What is cortisol and what does a high or low result mean? Why timing matters, why “adrenal fatigue” is not a recognised diagnosis, and when testing is useful.
Cortisol is often called the “stress hormone”, but it does far more than respond to stress. Produced by your adrenal glands, cortisol helps regulate your metabolism, blood pressure, immune response, and your natural sleep-wake cycle. A cortisol blood test can help investigate persistent fatigue, unexplained weight changes, and questions around adrenal function.
What does cortisol do?
Cortisol follows a natural daily rhythm, typically highest shortly after waking and gradually falling through the day, reaching its lowest point around midnight. This rhythm helps regulate energy, alertness and sleep. Cortisol also rises in response to physical or psychological stress, helping mobilise energy for a “fight or flight” response, and plays a role in regulating blood sugar, blood pressure and inflammation.
Why timing matters so much
Because cortisol naturally rises and falls across the day, a single result is only meaningful when interpreted alongside the time it was taken. A morning cortisol test is the most common starting point, since levels are naturally at their highest and any deficiency is easier to detect. Some assessments use multiple samples across the day to map your cortisol rhythm more fully.
What can high cortisol indicate?
Persistently high cortisol can be linked to chronic stress, and in less common cases, an underlying condition affecting the adrenal or pituitary glands. Symptoms sometimes associated with high cortisol include weight gain (particularly around the face and abdomen), high blood pressure, poor sleep, and mood changes.
What can low cortisol indicate?
Low cortisol can be linked to adrenal insufficiency, causing fatigue, low blood pressure, weight loss, and low mood. Significant adrenal insufficiency is a genuine medical condition requiring prompt diagnosis and treatment by a doctor; this is not something to self-diagnose from a single blood test.
A word on "adrenal fatigue"
You may have come across the term "adrenal fatigue" used to describe tiredness attributed to overworked adrenal glands. This isn’t a recognised medical diagnosis, and using it to explain fatigue can sometimes delay identifying the actual underlying cause; whether that’s a genuine adrenal or hormonal condition, or something else entirely, like thyroid problems, low iron, or poor sleep. If you’re experiencing persistent fatigue, a broader assessment is usually more useful than testing cortisol alone.
When is cortisol testing useful?
Cortisol testing is most useful when there’s a specific clinical reason to investigate adrenal function; for example, alongside other symptoms suggestive of an adrenal or pituitary condition. For general fatigue without other specific signs, a broader panel covering thyroid, iron, vitamins and blood sugar is often more informative as a starting point.
Getting tested
Cortisol is included in our Hormone Profile testing. If your main concern is general fatigue, our guide to fatigue and which tests to check may be a more useful starting point.
Not sure which test to choose? Use our free Test Finder for a personalised recommendation in under a minute.
Frequently asked questions
Is "adrenal fatigue" a real diagnosis?
No; it’s not a recognised medical diagnosis. Persistent fatigue is best investigated with a broader panel and a doctor’s assessment rather than assumed to be adrenal-related.
What time of day should I test cortisol?
Morning is the standard time, as levels are naturally highest then and results are easiest to interpret.
Can stress alone raise my cortisol permanently?
Cortisol rises temporarily with stress, but persistently high results are usually investigated further rather than attributed to stress alone.
Should I test cortisol if I just feel tired?
A broader panel covering thyroid, iron, vitamins and blood sugar is usually a more useful starting point for general fatigue without other specific signs.
How quickly will I get my results?
Most results are available within 1–2 working days and are reviewed by a GMC-registered doctor before you receive them.
Important note
This article is for general education only and is not medical advice or a diagnosis. Cortisol testing and interpretation can be complex, and abnormal results should always be discussed with your GP or an endocrinologist. Trupoint Health blood tests are analysed by a UKAS-accredited laboratory and reviewed by a GMC-registered doctor.
Medically reviewed by: ____________ (GMC-registered), pending review.
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