.

If cortisol is low (adrenal insufficiency)

• Cortisol replacement therapy: Hydrocortisone (typically 15–25 mg daily in divided doses) is the standard replacement. The largest dose is given in the morning to mimic the natural diurnal rhythm.
• Stress dosing: Patients with adrenal insufficiency must increase their hydrocortisone dose during illness, surgery, or physical stress ("sick day rules"). They should carry a steroid emergency card and wear a medical alert bracelet.
• Emergency injection: All patients should have an intramuscular hydrocortisone injection kit and know when and how to use it.
• Never stop corticosteroids abruptly: If you are on long-term prednisolone or other corticosteroids, tapering must be gradual under medical supervision to allow the HPA axis to recover.

When Should You Get Tested?

Consider cortisol testing if you experience:

• Unexplained severe fatigue that does not improve with rest
• Unintentional weight loss with nausea and low appetite
• Dizziness or fainting, particularly on standing
• Skin darkening (hyperpigmentation)
• Significant central weight gain with thin limbs and easy bruising
• Purple stretch marks, moon face, or buffalo hump
• Muscle weakness
• High blood pressure alongside weight gain and diabetes
• Salt cravings or low blood pressure
• Long-term corticosteroid use (to check HPA axis suppression)

For a reliable morning cortisol, the blood sample should be taken between 7:00 and 9:00 AM. Avoid intense exercise and excessive stress on the morning of the test.

Which Lola Health Tests Include Cortisol?

Cortisol is available in Lola Health's hormone and comprehensive panels:

• Peak Insights (70 Biomarkers) — Our most comprehensive panel, including morning cortisol alongside thyroid, metabolic, and nutritional markers.
• Vital Check (56 Biomarkers) — Broad health screen including cortisol and other hormonal markers.

All Lola Health tests use a venous blood draw at one of our nationwide partner clinics, with appointments available from early morning to ensure accurate cortisol measurement. Results are reviewed by a doctor and returned within 4 working days.