Cortisol: The Stress Hormone Explained
Cortisol is often called the stress hormone, but it does much more than respond to stress. It helps regulate metabolism, blood sugar, blood pressure, and the immune system, and it follows a strong daily rhythm that shapes how it is measured and interpreted.
What cortisol is
Cortisol is a steroid hormone in the glucocorticoid family, made from cholesterol. It is one of the body's main tools for managing energy, responding to physical or emotional demands, and keeping many systems in balance. Most cortisol in the blood travels bound to a carrier protein, with a smaller free fraction that is active in tissues. This is worth knowing because different tests measure different fractions, which is one reason results from different methods are not directly interchangeable.
Where it is produced
Cortisol is produced by the adrenal glands, two small glands that sit on top of the kidneys. Specifically, it is made in the outer layer of the adrenal gland, called the adrenal cortex. The same region also produces other steroid hormones, but cortisol is the body's primary glucocorticoid.
The HPA axis: how cortisol is controlled
Cortisol is governed by a three-part control chain called the hypothalamic-pituitary-adrenal (HPA) axis, which behaves like a thermostat.
- The hypothalamus releases corticotropin-releasing hormone (CRH).
- CRH prompts the pituitary to release adrenocorticotropic hormone (ACTH) into the bloodstream.
- ACTH signals the adrenal cortex to make and release cortisol.
- As cortisol rises, it feeds back to suppress CRH and ACTH, a negative feedback loop that keeps levels from climbing unchecked.
This feedback design is what allows clinicians to test the axis cleverly. Giving a small dose of a synthetic glucocorticoid should suppress the axis and lower cortisol in a healthy person (a suppression test), while giving synthetic ACTH should stimulate the adrenal glands to release more cortisol (a stimulation test). How the axis responds helps locate where a problem sits, whether at the level of the adrenal glands, the pituitary, or the hypothalamus.
The diurnal rhythm
Cortisol does not stay constant through the day. It follows a circadian rhythm, generally rising in the hours before waking, peaking in the early morning, and then declining across the day to its lowest point around the middle of the night. On top of this daily curve, cortisol is released in short pulses and can spike briefly in response to stress, exercise, or a meal.
This rhythm has a practical consequence: the same number means very different things depending on the time it was measured. A level that is expected in the early morning could be unexpectedly high if found late at night. For this reason, the timing of a sample is part of the result, not just background detail.
Cortisol and the fight-or-flight response
Cortisol is one of two main stress hormones, working alongside the faster-acting adrenaline. When the body faces a demand, adrenaline produces the immediate surge, a racing heart and heightened alertness within seconds, while cortisol acts more slowly and lasts longer. Cortisol helps sustain the response by keeping blood sugar available to muscles and the brain, dampening processes that are not urgent in the moment, and supporting blood pressure. Once the demand passes, the negative feedback loop normally brings cortisol back down.
This system is well suited to short bursts of stress. Researchers continue to study how the body responds when the HPA axis is activated repeatedly or for long stretches, and how that relates to sleep, mood, and metabolic health. Much in this area is still an active field of investigation rather than settled fact, so claims that go beyond the established physiology should be treated with caution.
What it does in the body
- Energy and metabolism: It helps raise and steady blood sugar and influences how the body uses fats, proteins, and carbohydrates.
- Stress response: During physical or emotional stress, cortisol helps mobilise energy and sharpen alertness.
- Blood pressure: It supports normal blood pressure and the body's response to low blood pressure.
- Immune and inflammation: It modulates inflammation and immune activity, which is why related synthetic glucocorticoids are used medically.
- Daily rhythm and waking: It helps coordinate the sleep-wake cycle, typically peaking in the early morning to support waking and alertness.
