- Cortisol is a hormone that follows a daily rhythm, usually highest in the morning and lower at night.
- Timing matters, so cortisol is often measured in the morning and sometimes again later in the day.
- It can be tested in blood, saliva, or urine depending on the question being asked.
- Reference ranges and units vary by lab and sample type, so read your own report.
- Very high or very low results may prompt further testing for specific conditions.
- A clinician interprets cortisol in context; do not self-diagnose or change medication.
A cortisol test measures the level of cortisol, a hormone made by your adrenal glands. Because cortisol plays a role in your stress response, metabolism, blood sugar, and blood pressure, doctors sometimes check it when certain symptoms or conditions are suspected. This guide explains how the test works and why the timing and interpretation are so important. It is educational information, not medical advice.
What cortisol is and why timing matters
Cortisol follows a daily pattern called a circadian rhythm. According to MedlinePlus, levels are typically highest in the early morning and fall to their lowest around midnight in people with a regular sleep schedule. Because of this swing, the time of your blood draw strongly affects the result. That is why a clinician may ask you to have blood drawn at a specific time, often around 8 a.m., and sometimes again in the afternoon or evening to compare.
How the test is done
Cortisol can be measured in different samples, and the choice depends on the clinical question. A blood test is common. Saliva testing, often collected late at night at home, is sometimes used to look at the low point of the rhythm. A 24-hour urine collection measures the total amount over a full day. The Endocrine Society notes that certain conditions are evaluated with dynamic tests, such as a suppression or stimulation test, where medication is given and cortisol is measured afterward to see how the glands respond.
Reference ranges vary
Reference ranges and units differ by laboratory, sample type, and time of day. A morning blood range will not match a late-night saliva range, and one lab’s numbers may differ from another’s. Always use the range printed on your own report.
| Sample and timing | What it generally looks at |
|---|---|
| Morning blood (around 8 a.m.) | The expected daily peak |
| Afternoon or evening blood | Whether levels fall as expected |
| Late-night saliva | The low point of the rhythm |
| 24-hour urine | Total cortisol output over a day |
What high or low results may suggest
Higher-than-expected cortisol can occur with stress, illness, pregnancy, certain medications, or, less commonly, conditions that cause the body to make too much cortisol. Lower-than-expected levels may point toward conditions where the adrenal glands make too little. However, many everyday factors, including acute stress, poor sleep, and some medicines such as corticosteroids or estrogen, can shift results. A single number rarely tells the whole story, which is why clinicians often repeat testing or add other tests. For related hormone context, see our explainer on testosterone testing.
What to expect and how to prepare
How you prepare depends on the type of test. For a blood draw, the sample is taken from a vein in your arm, and your clinician may ask you to come in at a specific time, rest beforehand, or note any medications you take. For late-night saliva collection, you typically follow instructions to avoid eating, drinking, or brushing your teeth for a short window before the sample. A 24-hour urine test asks you to collect all urine over a full day into a provided container. Because so many things nudge cortisol up or down, sharing an accurate list of your medications and supplements, your sleep pattern, and any recent illness helps your clinician read the result correctly. Steroid medicines, in particular, can strongly affect cortisol testing, so mention them even if you use a cream, inhaler, or joint injection.
Why a clinician interprets the result
Cortisol results are easy to misread without training. Timing, medications, and stress all matter, and abnormal results usually lead to more specific follow-up rather than a diagnosis on their own. Do not stop or adjust any medication, especially steroids, based on a cortisol number. Bring questions to the clinician who ordered the test. You can find more explainers in our wellness guide hub, including our ferritin test overview.
Frequently asked questions
Why was my blood drawn in the morning?
Morning is often when cortisol peaks, so an 8 a.m. draw gives a standard reference point. Your clinician may compare it to a later sample.
Do I need to fast?
Fasting is not always required, but follow the specific instructions from your lab or clinician, including any about timing, medications, and rest before the test.
Can stress change my result?
Yes. Physical and emotional stress, illness, and poor sleep can raise cortisol temporarily, which is one reason results are interpreted in context.
What is a suppression or stimulation test?
These are dynamic tests where medication is given and cortisol is measured afterward to see how your glands respond. They are ordered and supervised by a clinician.
Why is my number different from an online chart?
Ranges depend on lab, sample type, and time of day. Compare only to the reference range on your own report.
What if my result is abnormal?
An abnormal result usually leads to repeat or additional testing rather than an immediate diagnosis. Your clinician will decide the next step.
This article is for general education and is not medical advice. Lab tests and their reference ranges vary by laboratory and person, and results can be affected by timing, stress, and medications. Talk to your doctor or a licensed clinician to interpret any test result and before making changes to your health routine.
Sources
- MedlinePlus (NIH) — cortisol test
- Mayo Clinic — cortisol and adrenal testing
- Endocrine Society — adrenal and cortisol evaluation