Cortisol is a steroid hormone produced in the adrenal glands. It is the body’s primary glucocorticoid. It is most commonly associated with the stress response where it rises to trigger gluconeogenesis (the production of glucose) lipolysis (the liberation of fatty acids from glycerol), and glycogenolysis (the breakdown of glycogen to produce glucose), essentially making energy more freely available. This is important for when a threat or an urgency requires a quick burst of energy. In conjunction with other hormones, it participates in the formation of memories associated with stress and trauma.
It’s also involved in circadian rhythms and the regulation of diurnal energy levels, generally increasing upon waking, and decreasing throughout the rest of the day to (hopefully) be lowest at bedtime. High evening cortisol is associated with insomnia, and a flattening of the normal daily curve is correlated with a wide range of health issues.
Cortisol has an anti-inflammatory effect and is used as a topical anti-inflammatory drug (hydrocortisone). While many health issues derive from inflammation, chronic suppression can reduce the efficacy of the immune system, and this may be an explanation for the link between chronic stress and susceptibility to getting sick.
Most of the body’s cortisol is bound to corticosteroid-binding globulins, which prevent binding to glucocorticoid receptors, and cortisol must be freed before it. So much like with testosterone, what’s most relevant is the amount of unbound cortisol. Cortisol also has a less active form: cortisone. Cortisol testing tends to be of serum or salivary cortisol, though these are only reflective of transient and highly variable effects. Thus, testing on multiple days, continuous monitoring, or other tests such as hair cortisol may be more indicative of longer-term levels.