How levels are regulated day to day
Beyond the HPA axis and circadian rhythm, several everyday factors nudge cortisol up or down. Stress, illness, and disrupted sleep tend to raise it. Pregnancy and oestrogen-containing medications can raise the carrier protein and therefore total measured cortisol without necessarily changing the active free fraction. Certain medicines, including glucocorticoids taken for other conditions, can suppress the body's own production. Clinicians keep all of this in mind when reading a result.
What high or low levels can be associated with
Cortisol that is persistently too high or too low can be associated with specific conditions, though everyday factors can also shift levels temporarily, so results are always interpreted carefully and usually confirmed with more than one test.
- Persistently high cortisol can be associated with a group of features sometimes described as Cushing's syndrome, which may include changes in weight distribution, skin changes, and high blood pressure. It can arise from the body overproducing cortisol or from taking glucocorticoid medicines.
- Persistently low cortisol can be associated with adrenal insufficiency, of which Addison's disease is one example. This is evaluated carefully because it can affect how the body copes with stress and illness.
These associations are qualitative and are named here only as examples. They are not something to self-diagnose. See the conditions index, and when thyroid symptoms overlap, our guides to the TSH test and hypothyroidism may also help. Discuss any concerns with a clinician.
How it is measured
Cortisol can be measured in more than one way, and the choice depends on the question being asked. Each method samples a different fluid or time window, so they answer different questions and are not interchangeable.
| Test type | What it samples | Often used to explore |
|---|---|---|
| Blood (serum) cortisol | Total cortisol at a single moment, usually timed to the morning peak | An overall snapshot, often paired with ACTH or used in stimulation tests (illustrative; varies by lab) |
| Salivary cortisol | The free, active fraction, commonly collected late at night | The shape of the daily rhythm, especially the expected nighttime low (illustrative; varies by lab) |
| 24-hour urine cortisol | The total cortisol excreted across a full day | Overall daily production, smoothing out short pulses (illustrative; varies by lab) |
Cortisol may be tested together with ACTH or as part of a suppression or stimulation test ordered by a clinician. Because results depend on timing, the method, and the carrier-protein effects described above, they are read in context rather than against a single fixed number. See the blood tests overview for more, browse the hormones library, check related symptoms, or look up terms in the glossary.
Illustrative daily pattern
The pattern below is illustrative only and varies by laboratory and method. Use the reference range printed on your own report and discuss it with your clinician.
| Timing | General pattern |
|---|---|
| Early morning | Typically highest (illustrative; varies by laboratory) |
| Late afternoon / evening | Lower (illustrative) |
| Middle of the night | Generally lowest (illustrative; ranges vary by laboratory and method) |
Frequently asked questions
Why is cortisol called the stress hormone?
Cortisol rises during physical or emotional stress to help mobilise energy and maintain alertness, though it also has many everyday regulatory roles in metabolism, blood pressure, and the immune system.
Why does the time of the test matter?
Cortisol follows a daily rhythm, generally highest in the morning and lowest at night, so the draw time shapes how a result is read.
Can stress before a blood draw affect the result?
Yes. Acute stress, illness, and disrupted sleep can temporarily change cortisol, which is one reason clinicians interpret results in context and may repeat the test.
Where are the adrenal glands?
They sit on top of each kidney, and the outer layer (the adrenal cortex) produces cortisol.
What is the difference between blood, saliva, and urine cortisol tests?
Blood gives a timed snapshot of total cortisol, saliva captures the free fraction and is often used to check the nighttime low, and a 24-hour urine collection reflects total daily production. A clinician chooses the method based on the question being asked.
What conditions are linked to abnormal cortisol?
Persistently high cortisol can be associated with Cushing's syndrome and persistently low cortisol with adrenal insufficiency, such as Addison's disease. These are evaluated carefully with repeated and often specialised testing rather than a single result.
Sources
- MedlinePlus. Cortisol Test. https://medlineplus.gov/lab-tests/cortisol-test/
- MedlinePlus. Endocrine Diseases. https://medlineplus.gov/endocrinediseases.html
- Hormone Health Network. https://www.hormone.org/