Cortisol is a steroid hormone produced by the adrenal glands in response to stress and low blood glucose. It regulates metabolism, blood pressure, immune function and the body's response to stress. Cortisol follows a clear daily pattern — highest in the morning and lowest at night.
A cortisol blood test measures the amount of the hormone circulating at the time of the draw. Because cortisol is highly time-dependent — and fluctuates with stress, illness, medications and exercise — interpreting the result correctly requires knowing when the sample was taken and what was happening around it.
This guide explains when cortisol tests are ordered, what normal looks like at different times of day, and what high or low results might mean.
Why cortisol varies through the day
Cortisol follows a circadian rhythm. It peaks within an hour of waking — a natural surge that prepares you for the day — and falls steadily through the afternoon, reaching its lowest point around midnight. This is why most cortisol tests are timed: an 8 a.m. draw gives a very different result from a 4 p.m. one, and both would be expected to differ from midnight.
Many labs therefore report an 'expected range' that depends on the time of collection. Always check that the time of your sample matches the reference range being used.
What do raised cortisol levels mean?
Persistently high cortisol is called hypercortisolaemia. The most serious cause is Cushing's syndrome — a condition where the adrenal glands produce too much cortisol, either spontaneously or due to prolonged use of corticosteroid medications.
Symptoms of raised cortisol include weight gain around the abdomen, a rounded face, stretch marks, high blood pressure, fatigue and mood changes. However, cortisol also rises with ordinary stress, poor sleep, obesity and strenuous exercise — so a raised result needs careful interpretation in context.
- Cushing's syndrome — excess cortisol production by the adrenal glands
- Prolonged use of steroid medications (prednisolone, dexamethasone)
- Obesity, depression, chronic stress
- Poorly controlled diabetes
- Acute illness or surgery
What do low cortisol levels mean?
Low cortisol can be equally serious. Addison's disease (primary adrenal insufficiency) is a condition where the adrenal glands fail to produce enough cortisol. Symptoms include fatigue, weight loss, low blood pressure, salt cravings and darkening of the skin.
Low cortisol can also result from the pituitary gland not signalling the adrenals correctly (secondary adrenal insufficiency) or from abruptly stopping long-term steroid medication.
Other cortisol tests your doctor may order
- 24-hour urinary free cortisol — measures total cortisol excreted over a full day, less affected by a single stressful moment
- Late-night salivary cortisol — tests whether the normal evening drop happens; useful for Cushing's screening
- ACTH stimulation test (Synacthen test) — measures how the adrenal glands respond to a synthetic signal; the most reliable test for adrenal insufficiency
- Low-dose dexamethasone suppression test — used to confirm or rule out Cushing's syndrome
Keeping track of your results
If your doctor is monitoring your cortisol over time — for example, during Cushing's treatment or while managing adrenal insufficiency — a trend matters more than any single value. Logging your cortisol results in Looms with the time of draw and any relevant context (medications, illness) gives your care team a much richer picture. If you've also had a thyroid panel ordered, our guide to thyroid blood test results explains how those two hormones often interact.
"A single cortisol value tells you where you are at one moment. The trend over time tells you whether the treatment is working."
Dr. Lina Haddad, CMO, Looms