In brief
Measures cortisol in blood, urine or saliva. It is used to help diagnose Cushing's syndrome and Addison's disease, which are both rare.
What it measures
Cortisol, a hormone made by the adrenal glands. Samples can be blood, a 24-hour urine collection, or late-night blood or saliva, sometimes after a dexamethasone tablet the night before.
Why it is done
To help diagnose Cushing's syndrome (too much cortisol) and Addison's disease (too little). It is not a test for adrenal fatigue, which is not a recognised medical condition.
Reading the results
- A dexamethasone tablet switches off the hormone ACTH, which helps tell Cushing's syndrome apart from high cortisol caused by stress.
- Late-night blood or saliva cortisol is often used when Cushing's syndrome is suspected.
Before the test
Follow your doctor's instructions on timing, because cortisol peaks at about 8 am and falls in the evening.
What can affect the result
- Physical and emotional stress, especially illness
- Pregnancy
- The contraceptive pill
- Long-term steroid medicines
Related topics
Sources
Test information summarised from the sources listed on this page, read on 13 and 19 October 2026. General education only, not medical advice. Spotted a mistake? Please tell us through the HuBu-Intel contact page.