A serum cortisol test, which is a standard blood draw from a vein in your arm, is the primary blood test used to measure cortisol levels. The sample is typically collected in the early morning, between 6 and 9 a.m., when cortisol naturally peaks. Results are reported in micrograms per deciliter (mcg/dL) or nanomoles per liter (nmol/L), and the timing of the draw matters enormously because cortisol fluctuates dramatically throughout the day.
How the Serum Cortisol Test Works
The test itself is straightforward: a healthcare provider draws blood, usually from a vein in your arm, and sends it to a lab. Most of the cortisol in your blood (about 80%) is bound to a carrier protein, with another 10% bound to albumin. Only the remaining fraction is “free” cortisol, which is the biologically active form your cells actually use. A standard serum cortisol test measures total cortisol, meaning both bound and free together.
In certain situations, total cortisol can be misleading. When carrier protein levels shift, as they do during severe illness, after major surgery, or in ICU patients, total cortisol may look low even though free cortisol is adequate. One study found that 23% of surgical patients would have been incorrectly diagnosed with adrenal insufficiency based on total cortisol alone. For most outpatient testing, though, total serum cortisol is reliable enough to guide the next steps.
Why Timing Changes Everything
Cortisol follows one of the strongest daily rhythms of any hormone. Levels are already high when you wake up, then surge another 50 to 60% in the 30 to 40 minutes after waking. They drop quickly over the next few hours and continue falling more gradually until they hit their lowest point around bedtime.
This is why a morning blood draw is standard. A cortisol level that looks perfectly normal at 7 a.m. could be flagged as abnormally high if drawn at midnight. When doctors suspect excess cortisol production, a late-night measurement (either blood or saliva) becomes useful precisely because cortisol should be at its lowest then. A sleeping midnight serum cortisol above 1.8 mcg/dL has shown 100% sensitivity for detecting Cushing’s syndrome in research.
How to Prepare for the Test
Your provider will give specific instructions, but general preparation focuses on minimizing anything that could artificially raise or lower your cortisol. You may be asked to rest quietly before the draw, since physical or emotional stress can spike cortisol within minutes. Some providers request that you avoid vigorous exercise the evening before.
Several categories of medication can shift your results. Antidepressants and antipsychotics tend to lower both baseline cortisol and cortisol responses to stimulation tests. Stimulant medications, like those used for ADHD, can raise baseline cortisol. Oral contraceptives increase the carrier protein that binds cortisol, which inflates total cortisol readings without actually changing the amount of active hormone in your system. Let your provider know about all medications you’re taking so they can interpret your results correctly or adjust the testing approach.
What the Results Mean
A morning serum cortisol at or above 18 mcg/dL (500 nmol/L) generally rules out adrenal insufficiency. Newer, more precise lab methods can use a threshold about 25 to 30% lower than that. The specific reference range on your lab report will reflect the assay your lab uses, so always compare your number to the range printed alongside it rather than to values you find online.
Because cortisol is released in pulses rather than a steady stream, a single blood draw can catch you at a peak or a trough that doesn’t represent your true average. That’s why an inconclusive result doesn’t automatically mean something is wrong. It often just means more testing is needed to get a clearer picture.
When a Single Blood Draw Isn’t Enough
If your baseline cortisol falls in a gray zone, the next step is usually an ACTH stimulation test. A synthetic version of the hormone that tells your adrenal glands to produce cortisol is injected, and your blood is drawn again 30 to 60 minutes later. In a healthy response, cortisol should rise above 15 to 16 mcg/dL (430 to 450 nmol/L). A blunted response suggests the adrenal glands aren’t producing cortisol adequately, pointing toward adrenal insufficiency.
For suspected Cushing’s syndrome (too much cortisol), the Endocrine Society recommends against using a random serum cortisol level as an initial screening tool. Instead, the preferred first tests are 24-hour urine free cortisol collection (at least two samples), late-night salivary cortisol (two samples), or a dexamethasone suppression test. In the suppression test, you take a small dose of a synthetic steroid at bedtime, and your blood cortisol is checked the next morning. In a healthy response, cortisol drops below 1.8 mcg/dL because the medication signals your body to stop producing its own. Cortisol that stays above that threshold suggests the normal feedback loop isn’t working, which is characteristic of Cushing’s syndrome.
Blood Cortisol vs. Other Cortisol Tests
A serum cortisol test isn’t the only way to measure cortisol. Salivary cortisol is collected by spitting into a tube at home, which makes it especially practical for late-night sampling since you don’t need to visit a lab. It measures free cortisol specifically, avoiding the carrier-protein issue that can complicate blood tests in certain patients.
A 24-hour urine cortisol test captures your total cortisol output over a full day, smoothing out the pulsatile spikes that can make a single blood draw misleading. You collect all urine for 24 hours in a provided container, and the lab measures the total cortisol excreted. This is particularly useful for detecting chronic overproduction.
Each test has its strengths. Blood cortisol is fast, widely available, and useful as a first look. Salivary cortisol is convenient and better for nighttime collection. Urine cortisol gives the most complete daily picture. Your provider will choose based on what they’re trying to rule in or out.