Updated: Jan 2, 2019
The hormone cortisol is actually one of several hormones that are grouped under the heading of corticosteroids but I will refer to them all as cortisol for simplicity.
Cortisol is released from the adrenal gland, which releases many other hormones such as adrenaline, the adrenal androgens such as DHEA and the mineralocorticoids, hormones involved in water balance, such as aldosterone.
As with many hormones, the signal to release cortisol comes from the hypothalamus, which via release of Corticotrophin Releasing Hormone (CRH) stimulates the pituitary gland to release adrenocorticotrophic hormone (ACTC). This signals the adrenal gland to release cortisol.
The general trigger for this process to occur is stress.
Cortisol has broad reaching effects in the body and, due to many of its effects, is often thought of as being a bad hormone. This is prevalent in both the athletic community as well as the general public due to endless advertisements claiming that it increases belly fat. But just as the effects of stress can be positive or negative, the effects of cortisol (which really mediate the effects of stress) can also be positive or negative. Simply, acute pulses of cortisol (caused by short term stress) tend to be beneficial while chronically elevated cortisol (caused by long term stress) is not.
In the short-term for example, cortisol works to generally mobilise energy. Fatty acids are released from fat cells, glucose is produced in the liver and even protein breakdown is stimulated. Even this latter effect is adaptive, helping muscle to rebuild and remodel itself in response to that stress. Acutely, cortisol does impair bone growth but this is primarily a way to conserve energy (and when the stress ends, the bone can rebuild). Cortisol pulses helps to form memories, which is why people tend to remember acutely stressful events and acts as an anti-inflammatory, explaining its use to treat injuries.
Perhaps confusingly, cortisol acutely impairs immune system function but this to makes sense: the immune system uses a stunning amount of energy and inhibiting it in the short-term helps to provide energy for whatever stressor is being dealt with. Cortisol also impairs the function of the reproductive system; in the short-term this makes perfect sense for the same reasons menstrual cycle dysfunction makes perfect sense when insufficient energy is available. But in the aggregate, all of these effects are extremely beneficial for the body and are part of what helps it to adapt to stress over time.
In contrast, when cortisol is elevated chronically, its effects become distinctly negative. Cortisol causes insulin resistance and leptin resistance in the brain, both of which can have negative effects on body weight and body fat regulation.
If insulin levels are high when cortisol is chronically elevated, visceral fat storage can be stimulated (hence the claim that cortisol leads to belly fat). With chronic stress, cortisol increases protein breakdown but without the recovery period, tissue never get a chance to repair themselves. In the long-term this causes muscle loss and other bodily damage. Bone production is inhibited continuously and eventually this will cause the loss of bone mineral density (BMD). Memory is impaired and immune system, reproductive function and sex drive are all inhibited as well.
The primary take home of this article is this: acute pulses of cortisol are generally beneficial and important to help the body adapt to stress over time while chronically elevated cortisol levels are bad. As well, while chronically elevated cortisol is extremely damaging to the body, having too little produced is equally bad. At the extremes this shows up in Addison's disease (where the body can't mount a stress response at all) or Cushing's disease (where cortisol is chronically elevated). People with Addison's may pass out simply standing up as their bodies can't raise blood pressure adequately and other stresses simply can't be handled. Cushing's patients are in a state of chronically elevated cortisol with all that implies. While disease states, there are situations that occur in response to stress that share similarities with them.