Cortisol is secreted by the adrenal glands, two small glands that sit on top of each kidney. More specifically, cortisol comes from the middle layer of the adrenal cortex, called the zona fasciculata. But the adrenal glands don’t act alone. They’re the final step in a chain of signals that starts in the brain, and cortisol itself plays a surprisingly wide role in everything from blood sugar to immune function.
Where Cortisol Is Made
Each adrenal gland has two main parts: an outer cortex and an inner medulla. The cortex is further divided into three layers, each producing different hormones. The outermost layer makes hormones that regulate sodium and potassium. The innermost layer produces certain sex hormones. The middle layer, the zona fasciculata, is where cortisol is made.
The raw material for cortisol is cholesterol. Your adrenal cells pull cholesterol from the bloodstream and shuttle it into their mitochondria, where a series of enzymes transform it step by step into cortisol. That transfer of cholesterol into the mitochondria is actually the bottleneck of the entire process. A specialized protein controls how much cholesterol gets through, which effectively sets the pace of cortisol production.
How Your Brain Controls Cortisol Release
Your adrenal glands don’t decide on their own when to release cortisol. They take orders from a signaling chain that starts in the hypothalamus, a small region at the base of the brain. When your brain detects stress or senses that cortisol levels are low, specialized neurons in the hypothalamus release a signaling hormone into a network of tiny blood vessels leading to the pituitary gland, which sits just below it.
The pituitary responds by releasing ACTH (adrenocorticotropic hormone) into the bloodstream. ACTH travels to the adrenal glands and triggers the zona fasciculata to produce and release cortisol. This three-step relay, from hypothalamus to pituitary to adrenal glands, is known as the HPA axis. It also has a built-in off switch: when cortisol levels in the blood rise high enough, both the hypothalamus and pituitary dial back their signaling, preventing overproduction.
Cortisol Follows a Daily Rhythm
Cortisol levels aren’t constant throughout the day. They follow a predictable pattern tied to your sleep-wake cycle. Levels are lowest around midnight and begin climbing in the early morning hours. Within 30 to 60 minutes of waking up, cortisol surges by 50% or more. This spike is called the cortisol awakening response, and it helps prepare your body and brain for the day ahead.
From that morning peak, cortisol gradually declines through the afternoon and evening. Standard blood test reference ranges reflect this pattern: morning levels (measured between 6 and 8 a.m.) typically fall between 10 and 20 mcg/dL, while afternoon levels (around 4 p.m.) drop to 3 to 10 mcg/dL. A cortisol level that would be perfectly normal at 7 a.m. could signal a problem if it showed up at 11 p.m.
What Cortisol Does in the Body
Cortisol is often called the “stress hormone,” but that label undersells it. Cortisol is involved in metabolism, immune regulation, and energy balance every hour of every day, not just during stressful moments.
Blood Sugar and Energy
One of cortisol’s primary jobs is keeping blood sugar available when your body needs fuel. It does this by ramping up glucose production in the liver while simultaneously reducing how much glucose your muscles and fat tissue absorb. It also amplifies the effects of other hormones that raise blood sugar, like glucagon and adrenaline. The net result is more circulating glucose, ready for quick use.
Protein and Fat
Cortisol breaks down muscle protein to free up amino acids, which the liver can then convert into glucose. At the same time, it promotes fat breakdown for energy. When cortisol stays elevated for long periods, though, fat distribution shifts. Subcutaneous fat (the kind just under your skin) decreases, while visceral fat (the deeper fat around your organs) increases. This is why chronic cortisol excess often produces a characteristic pattern of central weight gain.
Immune Function
At normal levels, cortisol helps keep the immune system balanced. It dials down the production of inflammatory signaling molecules and limits the activity of both first-line immune cells (like those that detect and engulf pathogens) and longer-term immune cells (like T and B cells that target specific threats). This is useful for preventing inflammation from spiraling out of control. It’s also why synthetic versions of cortisol are used as anti-inflammatory medications. But when cortisol stays high for too long, this immune suppression becomes a liability, leaving the body more vulnerable to infections.
Cortisol During Acute and Chronic Stress
Short-term stress, like a job interview or a near-miss in traffic, triggers a rapid cortisol spike that resolves within hours. This is the system working as designed: a burst of energy, heightened alertness, then a return to baseline. The morning cortisol awakening response can also spike higher than usual in anticipation of a stressful event, like an exam later that day.
Chronic stress tells a different story. Prolonged periods of pressure, whether from work, illness, or life circumstances, keep cortisol elevated or disrupt its normal daily rhythm. Research on medical students preparing for national exams found significant cortisol elevations during both the weeks of preparation (chronic stress) and the exams themselves (acute stress), though the patterns differed. The awakening response reflected short-term anticipatory stress more than the long-term grind of studying.
What Happens When Cortisol Levels Go Wrong
When the adrenal glands produce too much cortisol over an extended period, the result is Cushing syndrome. Common signs include a round, fuller face, a fat pad on the upper back, thinning skin that bruises easily, and purple stretch marks. The cause is sometimes a tumor on the pituitary gland that overproduces ACTH, sometimes a tumor on the adrenal gland itself, and sometimes long-term use of corticosteroid medications. Diagnosis typically involves 24-hour urine collection, blood tests for cortisol and ACTH, and late-night saliva tests. Because cortisol normally drops in the evening, a saliva sample that remains high at night is a red flag.
The opposite problem, too little cortisol, occurs in Addison’s disease (primary adrenal insufficiency). This happens when the adrenal cortex is damaged, often by the body’s own immune system attacking it. Symptoms include severe fatigue, weight loss, low blood pressure, and darkening of the skin. Without enough cortisol, the body struggles to maintain blood sugar, manage stress, and regulate blood pressure, which can become life-threatening during illness or injury if untreated.