Normal cortisol levels: what the ranges mean for each test

There is no single normal cortisol level. What counts as normal depends on the type of test, the time of day the sample was taken and the laboratory that measured it. For a blood test at 8 am, one US reference gives 5 to 25 µg/dL, which is 138 to 690 nmol/L.¹ A UK hospital laboratory quotes 133 to 537 nmol/L for samples taken between 6 and 10 am, and below 50 nmol/L at midnight.² This guide sets out typical values for each kind of test, the thresholds doctors use, and why the range printed on your own report is the one that counts.

Why there is no single normal range

Three things move the goalposts.

• Time of day. In a healthy reference group, blood cortisol peaked at about 15.5 µg/dL at around 8.30 am and fell below 2 µg/dL around midnight.³ A result means little without the time it was taken. One NHS laboratory states that a random cortisol, taken at a time other than 9 am, cannot be interpreted.⁴ See the cortisol daily rhythm.
• The laboratory's method. Five automated immunoassays read 1.08 to 1.36 times higher than a mass spectrometry reference method.⁵ When one NHS laboratory changed its assay, it warned that results would read about 20 per cent lower.²
• Hormones and medicines. Oral oestrogen raises the protein that carries cortisol in the blood. In one study, women taking it had 67 per cent higher total blood cortisol.⁶ Steroid medicines, including inhalers, creams and injections, can lower a morning result.⁷

So treat every range below as an example. Compare your own result with the range on your report and, if you have symptoms, discuss it with the doctor who ordered the test.

Morning blood cortisol: typical ranges

Blood (serum) cortisol is usually measured in the morning. These published examples each apply only to that laboratory and its assay.

The µg/dL figures for the UK laboratories are our own conversion, at 27.59 nmol/L per µg/dL.¹⁰ Notice how far the lower limits differ, from 101 to 185 nmol/L, for samples taken at much the same time of morning.⁸⁹

What doctors look for in a low morning result

For anyone aged 16 or over with suspected adrenal insufficiency, NICE uses a blood cortisol taken between 8 and 9 am, read against decision thresholds rather than a reference range:⁷

• Below 150 nmol/L: the person may have adrenal insufficiency and should be referred to a hormone specialist (endocrinologist).
• 150 to 300 nmol/L: uncertain; consider repeating the test.
• Above 300 nmol/L: adrenal insufficiency is very unlikely.

These cut-offs apply only to modern immunoassays. NICE also advises stopping oral oestrogen for six weeks before the test, and waiting four weeks after a steroid injection into a muscle or joint.⁷ NICE's guidance for GPs adds that a result below 100 nmol/L makes adrenal insufficiency highly likely and calls for hospital admission.¹¹

A low or borderline result is followed by a Synacthen test, in which cortisol is measured before and after an injection of synthetic ACTH. With healthy adrenal glands it rises above 500 to 550 nmol/L, and a peak below 500 nmol/L is diagnostic of primary adrenal insufficiency, although exact cut-offs depend on the local assay.¹¹ See the Synacthen test.

Very low cortisol can turn into an adrenal crisis, which is an emergency. If someone with known or suspected adrenal insufficiency suddenly gets worse, with severe dizziness, vomiting, confusion, drowsiness or collapse, call 999.¹²

Normal saliva cortisol, late at night and in the morning

A morning blood cortisol is not one of the recommended first tests for Cushing's syndrome. Instead, doctors check whether cortisol falls as it should, using late-night saliva, 24-hour urine or a dexamethasone suppression test.¹³ Saliva measures free cortisol, so it is not affected by the oestrogen effect on blood tests.⁵

In healthy people with regular sleep, saliva cortisol at bedtime or between 11 pm and midnight is below 4 nmol/L (145 ng/dL).¹³ One NHS laboratory uses these locally derived ranges:¹⁴

Levels around waking are harder to pin down. Saliva cortisol normally rises by 50 to 160 per cent in the first 30 minutes after you wake, an average rise of about 9 nmol/L.¹⁵ There are no agreed norms for the absolute level: studies report 4.7 to 18.5 nmol/L on waking and 8.6 to 21.9 nmol/L half an hour later.¹⁵ Because results vary from night to night, the Endocrine Society recommends at least two late-night samples.¹³ See the saliva cortisol test.

Normal 24-hour urine cortisol

A 24-hour urine collection measures the free cortisol you pass over a whole day. One US reference gives normal as below 124 nmol (45 µg) per 24 hours for women and below 165 nmol (60 µg) for men.¹⁶ A study of 152 people with Cushing's disease used a range of 30 to 145 nmol per 24 hours, specific to its assay.¹⁷

The same study showed why one collection is rarely enough: results varied by about 52 per cent from one collection to the next in the same person.¹⁷ Guidelines recommend at least two collections.¹³ See the urine cortisol test.

The dexamethasone suppression test cut-off

Here, normal means cortisol switching off. You take a low dose (1 mg) of dexamethasone, a synthetic steroid, at night and have blood taken the next morning. In a healthy person the 8 am cortisol is then below 50 nmol/L (1.8 µg/dL).¹⁸ A result above that is the positive cut-off used when investigating Cushing's syndrome.¹³ Oral oestrogen can make the test misleading: false-positive rates reach 50 per cent in women on the combined pill, so oestrogen is stopped for six weeks beforehand where possible.¹³ See the dexamethasone suppression test.

Normal hair cortisol: why every laboratory's range differs

Hair cortisol is reported in picograms per milligram of hair (pg/mg). Some papers use ng/g, which gives the same number.¹⁹ There is no single normal range. Reported values for healthy adults differ more than tenfold depending on the laboratory method:²⁰¹⁹

So a hair result can only be read against the range of the laboratory that measured it. What hair offers instead of a universal range is consistency. One sample averages roughly the last three months,²³ and the same people's hair cortisol correlated at r = 0.84 from one month to the next, against 0.38 to 0.61 week to week for saliva and 0.59 to 0.68 for 24-hour urine.²⁴ That makes hair the most accurate way to measure your long-term cortisol from a single sample, and makes repeat results from the same laboratory meaningful to compare.

A hair result above the range means more cortisol than most healthy adults over the months tested; below it means less. It does not diagnose or rule out Cushing's syndrome or any other condition, and hair is not part of the diagnostic guidelines.¹³ See hair cortisol results explained.

Converting between nmol/L and µg/dL

UK and European laboratories report blood and saliva cortisol in nanomoles per litre (nmol/L). US laboratories usually use micrograms per decilitre (µg/dL). To convert µg/dL to nmol/L, multiply by 27.59; to go the other way, divide by 27.59.¹⁰ So 18 µg/dL is about 497 nmol/L, usually rounded to 500.¹⁰ Our cortisol unit converter does the sum for you. Hair results in pg/mg measure something different, an average over months, and cannot be converted into a blood value.