Cortisol is often called the "stress hormone" because of its crucial role in your body's response to stress. However, this powerful hormone does far more than just manage stress – it influences nearly every organ system in your body. A cortisol blood test measures the level of this vital hormone, helping to diagnose conditions related to adrenal gland function and stress response. This guide explains how cortisol testing works and what your results may reveal about your health.

What is Cortisol?

Cortisol is a steroid hormone produced by the adrenal glands, which sit on top of your kidneys. It belongs to a class of hormones called glucocorticoids and plays essential roles in numerous bodily functions:

Stress response – Cortisol helps your body respond to physical and emotional stress

Blood sugar regulation – It helps maintain blood glucose levels between meals

Metabolism – Cortisol influences how your body uses carbohydrates, fats, and proteins

Inflammation control – It has powerful anti-inflammatory effects

Blood pressure regulation – Cortisol helps maintain cardiovascular function

Sleep-wake cycle – Levels naturally fluctuate throughout the day, peaking in the morning

Immune function – It modulates immune system responses

Cortisol production is controlled by a complex feedback system involving the hypothalamus, pituitary gland, and adrenal glands – known as the HPA axis (hypothalamic-pituitary-adrenal axis).

The Natural Rhythm of Cortisol

One of the most important things to understand about cortisol is that its levels are not constant throughout the day. Cortisol follows a diurnal (daily) rhythm:

Morning peak – Cortisol levels are highest shortly after waking, typically between 6-8 AM. This "cortisol awakening response" helps you feel alert and energised.

Gradual decline – Levels decrease throughout the day

Evening low – Cortisol reaches its lowest point around midnight

Night-time rise – Levels begin rising again in the early hours of the morning

This natural rhythm is why the timing of your cortisol blood test is so important for accurate interpretation.

Why is a Cortisol Blood Test Performed?

Healthcare providers may recommend a cortisol blood test to investigate various conditions:

Cushing's Syndrome

This condition occurs when your body is exposed to high cortisol levels for an extended period. It can be caused by the adrenal glands producing too much cortisol, pituitary tumours, or long-term use of corticosteroid medications. Symptoms include weight gain (particularly around the face and abdomen), thin skin, easy bruising, muscle weakness, and high blood pressure.

Addison's Disease (Adrenal Insufficiency)

In this condition, the adrenal glands do not produce enough cortisol. Symptoms include fatigue, weight loss, low blood pressure, darkening of the skin, and salt cravings. Addison's disease can be life-threatening if not properly managed.

Adrenal Fatigue and Chronic Stress

While "adrenal fatigue" is not a recognised medical diagnosis, prolonged stress can affect cortisol production patterns. Testing can help identify whether cortisol levels are abnormal in people experiencing chronic fatigue, sleep problems, or difficulty coping with stress.

Pituitary Disorders

Since the pituitary gland controls cortisol production through ACTH (adrenocorticotropic hormone), pituitary tumours or dysfunction can affect cortisol levels.

Investigating Unexplained Symptoms

Cortisol testing may be ordered if you experience symptoms like unexplained weight changes, persistent fatigue, muscle weakness, or abnormal blood pressure.

How Does the Cortisol Blood Test Work?

A cortisol blood test measures the concentration of cortisol in your blood sample. Because cortisol levels fluctuate throughout the day, the timing of the test is crucial:

Morning Cortisol Test

Most commonly, blood is drawn in the early morning (usually between 7-9 AM) when cortisol levels should be at their peak. This is often the first-line test for assessing adrenal function.

Afternoon or Evening Cortisol Test

Testing later in the day can help identify conditions like Cushing's syndrome, where cortisol levels remain elevated when they should naturally be declining.

Multiple Time Points

Sometimes cortisol is measured at different times throughout the day to assess the overall pattern of cortisol production.

For comprehensive information about blood testing procedures, read our guide on blood testing procedures, results, and after effects.

