Short answer: the cortisol blood test measures the main stress hormone made by the adrenal glands. Cortisol has a strong daily rhythm, highest soon after waking and lowest around midnight, so the time of the sample changes the result completely. A single blood value is used mainly to look for too little cortisol; other tests, such as late-night saliva or a suppression test, are used to look for too much. nmol/L = µg/dL × 27.59.
At a glance
| Measures | Cortisol in serum or plasma at the time of the sample |
|---|---|
| Units | µg/dL (US) or nmol/L (Europe); 1 µg/dL = 27.59 nmol/L |
| LOINC | 2143-6 (serum or plasma) |
| When taken | Usually early morning, unless your doctor asks otherwise |
| Changed by | Time of day, steroid medicines, oestrogen (pill, pregnancy), acute illness, night shifts |
| Keep | Value, unit, range, date and the exact time of the sample |
What the test measures
Cortisol helps the body respond to stress, keeps blood pressure and blood sugar steady and calms inflammation. Its level is controlled by the pituitary gland through ACTH. The blood test measures total cortisol, most of it bound to a carrier protein. Our guide to cortisol and stress explains the daily rhythm in more detail.
What can change the result
- The time: a morning value and an evening value from the same healthy person can differ several-fold.
- Steroid medicines: prednisolone, hydrocortisone and some creams, inhalers and joint injections can suppress the body's own cortisol or be measured by the test. Tell the laboratory and your doctor what you use.
- Oestrogen: the contraceptive pill and pregnancy raise the carrier protein and therefore total cortisol.
- Illness, poor sleep and night work shift the rhythm.
Other cortisol tests
Doctors may ask for a late-night saliva cortisol, a 24-hour urine free cortisol or a dexamethasone suppression test when they look for excess cortisol, and an ACTH stimulation test when they look for too little. Each answers a different question, so a blood value is not compared directly with a saliva or urine value.
How the result is written on the report
| Result on the report | LOINC code | Usual unit | Also seen as |
|---|---|---|---|
| Cortisol, serum or plasma | 2143-6 | µg/dL | nmol/L (×27.59) |
nmol/L = µg/dL × 27.59: 15 µg/dL ≈ 414 nmol/L. Laboratories print ranges for morning and sometimes afternoon samples; use the range on your report and the time of your sample.
Keeping the results in your record
Write the sample time next to every cortisol result. In Anpheros Daily each report keeps its date, time, unit and range, and the lab results tracker charts repeated values without an account.
What this page does not do
It explains what the test is. It does not interpret your result or diagnose an adrenal condition, which needs your history and usually dynamic tests chosen by your doctor.
Sources
Frequently asked questions
Why is the cortisol test done in the morning?
Cortisol is highest soon after waking. A morning sample is the standard point of comparison, especially when doctors check whether the body makes enough.
Can stress raise my cortisol test?
Short-term stress, pain or illness can raise cortisol at the moment of the sample. That is one reason a single value is read with your situation in mind.
How do I convert cortisol from µg/dL to nmol/L?
Multiply by 27.59. For example, 10 µg/dL is about 276 nmol/L.
Do I need to stop steroid medicines before the test?
Do not stop any medicine on your own. Tell your doctor what you use, including creams and inhalers; they decide whether and how to prepare.
Try it in Anpheros Daily
Photograph your lab report and Anpheros Daily lines up each value with its reference range, so you can compare reports from different laboratories.
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