Cortisol µg/dL to nmol/L Converter

Cortisol in two units: the short answer

To convert cortisol from µg/dL (mcg/dL) to nmol/L, multiply by 27.59. To convert nmol/L back to µg/dL, divide by 27.59 (or multiply by 0.0362). A morning cortisol of 10 µg/dL is therefore about 276 nmol/L, and 500 nmol/L is about 18.1 µg/dL. The converter above handles both directions and shows typical morning values and the cut-off used after a dexamethasone suppression test.

Cortisol is measured to look for two opposite problems – too little cortisol (adrenal insufficiency) and too much (Cushing's syndrome) – and both are diagnosed with dynamic tests whose cut-offs are quoted in one unit or the other. A "1.8" threshold in the US and a "50" threshold in the UK are the same number after conversion. Because cortisol varies so much through the day, the time of the sample matters as much as the unit. This guide explains the conversion, the daily rhythm, the main tests and their thresholds, and the many factors that alter cortisol results.

What cortisol does

Cortisol is the main glucocorticoid hormone, produced by the outer layer (cortex) of the adrenal glands. It is essential for life. Cortisol:

• maintains blood pressure and the response of blood vessels to adrenaline
• raises blood glucose by stimulating glucose production in the liver
• regulates the immune response and limits inflammation
• helps the body respond to physical stress such as infection, injury and surgery
• influences mood, sleep, bone turnover and fluid balance

Cortisol production is controlled by the hypothalamic–pituitary–adrenal (HPA) axis. The hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the pituitary to release adrenocorticotropic hormone (ACTH), which in turn stimulates the adrenal glands to make cortisol. Cortisol then feeds back to suppress CRH and ACTH.

About 90% of cortisol in the blood is bound to cortisol-binding globulin (CBG) and albumin. Standard blood tests measure total cortisol, so anything that changes CBG – such as oestrogen-containing medicines or pregnancy – changes the result.

µg/dL and nmol/L explained

• µg/dL (micrograms per deciliter), also written mcg/dL – the usual US unit.
• nmol/L (nanomoles per liter) – the SI unit used in the UK, Europe, Australia and many other countries.

Salivary and urinary cortisol use different units (often nmol/L or µg/dL for saliva, and µg/24 h or nmol/24 h for urine) and have different reference ranges.

Why the factor is 27.59

Cortisol (hydrocortisone) has a molar mass of 362.46 g/mol. A concentration of 1 µg/dL is 10 µg per liter, or 10,000 ng per liter, which is 10,000 ÷ 362.46 = 27.59 nmol/L. Conversely, 1 nmol/L = 0.0362 µg/dL.

How to convert cortisol step by step

1. Check the unit and the sample type (blood, saliva or urine).
2. For µg/dL to nmol/L, multiply by 27.59.
3. For nmol/L to µg/dL, divide by 27.59.
4. Round nmol/L to whole numbers and µg/dL to one decimal place.

Worked examples

• 1.8 µg/dL × 27.59 = 50 nmol/L – the normal suppression cut-off after an overnight dexamethasone test.
• 3 µg/dL × 27.59 = 83 nmol/L – a morning level that strongly suggests adrenal insufficiency.
• 5 µg/dL × 27.59 = 138 nmol/L.
• 15 µg/dL × 27.59 = 414 nmol/L.
• 18 µg/dL × 27.59 = 497 nmol/L – the classic peak threshold in a Synacthen (ACTH stimulation) test.
• 100 nmol/L ÷ 27.59 = 3.6 µg/dL.
• 450 nmol/L ÷ 27.59 = 16.3 µg/dL.

Mental shortcut

Multiply µg/dL by 28 (then subtract a little), or divide nmol/L by 28. For 12 µg/dL: 12 × 28 = 336; the exact value is 331 nmol/L.

Cortisol conversion chart

Normal cortisol levels and the daily rhythm

Cortisol follows a strong circadian rhythm. It starts rising in the early hours of the morning, peaks within about 30–60 minutes of waking, falls through the day and reaches its lowest point around midnight. In someone with a normal sleep pattern:

Laboratory reference ranges for morning cortisol vary considerably with assay type. Newer, more specific immunoassays and LC-MS/MS methods give lower values than older assays, and some laboratories have revised their cut-offs downwards as a result.

