Cortisol tests.
A number needs its context.

Why timing matters

Cortisol follows a daily rhythm: higher in the morning, lower late at night. It also rises briefly with stress, illness, exercise and some medicines. A single measurement without its time of day, collection method and medical context can be misread. That is why clinicians choose the test for the question they are asking, not the other way around.

The tests clinicians use

Which test is used depends on the clinical question.

• Late-night salivary cortisol. Used when Cushing’s syndrome is suspected. Cortisol is normally low late at night, so a higher late-night level can be informative.
• 24-hour urinary free cortisol. Urine is collected over a full day to measure cortisol the body releases over time.
• Dexamethasone suppression test. A low dose of a steroid medicine is taken, then cortisol is measured to see whether it falls as expected.
• Morning blood cortisol and ACTH stimulation test. Used when low cortisol (adrenal insufficiency) is suspected. A clinician interprets them alongside symptoms and history.

For suspected Cushing’s syndrome, the Endocrine Society’s guideline describes screening with one or more of the first three tests above, and it does not recommend a random blood cortisol as a screening test. Abnormal screening results generally require further evaluation before anyone reaches a conclusion.

What can affect a result

• Glucocorticoid medicines (including some creams, inhalers and injections, depending on the situation) and estrogen-containing medicines.
• Pregnancy and acute illness.
• Disrupted sleep or shift work.
• Intense stress or exercise right before collection.
• Collection mistakes, such as the wrong time of day or contaminated saliva.

What about at-home kits?

Some companies sell kits that collect saliva or blood at home and send it to a laboratory. They can produce a number. A number from a consumer kit is not a diagnosis, and its meaning depends on the timing, the method, the laboratory’s reference range and your medical context. Quality and validation vary between companies. Results should not be used to start a supplement or change a medicine on your own.

Should I test because of a video?

Questions to bring to a clinician

• What question would this test answer for me?
• Which test is appropriate, and how and when should it be collected?
• Could my medicines or sleep affect the result?
• If the result is abnormal or normal, what happens next?

Use the preparation checklist on our High cortisol and Low cortisol pages to organize your notes. It does not score symptoms or interpret results.

To organize your notes before an appointment, use the free preparation checklist on our High cortisol and Low cortisol pages. It does not score symptoms or interpret results.

Quick answers

Also read: High Cortisol · Low Cortisol · Cortisol and Sleep

Sources

1. Endocrine Society: diagnosis of Cushing’s syndrome (2008 guideline).
2. NIDDK: Cushing’s syndrome, diagnosis.
3. NIDDK: adrenal insufficiency and Addison’s disease, diagnosis.

This page does not interpret personal results. Introductory education, not a complete clinical reference.