Cortisol test: what a cortisol measurement can and cannot tell you

Cortisol is a hormone made by your adrenal glands. It helps keep your blood sugar steady, helps regulate how your body responds to strain, and follows a strong daily rhythm: high in the early morning, low around midnight.

Measuring cortisol is a real, recognised test. Doctors use it to detect two rare conditions: Cushing's syndrome (too much cortisol) and adrenal insufficiency, or Addison's disease (too little cortisol). These are serious illnesses and the test is essential for them.

But look at what that test actually involves. The Endocrine Society guideline prescribes: two late-night saliva samples, two 24-hour urine collections, or a dexamethasone suppression test where you take a tablet the night before. None of these is a blood draw you can simply walk in for. For Addison's disease, a morning blood draw between 06:00 and 10:00 is a legitimate first step, but only alongside ACTH, and only when a doctor already suspects the disease.

This is the core problem, and it is arithmetic, not opinion.

Cortisol fluctuates enormously, even in healthy people doing nothing unusual. Clinical chemistry expresses this as the reference change value: how much a value must change before you can say with 95% confidence that something genuinely changed. For cortisol in blood, that is around 47%. For salivary cortisol it runs from 96% to 245%, depending on the time of day.

Put plainly: your cortisol has to roughly double before anyone can establish that anything changed at all. Everything below that is noise. We could draw your blood twice, an hour apart, and get two numbers tens of percent apart, both entirely normal, and both meaningless.

On top of that, the things that shift your cortisol are precisely the things many people in our audience bring with them:

• the contraceptive pill and other oestrogen-containing contraception, which raise a binding protein in your blood so your measured total cortisol goes up while nothing is actually wrong
• pregnancy, where values towards the end can run up to three times higher
• night shifts and an irregular sleep pattern, which overturn the daily rhythm the test leans on
• the needle itself: the stress of the blood draw raises your cortisol at the very moment of measurement

Most people looking for a cortisol test have come across "adrenal fatigue" somewhere along the way: the idea that chronic stress exhausts your adrenal glands so they start producing too little cortisol.

That condition does not exist. The term was coined in 1998 by a chiropractor, not a doctor. A 2016 systematic review searched 3,470 publications, kept 58, and concluded there is no substantiation for it whatsoever. In fact, in most fatigued participants the cortisol measurements were simply normal. The Endocrine Society is blunt: no scientific proof exists, and there is no test that can detect it. Dutch and Belgian endocrinologists reject the diagnosis just as clearly.

This is not only about wasted money. The treatments sold alongside it can cause harm. A study in Mayo Clinic Proceedings tested twelve over-the-counter "adrenal support" supplements: all twelve contained thyroid hormone that was not on the label, and a quarter contained a steroid. Taking adrenal hormones you do not need can suppress your own hormone production. And the endocrinologists' professional body warns that treating "adrenal fatigue" can mask genuine adrenal insufficiency.

Fatigue is one of the most common reasons people see a GP, and what it turns out to be has been studied properly. Among people presenting with tiredness, around 2.8% have anaemia and 0.6% a malignancy; in total roughly 4% have a serious physical illness. Hormonal causes together account for about 3%, and cortisol disorders are only a fraction of that.

The largest categories are quite different: musculoskeletal complaints, infections, and psychological or social causes. In more than half of people, no physical explanation is found even after a year.

More testing does not help. A Dutch randomised trial in people with unexplained fatigue compared a limited blood panel with an expanded one. The expanded panel found 1.1 percentage points more genuine abnormalities, but produced 33.5 percentage points more false positives: from 22% to 55.5%. You are not buying an answer, you are buying false alarms. That is exactly why the Dutch primary-care guideline deliberately keeps the list short.

For persistent fatigue, this is the list the Dutch primary-care diagnostics guideline starts with. Short, and for a reason:

• Haemoglobin, and if abnormal your iron stores via ferritin: anaemia is the most commonly found physical cause
• CRP: an inflammatory marker, which makes an infection or inflammatory disease less likely
• Fasting glucose: to rule out diabetes
• TSH: your thyroid, which genuinely is a common and treatable cause of fatigue

And to be honest: for a good share of people the answer is no blood test at all. If you feel burned out, that is a clinical diagnosis your GP makes from your story. The Dutch guideline on work-related stress and burnout is unambiguous, and says of additional investigation, literally: "not indicated". There is no blood value for burnout. What does help is day structure, paced recovery and addressing the cause, together with your GP or occupational physician.

We would rather earn nothing from you than sell you a test that does not answer your question.

There genuinely are situations where measuring cortisol is the right thing to do. If you recognise yourself here, you can order the test below:

• your GP or specialist asked for a cortisol measurement. Then you are here to have the test done, not to diagnose yourself, and we are glad to help
• you already have a known adrenal or pituitary condition and you are monitoring your values
• you use or have stopped corticosteroids (prednisone, dexamethasone, including creams and inhalers), where your own cortisol production may be suppressed

If you order without a doctor's request, book your draw between 06:00 and 10:00 in the morning, fasting and without heavy exertion beforehand. Outside that window your result cannot be compared against the correct reference range.

One thing holds even then: a single cortisol value is never a diagnosis. An abnormal result means your GP has to redo the work properly, with the right test.

We think you should be able to check what we base this on.

• Nieman LK et al. The Diagnosis of Cushing's Syndrome: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2008;93(5):1526-40. PMID 18334580. (Explicitly recommends against random serum cortisol.)
• Bornstein SR et al. Diagnosis and Treatment of Primary Adrenal Insufficiency: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2016;101(2):364-89. PMID 26760044. ("There is no evidence to support the use of random cortisol"; not suitable for community-based screening.)
• Cadegiani FA, Kater CE. Adrenal fatigue does not exist: a systematic review. BMC Endocr Disord. 2016;16(1):48. PMID 27557747.
• Akturk HK et al. Over-the-Counter "Adrenal Support" Supplements Contain Thyroid and Steroid-Based Adrenal Hormones. Mayo Clin Proc. 2018;93(3):284-90. PMID 29502560.
• Koch H et al. Ordering blood tests for patients with unexplained fatigue in general practice: what does it yield? Br J Gen Pract. 2009;59(561):e93-100. PMID 19341544.
• Nijrolder I et al. Diagnoses during follow-up of patients presenting with fatigue in primary care. CMAJ. 2009;181(10):683-7. PMID 19858240.
• NHG-Standaard Overspanning en burn-out (M110). Additional investigation: "not indicated".