What is cortisol?

In one line

Cortisol is a hormone made by the adrenal glands above the kidneys that helps the body cope with stress by controlling blood sugar, blood pressure and inflammation.

What is cortisol?
The word people call the 'stress hormone'
At the gym, people call cortisol the 'stress hormone' and often talk about it as a bad hormone. What is it really, and what does it do in the body?
In one line
A hormone that helps the body cope with stress
Cortisol is a hormone, a substance that carries signals inside the body. When the body is under stress, more of it is released, and it adjusts many body functions so the body can hold up. Stress here means not only mental tension but also physical strain such as an injury or shivering in the cold.
What a hormone is
A chemical signal that travels in the blood
A hormone is a substance made in one part of the body that travels around the body in the blood and tells a distant organ what to do. It works more slowly than a nerve signal, but its effect lasts longer. Also, having too much or too little of a hormone causes problems.
Where it is made
The adrenal glands, small organs above the kidneys
Cortisol is made in the outer layer of the 'adrenal glands', small organs that sit one on top of each kidney. How much is made is set by the 'pituitary gland', a pea-sized organ hanging under the brain, which sends a signal hormone. When there is enough cortisol, this signal drops.
What it does
It controls blood sugar, blood pressure and inflammation
Cortisol affects almost every organ in the body. It raises 'blood sugar', the amount of sugar in the blood, keeps blood pressure up, and calms 'inflammation', the swelling and heat around an injury. That is why drugs modeled on cortisol are used to calm inflammation.
How it frees up fuel
It breaks down stored energy
When stress lasts a long time, cortisol makes the body pull out its stored fuel. It breaks apart sugar stored in the liver and muscles, fat, and even muscle protein. This splitting apart is called 'breakdown', and it is why cortisol comes up in talk about muscle.
Daily rhythm
Highest in the morning, lowest around midnight
Cortisol rises and falls in a steady daily rhythm. It is highest in the morning, drops through the afternoon, and is lowest around midnight. That is why clinics usually take the blood sample early in the morning.
Exercise and cortisol
The harder the exercise, the more it rises
Exercise is a stress for the body, so cortisol rises. In a study that changed only the intensity of 30 minutes of nonstop exercise, easy exercise barely raised cortisol. The harder the exercise, the more it rose, by more than 80% at the hardest level. Cortisol also rises right after strength training.
Too much or too little
Both lead to illness
An illness where cortisol stays too high for a long time is called 'Cushing's syndrome'. People gain weight, the face gets round, the arms and legs get thin and the muscles get weak. In 'Addison's disease', where the adrenal glands cannot make enough cortisol, people feel tired all the time, have weak muscles and lose weight.
Common myth
Is less cortisol always better?
No. Cortisol is a hormone you need to stay alive. People whose adrenal glands cannot make it, as in Addison's disease, usually replace it with medicine for life. A sudden severe shortage drops blood pressure and clouds the mind, an emergency. Problems come when there is too little, or too much for a long time.
Summary
An essential hormone for coping with stress
Cortisol is a hormone made by the adrenal glands that helps the body cope with stress by controlling blood sugar, blood pressure and inflammation. It is not a bad hormone but an essential one, and problems come when there is too little or too much for a long time. Related words: adrenal glands, Cushing's syndrome, growth hormone.