Cortisol

Blood test · morning draw (about 8 AM)

Also known as: Morning cortisol, Serum cortisol

There is no single guideline cutoff for a morning cortisol. It is read with a clinician as a first clue to adrenal problems, and a diagnosis needs a stimulation or suppression test.

Cortisol is the main stress hormone made by the adrenal glands. Its level follows a strong daily rhythm, highest in the early morning and lowest around midnight. A morning cortisol is a first clue to adrenal problems, not a diagnosis.

What it tells you

The test measures total cortisol. Most cortisol in the blood rides on a carrier protein, and only a small part is free and active.
Anything that raises the carrier protein, such as estrogen or pregnancy, raises total cortisol without changing the active part.
A low morning value can be a clue to adrenal insufficiency, where the adrenals make too little. Confirming it needs a stimulation test, which checks how the adrenals respond to a hormone signal.
A random cortisol is not a test for Cushing's syndrome, where the body makes too much. Guidelines recommend against using it that way.
Cushing's syndrome is tested in other ways: an overnight dexamethasone suppression test, a urine collection or late-night saliva samples.
There is no valid test for "adrenal fatigue." The Endocrine Society says there is no proof it is a real medical condition.

What can throw it off

Time of day matters most. A later draw reads lower than an early-morning one.
Shift work and irregular sleep disrupt the daily rhythm.
Oral estrogen, including the birth control pill and oral hormone therapy, raises total cortisol. Pregnancy raises it too.
Prednisolone, a steroid medicine, is read as cortisol by many tests and gives a falsely high result.
Steroid medicines taken any way, including joint injections and nasal sprays, can suppress the adrenals and truly lower cortisol.
Acute illness, surgery and pain raise it. Depression, heavy drinking and severe obesity can also push it up.
Biotin supplements can make many cortisol tests read falsely high. Stop biotin for a few days before the draw, and tell whoever orders the test.
Test methods differ. Older methods read higher than newer ones.

Evidence

Screening for adrenal insufficiency

Early human evidenceCEBM Level 4

The guideline rates the evidence for a morning cortisol as very low quality. It allows it as a first step only when a stimulation test is not practical. It finds no evidence that a random cortisol can rule the condition out.

Confirming adrenal insufficiency

Strong human evidenceCEBM Level 1a

In a pooled analysis of thirteen studies, stimulation tests performed well at finding adrenal insufficiency caused by the pituitary. The guideline calls the standard stimulation test the gold standard. This is the test that follows a low morning value.

Screening for Cushing's syndrome

Strong human evidenceCEBM Level 1a

Guidelines recommend against a random cortisol for this. In a review of over two dozen studies, the dexamethasone, urine and saliva tests each performed well. But those studies came from specialist settings with many Cushing's cases, so accuracy in everyday care is unclear.

Limits

One morning value is a snapshot of a moving target. Timing, sleep, stress and illness all change it.
It measures total cortisol, not the free, active part. Estrogen and pregnancy distort it.
Neither a low nor a high result is a diagnosis. Low needs a stimulation test; high needs a suppression, urine or saliva test.
Cutoffs depend on the test method, and newer methods read lower than older ones.
It does not measure stress, burnout or "adrenal fatigue." No valid test exists for these.
Steroid medicines confuse it both ways. Some read as cortisol, and any route can suppress the adrenals.

References

Reviewed by Shubham Gupta, MD — Founder & President · November 5, 2026

This page is for education only. It is not medical advice. Decisions about any treatment belong with a physician who knows your history.