What is cortisol?

Cortisol is a steroid hormone, made from cholesterol. Like all hormones it is a chemical messenger that travels in the blood, carrying signals through the body. By hitching a ride in the blood, cortisol molecules quickly spread out through the body, carrying their message to cells in many different organs and tissues.

Nearly every cell in the body has receptors that can detect cortisol. But different types of cell react differently to its message.

Cortisol is made and released by the adrenal glands. There are two adrenal glands, one on top of each kidney. The adrenal glands secrete a variety of different hormones into the blood, of which cortisol is just one. 

What does cortisol do?

Cortisol has many roles including helping us get ready to wake up in the morning, and getting us ready to respond to stress.

Like most hormones, cortisol’s signal affects many different organs and body tissues. Different cell types respond to its signal in different ways, with these different responses working together to create cortisol’s overall effect on the body.

One important role of cortisol is to help keep the body environment stable (this process is known as homeostasis). It does this by helping to regulate blood sugar levels, salt and water balance, and blood pressure.

Cortisol also acts as an anti-inflammatory, affects memory, and helps development of the foetus during pregnancy.

But cortisol is perhaps most famous for its role in the stress response. When we feel scared or in danger, the hormone adrenalin is released (this gets our hearts pumping), and so are large amounts of cortisol. These high levels of cortisol help get our bodies ready for a ‘fight or flight’ response. For example, they increase our blood sugar levels to provide extra energy for our muscles.

Daily rhythms

Our cortisol levels have a daily rhythm. For most people, levels of cortisol are highest in the morning, just before we wake up. They then fall during the day and are lowest at night while we sleep.

But cortisol’s daily rhythm isn’t connected to day and night – instead it is connected to our personal pattern of activity. This means it’s not the same for everyone. Night-shift workers who sleep in the daytime have a different rhythm: their cortisol may be high in the late afternoon (when they wake up), fall during the night (while they work), and be lowest in the daytime (while they sleep).

When people travel to a different time zone it can take a while for their body’s cortisol rhythm to adjust. This is one of the causes of jet lag. 

Turn it up! Stress response

As well as this regular rhythm, extra cortisol can be released in response to physical or emotional stress. Whether it’s a scary exam or an angry bear, your body reacts in a similar way. This stress response is controlled by part of the brain called the hypothalamus. The hypothalamus is a long way from the adrenal glands, but it is able to contact them through a chain of hormonal messaging.

First, the hypothalamus releases a hormone called corticotrophin-releasing hormone (CRH) into a blood vessel that takes it straight to the pituitary gland. The pituitary gland, tucked just under the hypothalamus, is the main communications link between the brain and endocrine system (the body’s hormonal communications network).

The pituitary gland detects the CRH from the hypothalamus, and in response it releases adrenocorticotrophic hormone (ACTH) into the bloodstream. Once in the bloodstream, ACTH can travel anywhere in the body – including down to the adrenal glands.

The adrenal glands have receptors for ACTH. When they pick up its signal, they release cortisol into the blood.

This sudden rise in cortisol levels helps boost our ‘fight or flight’ response. The body reacts by getting us and our muscles ready for action:

• The liver releases glucose into the blood, providing extra energy for muscles
• Breathing increases, providing extra oxygen for muscles and removing carbon dioxide
• Heart rate increases, speeding up the delivery of resources and removal of waste
• Blood pressure increases, with more blood directed to the muscles
• Sweating increases, ready to cool off the heat produced by hard-working muscles

Our sight and hearing become sharper to help us focus on the threat. And digestion slows down as the body focuses its resources on things that might be useful in a fight.

This stress response is really helpful if you are facing a bear, but is not helpful in an exam situation. If you ever feel symptoms like these around exams, try checking out this useful list of tips for coping with exam stress  from the UK charity Mind.

Turn it down! Negative feedback loop

Cortisol has the neat trick of being able to stop its own production.

It does this by use of a negative feedback loop. A negative feedback loop happens when the outcome of a process acts to slow down or stop that same process from happening.

As explained above, in times of stress the hypothalamus tells the pituitary gland to tell the adrenal glands to release cortisol into the blood. The outcome of this chain of messaging is that cortisol levels in the blood rise.

This outcome (higher levels of cortisol in the blood) is detected by the hypothalamus and the pituitary gland. In response, they slow down their hormonal signalling. Which means the adrenal glands then slow down the production of cortisol.

All of this means that the cortisol released during a stress response does not only prepare the body for action, but also signals to the brain that it has been successfully released and that its levels can now be reduced. This is important because having high levels of cortisol for too long can be damaging to the body.

Feedback loops like this are really common in biology. For a more detailed explanation of how they work, and more examples, see our article What is a Feedback Loop .

What happens if you have too much cortisol

If cortisol levels are too high for too long, people can develop a condition called Cushing’s Syndrome.

This can happen due to problems at any stage of the cortisol control pathway. The problem might be in the hypothalamus, pituitary gland, or adrenal gland. For example, a pituitary tumour might release too much CRH, leading to the adrenal glands releasing too much cortisol. Or an adrenal tumour might release too much cortisol directly.

Treatment for high cortisol levels will depend on the cause. If there is a tumour, it might be removed, or destroyed using radiotherapy. In some cases, treatment may also include using drugs to reduce the effect of cortisol on the body.

Because cortisol has many roles in the body, Cushing’s Syndrome affects the body in a wide variety of ways. This usually includes:

• Rapid weight gain in the upper body and face
• Muscle loss, leading to muscle weakness and thin arms and legs
• Weakened skin and bones, causing bruises, stretch marks, aches, and/or osteoporosis
• High blood pressure, high blood sugar, and a flushed face
• Feeling thirsty and producing a lot of urine (going to the toilet a lot)
• Feeling tired, and experiencing mood changes (irritability, depression, and/or anxiety)

Similar problems can also be caused by taking too much steroid medications.

The connection between cortisol and mental health problems is still being explored – the link is not at all well understood. Finding out more about why these symptoms happen might make it easier to work out how they can best be treated.  

If high cortisol levels continue for a long time, this can affect or stop the menstrual cycle (periods). It can also lead to low sex drive.

What happens if you have too little cortisol?

Problems with the hypothalamus, pituitary gland and adrenal glands can also lead to too little cortisol being released.

The most common cause of too-little cortisol is a condition called Addison’s Disease. This is usually caused by the body’s own immune system attacking the adrenal glands, damaging them so they stop working properly.

Addison’s disease can be treated with a hormone replacement treatment, but replacing cortisol is a challenge because its natural levels change so much during the day. People also need to take extra cortisol if they are ill or under increased stress. The adrenal glands do not recover, therefore these treatments need to be taken regularly for the rest of the person’s life.

People with too little cortisol usually have:

• Weight loss (the opposite of Cushing’s Syndrome)
• Muscle loss and weakness, similar to that seen in Cushing’s syndrome
• Low blood pressure and low blood sugar (the opposite of Cushing’s Syndrome)
• Feeling tired, with a lack of energy (in medicine this is called fatigue)

If the problem continues for a long time, people may also experience dizziness, fainting and cramps. They may also find that areas of their skin, lips or gums become dark in colour. 

More about fight or flight

To find out more about the body’s Fight or Flight response and how cortisol works alongside the hormone adrenaline, see our article about adrenaline.