What is cortisol? The hormone explained

Cortisol is a hormone made by your adrenal glands, two small glands that sit above your kidneys. It is often called the stress hormone, but stress is only one of its jobs. Your body makes cortisol every day, in a steady rhythm, and uses it to manage energy, blood pressure and inflammation.

This page is the starting point for the site. It explains what cortisol is, what controls it, why the level changes and when a level is worth a conversation with a clinician. Each topic links to a page that goes further.

Cortisol

One blood draw for one cortisol result. It can’t diagnose or rule out an adrenal condition by itself.

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What cortisol is

A hormone is a chemical messenger that travels in your blood and tells cells and organs what to do. Cortisol is a steroid hormone. It belongs to the group called glucocorticoids, and a 2018 review calls it the main glucocorticoid in humans (Front Behav Neurosci, 2018).

MedlinePlus says cortisol affects almost every organ and tissue in the body. That is why it matters when the level is far too high or too low for a long time. Your adrenal glands make several other hormones too, including aldosterone, adrenaline and noradrenaline (MedlinePlus). Cortisol is only one of them.

When people ask what “cortisol” or “cortisol levels” mean, they usually mean the amount of this hormone measured in blood, saliva or urine. Most cortisol sits in the blood, and MedlinePlus says some passes into urine and saliva, where it can be measured to estimate the blood level.

What cortisol does

The short version, from MedlinePlus and NIDDK, is that cortisol helps your body:

• respond to stress
• reduce inflammation
• control blood sugar and metabolism, which is how your body uses food for energy
• control blood pressure

Cortisol is not only about emergencies. A 2017 review says the rise toward the end of the night frees up stored energy for the day ahead, and that its actions on the stress response, metabolism, blood vessels, immune system, sleep and memory are shaped by the time of day (Endocrine Reviews, 2017). Our page on what cortisol does to your body takes each system in turn. For how it compares with the other stress hormone, see cortisol vs adrenaline.

How your body controls it

Three structures work as a chain, called the HPA axis. The hypothalamus, a part of the brain, makes a hormone that tells the pituitary gland how much ACTH to make. ACTH tells the adrenal glands to make cortisol. When cortisol in the blood rises, it signals the hypothalamus to ease off, which keeps the system in balance (MedlinePlus).

A problem at any step can leave cortisol too high or too low. That is why clinicians often measure ACTH alongside cortisol. Read more in the HPA axis and the ACTH blood test.

How it changes through the day

In healthy people who sleep at night, cortisol begins rising at about 4 a.m., peaks in the morning and then falls to its lowest point at around midnight (Endocrine Reviews, 2024).

This is why a cortisol value always needs a time attached. MedlinePlus says blood samples are usually taken twice in a day, once in the morning when cortisol is normally highest and again around 4 p.m., when it is normally much lower. Two pages go further: when cortisol is highest and the cortisol awakening response, the sharp rise in the first hour after waking.

What moves cortisol up or down

Many ordinary things change a cortisol result. MedlinePlus lists stress, pregnancy, exercise, hot and cold temperatures and some medicines, such as birth control pills. It says the most common cause of an abnormal result is taking steroid medicines for a long time, or stopping them suddenly.

Labs also differ. They use different methods and print their own ranges, so the range on your own report is the one to use. Our normal cortisol levels page gives published ranges by time of day, and the cortisol units converter helps if your report is in a different unit.

When cortisol is too high or too low

NIDDK describes Cushing’s syndrome as a disorder that develops when the body makes too much cortisol over a long period. The most common cause is long-term, high-dose use of cortisol-like medicines called glucocorticoids. Adrenal insufficiency is the opposite problem: the body doesn’t make enough cortisol. Addison’s disease is one form of it.

MedlinePlus lists these symptoms:

• Too much cortisol: weight gain, thin arms and legs, a round face, easy bruising, wide purple streaks on the skin and muscle weakness.
• Too little cortisol: long-lasting fatigue, muscle weakness, loss of appetite, weight loss and abdominal pain.

Neither list proves anything. Each symptom has many other causes. If a symptom or a result worries you, take it to a clinician. See high cortisol and low cortisol for the detail.

If you take a steroid medicine, never stop it or change the dose on your own. Ask your prescriber.

Some signs need urgent care and not a test. NIDDK says that if you have adrenal insufficiency and symptoms of adrenal crisis, you need emergency medical treatment. It lists sudden, severe pain in the lower back, abdomen or legs, vomiting and diarrhea, weakness, confusion and loss of consciousness. It also says a severe infection, or severe diarrhea or vomiting that stops you keeping steroid pills down, needs immediate medical attention. If you have adrenal insufficiency or take a steroid medicine and have any of these, call 911 or go to an emergency room. Don’t wait for a test or an appointment. If you aren’t sure how serious your symptoms are, get urgent care rather than wait.

What cortisol is not

Cortisol is not a diagnosis, and a “high cortisol” label isn’t one either. “Adrenal fatigue” is sometimes blamed on cortisol, but it is not a recognized medical condition. A 2024 paper in the Journal of the Endocrine Society says it has never been scientifically proven to exist, and that the saliva cortisol profiles sold for it have never been validated (J Endocr Soc, 2024). See adrenal fatigue for what the term means and what to ask a clinician instead.

How cortisol is tested

MedlinePlus says a cortisol test can use blood, urine or saliva. A blood draw takes less than five minutes. Tests for too much cortisol in particular are different from a simple blood draw, and a clinician usually arranges them. A single result can’t diagnose or rule out Cushing’s syndrome or adrenal insufficiency; MedlinePlus says a cortisol test alone can’t identify the cause of an abnormal level and more testing is usually needed.

If you want a number to bring to a clinician, our cortisol test guide explains which test answers which question. The price boxes on this page show the single blood test.

Questions people ask

What does low cortisol mean?

It can mean two different things. A result under a lab’s range is a finding that needs context, such as the time of day and any medicines. The condition, adrenal insufficiency, is a medical diagnosis made with more than one test. Read low cortisol for the difference and for the emergency signs.

Is cortisol only about stress?

No. Stress raises cortisol, but the body also makes it on a daily schedule with no stressful event at all. See is cortisol good or bad for why you need it.

Is cortisol the same as cortisone?

No. Cortisol is the active hormone. Cortisone is a related, inactive form that the body converts back and forth. See cortisone vs cortisol.

Keep reading

• What cortisol does

  The jobs it does in stress, energy, inflammation and blood pressure.

• Is cortisol good or bad?

  Why the answer depends on how much, when and for how long.

• The HPA axis

  The brain-to-adrenal chain that controls cortisol.

• When is cortisol highest?

  The daily rhythm, from the early-morning peak to the midnight low.

• Cortisol vs adrenaline

  Two stress hormones with different speeds and jobs.

• How long cortisol stays in your system

  Half-life, and what it does and doesn't tell you.

• Cortisol test guide

  Which test answers which question.