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. So we are emphatically not saying that measuring cortisol is nonsense.

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 measured cortisol are precisely the things many women bring with them:

• a disrupted sleep pattern, whether from night shifts, a child waking up, or night sweats in perimenopause: the daily rhythm the test leans on collapses
• the needle itself: the stress of the blood draw raises your cortisol at the very moment of measurement
• oestrogen-containing contraception and pregnancy, which is a story of its own. We explain it below, because it is the main reason we advise you against this test

Cortisol does not float around freely in your blood. Most of it is stuck to a carrier protein, cortisol-binding globulin (CBG). Only the small, free fraction actually does anything in your body. An ordinary blood test measures total cortisol: the bound fraction plus the free fraction together.

Oestrogen you swallow, as in the combined pill and in some tablet forms of hormone therapy, prompts your liver to make more CBG. More carrier protein means more bound cortisol, and therefore a higher total. But the free, active cortisol barely changes. Read that again slowly, because it is the whole point: the number goes up, the hormone that does the work does not. Nothing is wrong with you. Something is wrong with what the test measures.

The size of that effect is not a footnote. The Endocrine Society guideline states, verbatim, that "false-positive rates for the overnight DST are seen in 50% of women taking the oral contraceptive pill". A small Dutch study (Vastbinder, 2016) looked at thirteen healthy volunteers on the pill, and in eight of them, just over 61%, cortisol failed to suppress on the very test designed to rule out Cushing's syndrome. These women were healthy. The test disagreed.

Now put that side by side. The women who most often end up looking for a "stress hormone" test are frequently women on the pill, women who are pregnant, women dealing with PCOS, or women sleeping badly through perimenopause. That is precisely the group whose result cannot be interpreted. We would be taking your money for a number we already know nobody can read. We do not want to do that.

The guideline does have a solution: withdrawing oestrogen roughly six weeks before the test. Read this part carefully: that is a decision for you and your doctor together, not for a webshop. We will never advise you to stop your contraception in order to get a prettier number. We tell you this so you know why we are not pushing a test on you here, and so you can have a well-informed conversation with your GP.

Then pregnancy. There, cortisol genuinely does rise, and substantially: urinary free cortisol excretion can run up to three times higher towards term, and those values overlap with what you see in genuine Cushing's syndrome. A cortisol result during pregnancy is therefore essentially uninterpretable outside specialist care.

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.

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 treating "adrenal fatigue" can mask genuine adrenal insufficiency.

Then the second phrase you see everywhere: "hormone balance", usually with a picture of a "cortisol face" attached. Be strict with that word. Cortisol genuinely does things in your body, and chronic stress genuinely has consequences; we are not denying that. But the link between your measured cortisol and how much stress you actually feel is very small in large studies: stress explains roughly 1 to 4% of the variation in cortisol between people. It takes hundreds of participants to make that effect visible at all. In you, one person, one tube, it says nothing. And a puffy face is not a diagnosis: a doctor recognises genuine Cushing's syndrome by a whole constellation of signs, not by one photo and not by one number.

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.

For persistent fatigue, the Dutch primary-care guideline therefore starts with a short list, and the shortness is exactly the point:

• Haemoglobin, and if abnormal your iron stores via ferritin: anaemia is the most commonly found physical cause of fatigue, and in menstruating women an iron store drained by heavy bleeding is one of the most frequently missed explanations
• CRP: an inflammatory marker, which makes an infection or inflammatory disease less likely
• Fasting glucose: to rule out diabetes
• TSH: your thyroid. Thyroid disorders are far more common in women than in men, and this genuinely is a common, 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 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 do have blood drawn, book it 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.

Always mention whether you use oestrogen-containing contraception, take hormone therapy, or are pregnant. Not because you are doing anything wrong, but because your doctor has to read your result differently. Without that information, a perfectly healthy woman can look ill on paper.

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. Also the source of "false-positive rates for the overnight DST are seen in 50% of women taking the oral contraceptive pill", the advice to withdraw oestrogen roughly six weeks before testing, and the up to threefold rise in urinary free cortisol in pregnancy.)
• Vastbinder M et al. The influence of oral contraceptives on overnight 1 mg dexamethasone suppression test. Neth J Med. 2016;74(4):158-61. PMID 27185774. (In eight of thirteen healthy volunteers using the oral contraceptive pill, just over 61%, cortisol failed to suppress.)
• 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/NVKC LESA Laboratoriumdiagnostiek. NHG-Standaard Overspanning en burn-out (M110). Additional investigation: "not indicated".