Cortisol is a steroid hormone made by the adrenal glands, two small glands that sit above the kidneys. It is best known as the body’s “stress hormone,” but that description is incomplete. Cortisol is essential for everyday life, not just for stressful situations.
Your body uses cortisol to help regulate blood sugar, blood pressure, metabolism, immune activity, and the sleep-wake cycle. It also helps the body respond appropriately when physical or psychological demands increase. The problem is not cortisol itself. Problems arise when cortisol production is too high or too low for too long.
Understanding what cortisol does requires looking at both its normal daily role and its response to stress.
What is cortisol?
Cortisol is a glucocorticoid, a type of steroid hormone that influences how the body uses energy and responds to changes in its environment. It is produced in the adrenal cortex, the outer portion of the adrenal glands.
Cortisol production is controlled by the hypothalamic-pituitary-adrenal (HPA) axis. This system connects parts of the brain with the pituitary gland and adrenal glands.
The process begins in the hypothalamus, which releases corticotropin-releasing hormone (CRH). CRH signals the pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH then travels through the bloodstream to the adrenal glands, where it stimulates cortisol production.
As cortisol levels rise, they feed back to the brain and pituitary gland, reducing the signals that stimulate further cortisol production. This negative-feedback system helps keep cortisol within an appropriate range.
Cortisol helps the body manage energy
One of cortisol’s most important jobs is making energy available when the body needs it.
Cortisol promotes the production of glucose, the body’s main blood sugar, particularly during fasting or increased demand. It helps the liver make glucose from substances such as amino acids and other metabolic compounds, a process called gluconeogenesis.
Cortisol also changes how the body handles fats and proteins. It can increase the availability of fatty acids from fat tissue and influence protein breakdown, providing raw materials that can be used for energy or glucose production.
These effects are useful during stress, illness, fasting, or other situations in which maintaining an adequate blood supply of glucose is important. Cortisol works alongside hormones such as adrenaline, glucagon, and insulin to coordinate the body’s energy supply.
Cortisol is part of the stress response
When you encounter a significant physical or psychological stressor, the body activates several systems at once. The sympathetic nervous system produces a rapid response involving chemicals such as adrenaline, while the HPA axis produces a slower hormonal response involving cortisol.
Cortisol helps sustain the body’s response after the initial surge of adrenaline. It increases the availability of fuel and helps maintain blood pressure and cardiovascular function. It also modifies immune and inflammatory activity, which can help prevent an excessive inflammatory response during acute stress.
This response is adaptive when it is temporary. Once the stressor passes, cortisol production normally decreases through the HPA axis’s feedback system.
Importantly, cortisol does not simply rise whenever you feel worried. Cortisol secretion is influenced by many factors, including physical illness, injury, sleep, activity, food intake, and the body’s internal clock.
Cortisol affects the immune system and inflammation
Cortisol has powerful effects on immune function. In general, it suppresses or modifies aspects of inflammation and immune activity.
This is one reason synthetic glucocorticoids, such as prednisone, are widely used as anti-inflammatory and immunosuppressive medications. They act on many of the same cellular pathways influenced by naturally produced cortisol, although their effects and clinical uses vary.
Cortisol’s immune effects are not simply “good” or “bad.” A controlled inflammatory response is essential for fighting infections and repairing tissue, but excessive or prolonged inflammation can damage healthy tissue. Cortisol helps regulate that balance.
Long-term exposure to unusually high levels of glucocorticoids, however, can impair aspects of immune function and produce other effects throughout the body.
Cortisol helps regulate blood pressure and circulation
Cortisol contributes to normal cardiovascular function. Among other effects, it helps blood vessels respond appropriately to signals that constrict them, supporting normal blood pressure.
When cortisol production is severely reduced, blood pressure can fall, sometimes dangerously. This is one reason adequate adrenal function is essential for maintaining circulation, particularly during physical stress such as infection or injury.
Cortisol therefore plays a role that extends beyond the classic “fight-or-flight” response: it helps the cardiovascular system function normally even under ordinary conditions.
Cortisol follows a daily rhythm
Cortisol is not normally released at a constant rate throughout the day. Its production follows a circadian rhythm controlled by the body’s internal biological clock.
For most people, cortisol levels are relatively high around the time of waking and then decline over the course of the day, reaching their lowest levels around the usual sleep period. The precise pattern varies among individuals and can be altered by sleep schedules, shift work, illness, and other factors.
This daily rhythm is one reason that a single cortisol measurement cannot always be interpreted in isolation. The time of day, the type of test, medications, and the clinical situation can all matter.
Cortisol also rises in response to particular challenges, temporarily overriding the ordinary daily pattern.