Because of the rhythm, a single random cortisol result is often hard to interpret. Morning samples (between about 8 and 9 a.m.) are used to screen for low cortisol; late-night samples and suppression tests are used to look for high cortisol.

Testing for low cortisol (adrenal insufficiency)

Adrenal insufficiency can be primary (the adrenal glands are damaged, most often by autoimmune disease – Addison's disease) or secondary (the pituitary does not make enough ACTH, often after long-term steroid treatment or pituitary disease).

Morning cortisol

A morning cortisol is the usual first test:

• Below about 3–5 µg/dL (83–140 nmol/L) – adrenal insufficiency is very likely.
• Above about 13–15 µg/dL (360–415 nmol/L) – adrenal insufficiency is very unlikely with most modern assays.
• In between – a stimulation test is needed.

Exact cut-offs depend on the assay and local guidance.

The ACTH stimulation (Synacthen) test

In the short Synacthen test, cortisol is measured before and 30 and/or 60 minutes after an injection of synthetic ACTH (tetracosactide, 250 µg). A healthy adrenal gland responds by raising cortisol. The traditional pass threshold is a peak cortisol of at least 18 µg/dL (500 nmol/L). With newer, more specific assays, lower thresholds – often around 14–15 µg/dL (about 400–430 nmol/L) – are increasingly used, and laboratories set their own cut-offs. This is a clear example of why the unit and the assay must both be known.

Symptoms of adrenal insufficiency

Symptoms develop gradually: tiredness, weakness, weight loss, poor appetite, nausea, abdominal pain, dizziness on standing, salt craving and, in primary adrenal insufficiency, darkening of the skin. Blood tests may show low sodium, high potassium or low glucose. An adrenal crisis – severe vomiting, low blood pressure, confusion and collapse – is a medical emergency treated with injected hydrocortisone and fluids, without waiting for test results.

Testing for high cortisol (Cushing's syndrome)

Cushing's syndrome is caused by prolonged exposure to excess cortisol. By far the most common cause is taking glucocorticoid medicines (such as prednisolone). Endogenous causes include a pituitary tumour making ACTH (Cushing's disease), adrenal tumours and, rarely, ACTH from tumours elsewhere.

Endocrine Society guidelines recommend at least one of three first-line screening tests:

• Overnight 1 mg dexamethasone suppression test. Dexamethasone 1 mg is taken at 11 p.m.–midnight, and cortisol is measured at 8–9 a.m. the next morning. In healthy people cortisol is suppressed to below 1.8 µg/dL (50 nmol/L). A higher value suggests possible Cushing's syndrome and needs further testing.
• Late-night salivary cortisol. Saliva collected at about 11 p.m. on two nights. Values above the laboratory's limit suggest loss of the normal night-time fall.
• 24-hour urinary free cortisol. Collected on at least two occasions; values above the laboratory's upper limit (often several-fold) suggest excess cortisol.

No single test is perfect, and abnormal results are usually confirmed with another test before imaging or further investigation.

Steroid medicines and adrenal suppression

Glucocorticoid medicines – prednisolone, prednisone, dexamethasone, hydrocortisone and others – are the most common cause of both excess cortisol effects and, after they are stopped, low cortisol. Taking them for more than a few weeks suppresses the HPA axis: the pituitary makes less ACTH, and the adrenal glands become less able to make cortisol on their own. This is called glucocorticoid-induced adrenal insufficiency.

The 2024 joint guideline from the European Society of Endocrinology and the Endocrine Society recommends:

• tapering long-term steroids rather than stopping them suddenly
• considering a morning cortisol measurement when the dose has been reduced to around physiological levels (roughly 4–6 mg of prednisolone per day), taken at least 24 hours after the last hydrocortisone or prednisolone dose
• using results to guide further tapering – a morning cortisol above about 300 nmol/L (11 µg/dL) generally suggests HPA recovery, while a very low value suggests ongoing suppression

These thresholds depend on the assay, and the guideline is written in nmol/L, so converting correctly is essential for clinicians working in µg/dL.

