Cortisol: what it is, what it tells you and when it’s tested

Cortisol is a hormone made by the adrenal glands, which sit on top of your kidneys. It helps keep blood sugar and blood pressure steady and helps you cope with illness or strain. It is high when you wake up and falls through the day. That is why a result only makes sense if you know what time the blood was taken.

When it’s tested

Doctors order it when they suspect the adrenal glands make too much cortisol (Cushing’s syndrome) or too little (adrenal insufficiency). For example, when the tummy and face get rounder, with wide reddish purple stretch marks, and blood pressure and blood sugar rise together. Or with severe tiredness, weight loss, low blood pressure and nausea.

It is also tested in people who have taken steroid medicines for a long time, before the dose is lowered, and in people with a lump on an adrenal gland found by chance on a scan.

It is not a prevention test for people who feel well: it changes too much with the time of day and the situation, and a single result is not enough for a diagnosis. Rosa does not test it. It does look at blood sugar, sodium and potassium, which shift when cortisol is truly out of place.

What it tells you, and what it does not

A very low morning cortisol can point to adrenal insufficiency. A high result on its own says little: Cushing’s needs different tests, such as late-evening saliva cortisol, a 24-hour urine collection or a blood test after a dexamethasone tablet.

Cortisol does not measure everyday stress. A hard week at work does not usually push blood cortisol out of the normal range, and a normal result does not mean you are not stressed.

‘Adrenal fatigue’ is not a recognised diagnosis: studies have not found an adrenal problem in people who feel exhausted. If you have been tired for weeks, the cause should be looked for elsewhere, with your doctor.

That is why it is read together with other tests:

• ACTH: Shows whether the problem starts in the pituitary or the adrenals
• Sodium: Falls when cortisol is short
• Potassium: Rises in adrenal insufficiency
• Blood sugar: Too much cortisol raises it

Normal cortisol levels

Normal levels depend on the time of the blood test and on the lab’s method. That is why the blood test is usually done in the morning, between 7 and 10.

A large Italian hospital lab gives 6 to 22 µg/dL for an adult in the morning (printed as 60 to 220 ng/mL). Other labs use slightly different limits. The range on your own report is what counts.

In the afternoon and evening cortisol is lower: the same number can be normal at 8 in the morning and high at 11 at night. Late-evening saliva and 24-hour urine have their own ranges, always given by the lab.

If your cortisol is high or low

• High cortisol: Often down to the time of the test, the pill or an illness. Cushing’s is rare.
• Low cortisol: Often down to the time of day or steroid medicines. Very low in the morning needs looking at soon.

What can skew the result

Many things shift cortisol without any adrenal problem. Tell your doctor about them before the test.

• The time of the test: It peaks when you wake and is lowest at night. A test taken late can look low.
• The pill and pregnancy: Oestrogen raises the protein that carries cortisol: the total goes up without a real excess.
• Steroid medicines: Tablets, but also inhalers, creams or joint injections, can lower your own cortisol or confuse the test. Do not stop them on your own.
• Illness or a sleepless night: Fever, severe pain, surgery or a night without sleep raise it, sometimes for a few days.

When to worry

Talk to your doctor if:

• Your morning cortisol is below the limit on your report, especially if you are tired, losing weight or have low blood pressure.
• You put on weight around your tummy and face while your arms and legs get thinner, or have wide reddish purple stretch marks or bruise easily.
• You have taken steroids for months and want to cut down: this is tapered with your doctor, never stopped suddenly.
• You have extreme weakness, vomiting that will not stop, severe tummy pain, fainting or confusion, especially if you take steroids or have an adrenal problem: call 112 or go to the emergency department.

Frequently asked questions

What is a normal morning cortisol level? It depends on the lab. A large Italian hospital lab gives 6 to 22 µg/dL for an adult in the morning. The range on your report and the time of the test are what count.

Does stress raise cortisol in a blood test? Sudden stress raises it briefly. Everyday stress does not usually push blood cortisol out of the normal range. A normal cortisol does not tell you how stressed you are.

Do I need to fast for a cortisol test? Labs often ask for an early morning test, after resting. Fasting depends on the other tests on the form. Follow your lab’s instructions.

Is a blood, saliva or urine cortisol test better? They answer different questions. Morning blood helps show whether you make too little. Late-evening saliva, 24-hour urine and the dexamethasone test look for too much. Your doctor chooses.

Is adrenal fatigue real? No, it is not a diagnosis recognised by endocrine societies. If you are always tired, talk to your doctor: the causes lie elsewhere, and they are worth looking for together.

Does Rosa test cortisol? No. Rosa measures blood sugar, HbA1c, sodium and potassium, which shift when cortisol is truly out of place. If your doctor wants cortisol, they order it separately.

Sources

• Bornstein and colleagues, diagnosis and treatment of primary adrenal insufficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism, 2016. PMID 26760044
• Nieman and colleagues, the diagnosis of Cushing’s syndrome: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism, 2008. PMID 18334580
• Cadegiani and Kater, adrenal fatigue does not exist: a systematic review. BMC Endocrine Disorders, 2016. PMID 27557747
• Fondazione Policlinico Universitario A. Gemelli IRCCS, clinical chemistry laboratory: list of tests and reference intervals, 2024. Policlinico Gemelli, elenco prestazioni CDS.002