Cortisol is a steroid hormone your adrenal glands produce to help regulate blood sugar, metabolism, inflammation, and your body’s response to stress. It’s often called the “stress hormone,” but that label undersells it. Cortisol is involved in dozens of processes that keep you alive and functioning every day, from the moment you wake up to how your body fights infection.
Where Cortisol Comes From
You have two adrenal glands, one sitting on top of each kidney. The outer layer of each gland, called the adrenal cortex, contains a specific zone that produces cortisol. Your body makes it from cholesterol, which is one reason cholesterol isn’t purely a villain in human biology.
Cortisol production is controlled by a communication chain between three parts of your brain and body: the hypothalamus (a small region at the base of your brain), the pituitary gland (just below it), and the adrenal glands. When your brain detects stress, your hypothalamus sends a chemical signal to the pituitary, which sends its own signal to the adrenal glands, which then release cortisol into your bloodstream. Once cortisol levels rise high enough, the hypothalamus detects this and stops sending the initial signal, shutting the whole chain down. This feedback loop is how your body keeps cortisol from spiraling out of control under normal circumstances.
How Cortisol Levels Change Throughout the Day
Cortisol follows a predictable daily rhythm. Levels are highest in the early morning, typically between 10 and 20 micrograms per deciliter (mcg/dL) when measured between 6 a.m. and 8 a.m. By late afternoon, around 4 p.m., they drop to roughly 3 to 10 mcg/dL. By midnight, cortisol hits its lowest point.
There’s also a distinct surge right after you wake up. Within 30 minutes of opening your eyes, cortisol spikes by about 38 to 75% above whatever your waking level was. This is called the cortisol awakening response, and it essentially primes your body for the day, boosting alertness, energy, and blood sugar availability. It’s one reason you can go from groggy to functional within half an hour of getting out of bed.
What Cortisol Does for Your Metabolism
One of cortisol’s most important jobs is making sure your brain has enough glucose. Your brain burns through glucose faster than any other organ, and cortisol ensures there’s always a supply, especially during stress when energy demands spike.
It does this primarily by telling your liver to produce new glucose from non-sugar sources. Cortisol triggers the breakdown of proteins in your muscles into amino acids and the breakdown of fats in your fat tissue into glycerol, then funnels both of those raw materials to the liver, where they’re converted into glucose and released into your bloodstream. Cortisol also counteracts insulin, the hormone that normally pulls glucose out of your blood and into cells. By partially blocking insulin’s effects, cortisol keeps more glucose circulating and available for your brain.
This system works well in short bursts. During a stressful event, you get a temporary energy boost. The problem comes when cortisol stays elevated for weeks or months. Chronic exposure leads to persistently high blood sugar, insulin resistance, and eventually the metabolic profile that sets the stage for type 2 diabetes.
How Cortisol Controls Inflammation
Cortisol is your body’s most powerful natural anti-inflammatory compound. When you’re injured or fighting an infection, your immune system launches an inflammatory response: swelling, redness, heat, pain. That response is necessary, but it can cause serious damage if it runs unchecked. Cortisol dials it down by suppressing the chemical messengers that drive inflammation and reducing the activity of immune cells.
This is why synthetic versions of cortisol (corticosteroids like prednisone and hydrocortisone cream) are so widely used in medicine. They exploit cortisol’s natural ability to calm overactive immune responses. Your own cortisol does the same thing on a smaller, more targeted scale every day. It helps resolve the minor inflammation from a tough workout, a paper cut, or a mild cold without you ever noticing.
Your body also has enzymes in different tissues that fine-tune how much active cortisol is available locally. Some tissues convert inactive forms of cortisol into active ones, ramping up the anti-inflammatory effect. Others do the reverse, deactivating cortisol to let the immune response proceed. This local regulation means cortisol’s effects aren’t uniform across your whole body at any given moment.
What Happens When Cortisol Stays Too High
Chronically elevated cortisol, whether from prolonged stress, certain tumors, or long-term use of corticosteroid medications, causes a recognizable pattern of changes throughout the body. The most extreme version is Cushing syndrome, which produces a distinctive set of physical signs:
- Weight redistribution: fat accumulates in the face (giving it a rounded, “moon” appearance), at the back of the neck, and between the shoulders, while the arms and legs become thinner as muscle breaks down
- Skin changes: easy bruising, slow wound healing, and purple or pink stretch marks across the belly, breasts, and hips
- Metabolic disruption: high blood pressure, high cholesterol, and prediabetes or type 2 diabetes
- Bone and muscle loss: weak bones prone to fractures and progressive muscle weakness
- Mood and energy changes: fatigue, mood swings, and reduced sex drive
Left untreated, Cushing syndrome can be fatal, primarily through cardiovascular complications and uncontrolled diabetes. But even cortisol levels that are chronically elevated without reaching Cushing-level extremes can gradually increase your risk of weight gain, high blood sugar, and weakened immunity over time.
What Happens When Cortisol Is Too Low
Too little cortisol is just as dangerous as too much. The most common cause is adrenal insufficiency, where the adrenal glands can’t produce enough cortisol. When this happens because the adrenal glands themselves are damaged, often by an autoimmune attack, it’s called Addison’s disease. It can also happen when the pituitary gland fails to send the signal that tells the adrenals to produce cortisol.
Symptoms of low cortisol come on slowly and are easy to miss early on. They include persistent fatigue, muscle weakness, unintentional weight loss, low blood pressure, dizziness, and salt cravings. Because these symptoms overlap with so many other conditions, adrenal insufficiency often goes undiagnosed for months. The most dangerous complication is an adrenal crisis, a sudden, severe drop in cortisol triggered by physical stress like illness or injury, which can cause dangerously low blood pressure and requires emergency treatment.
How Cortisol Levels Are Tested
If your doctor suspects a cortisol problem, there are three main ways to measure it. Blood tests are the most common. Because cortisol levels swing so dramatically throughout the day, blood is typically drawn twice: once in the early morning and again around 4 p.m. Comparing those two values reveals whether your cortisol is following its normal daily rhythm.
Saliva tests offer a more convenient alternative. You collect samples at home at different times of day, which gives your doctor a picture of how your cortisol fluctuates without multiple lab visits. A 24-hour urine collection captures the total amount of cortisol your body produces over an entire day, which is useful for spotting overproduction that might not show up in a single blood draw.
All three methods are considered accurate estimates of your cortisol status. The choice depends on what your doctor is looking for. Blood tests are standard for initial screening, saliva tests are useful for tracking patterns, and urine tests are particularly helpful when Cushing syndrome is suspected.