Cortisol: what it does, and what high and low levels mean

What is cortisol?

Cortisol is a steroid hormone produced by the adrenal glands, which sit on top of the kidneys. It’s often called “the stress hormone”, but that undersells it: cortisol is essential for life. Among other things, it helps to:

• keep blood sugar steady, especially between meals and overnight
• maintain blood pressure
• regulate metabolism – how the body uses fat, protein and carbohydrate
• damp down inflammation and regulate the immune system
• shape the sleep–wake cycle
• mobilise energy when you face a physical or mental challenge

What is the stress hormone and how does it work?

When you face a challenge – a deadline, an argument, a near miss on the road – two systems respond. Adrenaline acts within seconds, giving you the racing heart and quick breathing of “fight or flight”. Cortisol follows over minutes, via a chain known as the HPA axis:

• the hypothalamus in the brain releases a hormone called CRH;
• that prompts the pituitary gland to release ACTH;
• ACTH tells the adrenal glands to make cortisol.

Cortisol then feeds back to the brain to switch the response off again. This short-term rise is normal and useful. Problems are more likely when stress goes on for weeks or months with little chance to recover – see Symptoms of stress.

Why does cortisol change throughout the day?

Cortisol follows a strong daily (circadian) rhythm. It rises sharply in roughly the first 30 to 45 minutes after you wake – the cortisol awakening response – which helps you get going in the morning. It then falls gradually through the day and is at its lowest around midnight, when you’re normally asleep.

On top of that rhythm come short-term spikes from exercise, a stressful event, a heavy meal, caffeine or a bad night’s sleep. Shift work and jet lag can shift the whole pattern.

How can you tell if cortisol is high?

Ordinary stress raises cortisol temporarily, but that isn’t the same as having “high cortisol” as a medical problem. Too much cortisol over a long time is called Cushing’s syndrome. It’s rare, and the most common cause is taking high doses of steroid medicines, such as prednisolone, for a long time. Sometimes it’s caused by a small, non-cancerous growth in the pituitary or adrenal glands.

According to the NHS, signs of Cushing’s syndrome include:

• more fat on the neck, upper back, chest and tummy, with thinner arms and legs
• a red, round face
• skin that bruises easily, and stretch marks on the tummy, hips, breasts and under the arms
• muscle weakness, especially at the top of the arms and legs
• low sex drive, and irregular periods or periods stopping
• changes in mood, such as feeling irritable, anxious or depressed
• excessive hair growth (hirsutism)

What are 10 warning signs of high cortisol?

Lists of “10 signs of high cortisol” on social media usually mix the genuine signs of Cushing’s syndrome with everyday symptoms – tiredness, weight gain, poor sleep, cravings, anxiety – that have many possible causes. Those everyday symptoms don’t mean your cortisol is high, and they can’t be fixed by targeting a hormone.

If you feel persistently tired, stressed or unwell, that’s a good reason to see your GP – who can look at the whole picture and decide whether any tests are needed.

What are the symptoms of low cortisol?

Low cortisol happens when the adrenal glands don’t make enough of it. The best-known cause is Addison’s disease, a rare condition. It can also happen after stopping long-term steroid treatment, or with problems in the pituitary gland. The NHS lists symptoms of Addison’s disease including:

• extreme tiredness and muscle weakness
• loss of appetite and weight loss
• feeling dizzy or light-headed when standing up
• strong cravings for salty food
• feeling or being sick, or tummy pain
• areas of skin that are darker than the surrounding skin
• joint pain, muscle cramps and headaches

Why does one cortisol test say so little?

Because cortisol changes so much over the day and with stress, a single blood or saliva result on its own tells you very little. That’s why doctors use specific tests depending on what they’re looking for – for example, a late-night saliva sample, a 24-hour urine collection or a suppression test for suspected Cushing’s, or a stimulation test for suspected Addison’s – and interpret them alongside your symptoms.

How do I check my cortisol level at home?

Home saliva and finger-prick kits are widely sold. They can measure cortisol at the moment you take the sample, but they can’t tell you whether you have a medical problem, and a “high” result after a stressful morning is entirely normal. If you’re worried, it’s better to see your GP, who can arrange the right test if it’s needed.

What are the signs of high cortisol at night?

Cortisol should be at its lowest around midnight. In Cushing’s syndrome, that night-time dip is lost – which is why a late-night saliva test is used to investigate it. But trouble getting to sleep, waking at 3am or feeling “wired but tired” are very common and usually down to stress, worry, caffeine, alcohol, screens or an irregular routine rather than a hormone problem.

How can you support a normal stress response?

You can’t – and shouldn’t – try to “lower” a hormone your body needs. What you can do is give your stress response the conditions to work normally. The best-supported steps are:

• Sleep: aim for regular bed and wake times, even at weekends.
• Regular meals: skipping meals and long gaps can add to the body’s stress load.
• Moderate exercise: the NHS recommends at least 150 minutes of moderate activity a week. Very intense training without enough recovery can raise cortisol.
• Caffeine and alcohol: cut back, particularly later in the day. Alcohol disrupts sleep, even if it helps you nod off.
• Breathing and relaxation: slow breathing exercises, such as those on the NHS website, can help you calm down in the moment.
• Daylight: getting outside in the morning helps set your body clock.
• Connection: talking to friends, family or a professional helps more than many people expect.

How do you reduce cortisol, according to the NHS?

The NHS doesn’t give advice on “reducing cortisol” as such. Its advice is about managing stress: talk to someone, try exercise and time for yourself, use simple time-management and breathing techniques, plan ahead for difficult days, set small achievable goals and avoid using alcohol, smoking or gambling to cope. If you’re struggling, the NHS advises seeing a GP.

What five foods lower cortisol?

No food has been shown to lower cortisol in a meaningful way, despite many lists online. What helps is a balanced diet with regular meals – plenty of fruit, vegetables, wholegrains, pulses, nuts and oily fish, as set out in the Eatwell Guide – and keeping caffeine, alcohol and very sugary foods in check.

Is cortisol belly real?

Not in the way it’s presented online. In Cushing’s syndrome, very high cortisol does lead to fat building up around the tummy, face and upper back – but that’s a medical condition with other clear signs. For most people, fat around the middle is down to the usual factors: diet, activity, sleep, age, genes and, for women, the hormonal changes of menopause.

“Cortisol cocktails” – drinks of juice, coconut water and salt that trend on social media – have not been shown to change cortisol levels or body fat. There is no product or drink that can target belly fat by changing your hormones.