The Cortisol Blood Test Procedure

Preparation

To ensure accurate results:

Schedule your test for the appropriate time of day as advised

Inform your healthcare provider about all medications, including corticosteroids, oral contraceptives, and herbal supplements

Try to avoid intense physical exercise before the test

Minimise stress before your appointment if possible

Some providers may ask you to avoid eating before a morning test

During the Test

A healthcare professional will draw blood from a vein in your arm. The procedure is quick and straightforward, taking just a few minutes.

After the Test

You can resume normal activities immediately. Results are typically available within a few days.

Understanding Your Cortisol Test Results

Cortisol levels are usually measured in nanomoles per litre (nmol/L) or micrograms per decilitre (μg/dL). Normal ranges vary depending on the time of day:

Morning Cortisol (7-9 AM):

Normal range: approximately 170-540 nmol/L (6-20 μg/dL)

Levels below 100 nmol/L may suggest adrenal insufficiency

Levels above 550 nmol/L may indicate excess cortisol production

Afternoon/Evening Cortisol:

Normal range: approximately 65-330 nmol/L (2.5-12 μg/dL)

Should be significantly lower than morning values

Reference ranges may vary between laboratories, so always compare your results to the specific ranges provided.

What Do Abnormal Cortisol Levels Mean?

High Cortisol Levels May Indicate:

Cushing's syndrome or Cushing's disease

Pituitary tumour producing excess ACTH

Adrenal tumour producing excess cortisol

Chronic stress

Depression or severe anxiety

Use of corticosteroid medications

Alcoholism

Severe illness or trauma

Low Cortisol Levels May Indicate:

Addison's disease (primary adrenal insufficiency)

Secondary adrenal insufficiency (pituitary problems)

Hypopituitarism

Damage to the adrenal glands

Sudden withdrawal from long-term corticosteroid use

Additional Tests That May Be Ordered

Cortisol blood testing is often performed alongside other tests for a complete picture:

ACTH test – Measures the hormone that stimulates cortisol production

24-hour urinary cortisol – Measures total cortisol excretion over a full day

Salivary cortisol – Can be collected at home at specific times

Dexamethasone suppression test – Assesses how cortisol responds to synthetic steroids

ACTH stimulation test – Evaluates adrenal gland response

Factors That Can Affect Cortisol Levels

Several factors can influence your cortisol results:

Time of day – The most significant factor

Stress – Physical or emotional stress elevates cortisol

Medications – Corticosteroids, oestrogen, and other drugs

Pregnancy – Cortisol levels naturally rise during pregnancy

Illness or infection – Acute illness raises cortisol

Sleep patterns – Disrupted sleep affects the cortisol rhythm

Shift work – Can alter normal cortisol patterns

Managing Cortisol and Stress

If you are concerned about stress and cortisol levels, lifestyle strategies can help maintain healthy balance:

Regular exercise – Moderate activity helps regulate cortisol

Adequate sleep – Aim for 7-9 hours of quality sleep

Stress management – Meditation, yoga, deep breathing

Balanced diet – Avoid excessive caffeine and sugar

Social connections – Strong relationships buffer stress effects

For a comprehensive assessment of your hormonal health, consider our health screening packages.

When to Seek Medical Advice

At Private Blood Tests London, we provide your raw cortisol test data. We recommend consulting your GP or an endocrinologist if:

Your cortisol levels are outside the normal range

You have symptoms suggesting adrenal dysfunction

You need guidance on interpreting your results

You require further testing or treatment

Book Your Cortisol Blood Test

Understanding your cortisol levels can provide valuable insights into your stress response and overall hormonal health. At Private Blood Tests London, we offer cortisol testing at our CQC-registered clinic in South Kensington. Our experienced team ensures accurate timing and a comfortable testing experience.

Learn more about [the link between stress, cortisol and your blood results](/blog/the-link-between-stress-cortisol-and-your-blood-test-results), [hormone imbalance symptoms](/blog/hormone-imbalance-symptoms-you-shouldnt-ignore), and explore our [health screening packages](/health-screening).

Take the first step in understanding your stress hormones – book your cortisol blood test today.

How does the Cortisol Blood Test Assess Your Stress Hormones?