Cortisol interacts with sleep and the body clock
The relationship between cortisol and sleep is closely tied to circadian biology. Cortisol normally increases toward morning, helping promote alertness and prepare the body for daytime activity.
Cortisol is only one part of this system. Other signals, including melatonin, also follow daily rhythms and help coordinate sleep and wakefulness.
Disrupted sleep can affect cortisol regulation, while disturbances in the body’s circadian system can alter the normal timing of cortisol secretion. This does not mean that every night of poor sleep causes a harmful cortisol imbalance; short-term fluctuations are a normal part of physiology.
Cortisol influences metabolism throughout the body
Cortisol affects several aspects of metabolism, including carbohydrate, fat, and protein metabolism.
In the short term, these effects help ensure that energy is available when demand increases. With prolonged exposure to excessive cortisol, however, the same metabolic actions can become harmful. Persistently elevated glucocorticoid activity can contribute to increased blood glucose, changes in fat distribution, muscle loss, and other metabolic effects.
Cortisol also interacts with insulin. Cortisol tends to oppose some of insulin’s actions, particularly its effects on glucose availability. This is useful during stress because the body needs to keep enough glucose available for tissues that depend heavily on it. Chronic excessive cortisol, however, can make blood sugar regulation more difficult.
What happens when cortisol is too high?
Cortisol levels can be temporarily elevated for normal reasons, including acute stress and illness. That is different from having excessive cortisol production over an extended period.
Persistently excessive cortisol can occur in conditions such as Cushing syndrome. Depending on the cause and severity, prolonged cortisol excess can lead to changes such as increased blood pressure and blood glucose, muscle weakness, easy bruising, changes in body-fat distribution, and thinning of the skin. It can also affect bone health, mood, and other body systems.
Taking glucocorticoid medications for a prolonged period can also produce effects similar to excess cortisol because these drugs act through related hormonal pathways.
It is therefore misleading to treat “high cortisol” as a single diagnosis. Cortisol naturally fluctuates, and symptoms alone cannot establish that cortisol is abnormally elevated.
What happens when cortisol is too low?
The opposite problem is inadequate cortisol production. The major condition associated with this is adrenal insufficiency, in which the adrenal glands do not produce enough cortisol.
Low cortisol can cause symptoms such as fatigue, weakness, low blood pressure, reduced appetite, weight loss, nausea, and abdominal discomfort. Some forms of adrenal insufficiency can also cause changes in skin pigmentation or abnormalities in other adrenal hormones.
A severe deficiency can become an adrenal crisis, a medical emergency characterized by profound weakness, low blood pressure, and potentially shock. The body may be unable to mount an adequate cortisol response to a major physical stressor such as infection, injury, or surgery.
Because cortisol is necessary for maintaining blood pressure, blood glucose, and the body’s response to stress, severe deficiency can be life-threatening.
Is stress always caused by “high cortisol”?
No. The relationship between stress and cortisol is more complicated than that.
Cortisol is one component of a coordinated stress response, and its levels change according to the type and timing of the stressor. A person can experience stress without having a persistently abnormal cortisol level.
Likewise, symptoms often attributed to “high cortisol”—such as tiredness, difficulty sleeping, or changes in weight—have many possible causes. These symptoms alone cannot tell you whether cortisol is too high, too low, or normal.
This distinction matters because commercial tests and wellness claims sometimes present cortisol as though it were a simple gauge of how stressed a person is. In reality, cortisol follows a complex pattern that depends on time of day, physiology, health status, medications, and the specific test being used.
How is cortisol measured?
Cortisol can be measured in blood, saliva, or urine, depending on what a clinician is investigating.
The appropriate test depends on the suspected problem. For example, evaluating possible cortisol excess may involve measurements designed to determine whether the body’s normal feedback system can suppress cortisol production, while evaluation of suspected adrenal insufficiency may focus on whether the adrenal glands can produce an adequate response to stimulation.
Because cortisol changes throughout the day, the timing and circumstances of testing are important. A result that is abnormal in one context may not mean the same thing in another.
A cortisol test should therefore be interpreted in relation to the reason it was ordered rather than as a general measure of health or stress.
Cortisol is essential—but balance matters
Cortisol is often portrayed as a hormone that should be minimized. That is not how human physiology works. Cortisol is necessary for normal metabolism, cardiovascular function, immune regulation, the stress response, and daily biological rhythms.
The healthy goal is not to eliminate cortisol or keep it as low as possible. It is to maintain appropriate cortisol production at the appropriate time and to preserve the body’s ability to increase or decrease production when circumstances change.
Both excessive and insufficient cortisol can interfere with normal body function. The key is regulation: the adrenal glands, brain, pituitary gland, and feedback mechanisms work together to adjust cortisol production to the body’s changing needs.