Sick-day rules

People with adrenal insufficiency – whether primary, secondary or steroid-induced – cannot increase cortisol during illness. They are taught "sick-day rules": typically doubling their usual daily hydrocortisone dose during a fever or illness requiring bed rest, and using an emergency injection of hydrocortisone if they are vomiting or collapsing. Carrying a steroid emergency card or wearing medical identification is strongly recommended. Cortisol blood tests are not used to decide doses in an emergency; treatment is given immediately.

Cortisol in pregnancy

Total cortisol rises progressively during pregnancy, roughly doubling or tripling by the third trimester, mainly because oestrogen increases CBG and because the placenta releases CRH. Free cortisol also rises. This makes Cushing's syndrome hard to diagnose in pregnancy: the dexamethasone suppression test is less reliable, and urinary free cortisol values within about three times the upper limit of normal may be seen in normal late pregnancy. Pregnant women with known adrenal insufficiency usually need a modest increase in their hydrocortisone dose in the third trimester and extra cover during labour.

Cortisol in critical illness

During severe illness, cortisol rises as part of the stress response, and random values of 20–50 µg/dL (550–1,380 nmol/L) are common. Interpreting cortisol in intensive care is difficult because CBG and albumin fall, which lowers total cortisol without necessarily reducing free cortisol. Guidelines on critical illness-related corticosteroid insufficiency note that no test reliably identifies which critically ill patients benefit from steroids, and decisions about hydrocortisone in septic shock are based mainly on the clinical response to fluids and vasopressors.

What affects cortisol results

• Time of day – the largest single influence.
• Shift work, jet lag and disrupted sleep – shift the rhythm and complicate interpretation.
• Oestrogen – combined oral contraceptives, oral HRT and pregnancy raise CBG and can increase total cortisol by 50% or more. Many guidelines recommend stopping oral oestrogen for about six weeks before dexamethasone testing when possible.
• Steroid medicines – prednisolone, hydrocortisone and some others cross-react with cortisol immunoassays; dexamethasone does not. Inhaled, injected, topical and nasal steroids can suppress the body's own cortisol.
• Medicines that speed dexamethasone breakdown – carbamazepine, phenytoin, rifampicin and others can cause false-positive dexamethasone tests.
• Acute illness, surgery and severe stress raise cortisol appropriately.
• Depression, alcohol use disorder, obesity and poorly controlled diabetes can cause mildly raised cortisol ("pseudo-Cushing's" states).
• Low albumin or CBG – in critical illness or liver disease, total cortisol can be low even when free cortisol is adequate.
• Biotin can interfere with some cortisol immunoassays.

"Adrenal fatigue" and cortisol testing

"Adrenal fatigue" is a term used by some practitioners to describe tiredness, poor sleep and low mood supposedly caused by adrenal glands worn out by chronic stress, often "diagnosed" with salivary cortisol profiles taken several times a day. It is not a recognised medical diagnosis. A systematic review of 58 studies found no substantiated evidence that adrenal fatigue exists, and the Endocrine Society advises against the concept. Treating it with hydrocortisone or adrenal extracts can itself suppress the HPA axis and cause genuine adrenal insufficiency when stopped.

Persistent fatigue deserves a proper assessment, which may include tests for anaemia, thyroid disease, diabetes, sleep disorders and depression, and – if there are suggestive features – a correctly timed morning cortisol or Synacthen test to look for true adrenal insufficiency.

When to see a doctor

See a doctor if you have symptoms of adrenal insufficiency – persistent tiredness, weight loss, dizziness on standing, salt craving or skin darkening – or features of Cushing's syndrome such as easy bruising, purple stretch marks, a rounder face, weight gain around the trunk, muscle weakness, high blood pressure or high blood sugar. Seek emergency care for vomiting, severe weakness, confusion or collapse in anyone with known adrenal insufficiency or recent long-term steroid use. Never stop long-term steroid medicines suddenly without medical advice, because this can cause an adrenal crisis.