Cortisol Test: Normal Range, Correct Timing and What High or Low Means
Cortisol is the body's main stress hormone, produced by the adrenal glands under pituitary control. It follows a strong daily rhythm, peaking around 8 a.m. at 5–25 µg/dL and falling to under 5 µg/dL by midnight. Because of this rhythm, the time of the sample determines whether the result means anything at all.
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Cortisol is a glucocorticoid released from the adrenal cortex under the control of ACTH from the pituitary. It raises blood glucose, mobilises fat and protein, suppresses inflammation and immune activity, and maintains blood pressure and vascular responsiveness. Life is not possible without it, which is why adrenal insufficiency is an emergency.
Its defining feature for testing is the circadian rhythm: cortisol peaks in the first hour after waking and falls progressively to its lowest point around midnight. This means a single random cortisol is almost uninterpretable. The standard measurements are an 8 a.m. serum cortisol for suspected deficiency, and for suspected excess a late-night salivary cortisol, an overnight dexamethasone suppression test or a 24-hour urinary free cortisol.
Loss of the rhythm is itself the abnormality in Cushing's syndrome — the midnight value fails to fall — which is why late-night sampling is so useful. It is also worth remembering that most cortisol circulates bound to cortisol-binding globulin, which rises on oestrogen therapy and in pregnancy and can raise total cortisol without any true excess.
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Frequently Asked Questions
Q: What is the normal cortisol level?
A: 5 to 25 µg/dL on an 8 a.m. serum sample, falling to 3 to 13 µg/dL by 4 p.m. and below 5 µg/dL at midnight. Because of this strong daily rhythm, a cortisol result is close to meaningless unless the sampling time is recorded.
Q: Why must cortisol be tested at 8 a.m.?
A: Cortisol peaks in the first hour after waking and falls steadily through the day, so the 8 a.m. value is the one against which the reference interval was established. A normal-looking afternoon value could represent either health or deficiency, and cannot distinguish between them.
Q: What are the symptoms of high cortisol?
A: Central weight gain with thin arms and legs, a rounded face, purple stretch marks on the abdomen, easy bruising, proximal muscle weakness making it hard to rise from a chair, new or worsening diabetes and hypertension, thin skin and, in women, irregular periods and excess hair growth.
Q: What are the symptoms of low cortisol?
A: Persistent fatigue, weight loss, poor appetite, nausea, dizziness on standing, salt craving, muscle and joint aches, and in primary adrenal insufficiency a characteristic darkening of skin creases, scars and gums. Low sodium with high potassium on a routine panel is an important clue.
Q: Can stress raise cortisol enough to affect the test?
A: Yes. Acute stress, pain, illness, surgery, poor sleep and even anxiety about the blood draw itself raise cortisol. This is why resting for 30 minutes before the sample matters, and why testing should be deferred during any acute illness.
Q: What is the dexamethasone suppression test?
A: You take 1 mg of dexamethasone at 11 p.m. and have cortisol measured at 8 a.m. the next morning. Normally the dexamethasone suppresses cortisol to below 1.8 µg/dL. Failure to suppress suggests Cushing's syndrome and leads to further confirmatory testing.
Q: Do steroid medicines affect cortisol results?
A: Yes, all of them — oral, inhaled, nasal, topical creams and joint injections. They suppress the body's own cortisol production and can make the test uninterpretable, and hydrocortisone interferes directly with the assay. Tell the laboratory about every steroid preparation you use, including ones you may not think of as medication.
Q: What is a salivary cortisol test?
A: It measures free cortisol in saliva, usually collected late at night when cortisol should be at its lowest. It is particularly useful for detecting Cushing's syndrome, where the normal night-time fall is lost. Collection must be done correctly — no eating, drinking, smoking or tooth-brushing for 30 minutes beforehand.
Q: Is adrenal fatigue a real diagnosis?
A: Adrenal fatigue is not a recognised medical diagnosis and is not supported by endocrine evidence. Adrenal insufficiency is real, serious and diagnosable with the tests described here. Persistent fatigue deserves proper evaluation — thyroid function, anaemia, B12, vitamin D, sleep disorders and depression are all far more common explanations.
Q: What is an adrenal crisis?
A: A life-threatening collapse of adrenal function, presenting with severe hypotension, vomiting, abdominal pain, weakness and confusion, often triggered by infection, surgery or abrupt steroid withdrawal in someone with adrenal insufficiency. It requires immediate intravenous hydrocortisone and fluids — treatment must not wait for laboratory confirmation.
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