Cortisol is a steroid hormone produced by the adrenal glands, two small organs situated above the kidneys. Often referred to as the “stress hormone,” cortisol is a glucocorticoid necessary for maintaining the body’s internal stability, or homeostasis. The body controls cortisol production through the Hypothalamic-Pituitary-Adrenal (HPA) axis, a complex communication system involving the brain and the adrenal glands.
How Cortisol Functions
Cortisol plays a broad role in regulating numerous bodily processes, acting as a metabolic signal, an immune modulator, and a circulatory regulator. Its main function is to manage metabolism by mobilizing energy stores throughout the body. It promotes gluconeogenesis, the liver’s process of creating new glucose from non-carbohydrate sources like amino acids, ensuring the brain and muscles have fuel.
The hormone also influences the breakdown of fats and proteins, making their components available for energy or repair. Cortisol is a potent anti-inflammatory agent, helping to suppress an overactive immune response and manage the effects of injury or infection.
Cortisol also contributes significantly to cardiovascular function by helping to maintain blood pressure and vascular tone. It works with other hormones to ensure that blood vessels remain responsive, which is necessary for a healthy circulatory system. Without sufficient cortisol, the body struggles to maintain stable blood pressure, especially during stress or illness.
The Importance of the Daily Rhythm
Understanding normal cortisol levels requires recognizing that the hormone follows a predictable 24-hour cycle known as the diurnal or circadian rhythm. Cortisol levels fluctuate dramatically throughout the day, driven by the body’s internal clock. Therefore, a single measurement taken at a random time is generally not useful for determining overall health status.
The daily rhythm is characterized by a sharp rise shortly after waking, often called the Cortisol Awakening Response (CAR). This peak, typically occurring within 30 to 45 minutes of waking, prepares the body for the demands of the upcoming day. Levels then gradually decline throughout the afternoon and evening, reaching their lowest point around midnight.
This peak-and-trough pattern reflects a healthy HPA axis and is crucial for regulating the sleep-wake cycle. A flattened diurnal curve, where morning levels are low and evening levels remain high, is often associated with dysregulation. A “normal” level is defined by a specific range that changes based on the time of day the sample was collected.
Age-Specific Normal Ranges
The concept of a normal cortisol range depends on the testing method, the time of day, and the laboratory’s specific reference values. For healthy adults, serum cortisol levels are highest in the morning, often falling within the range of 6 to 23 micrograms per deciliter (mcg/dL) at 8 a.m. By mid-afternoon, around 4 p.m., the expected range drops significantly to about 3 to 10 mcg/dL.
In infants and children, the cortisol rhythm is still maturing, with the adult pattern generally stabilizing by around age six. Serum cortisol concentrations in healthy children are often similar to adult ranges, with morning levels falling between 5 and 25 mcg/dL. Levels in newborns and preterm infants are highly variable and difficult to define due to the physiological stress of the postnatal period.
Older adults often show changes in their cortisol profile that reflect altered HPA axis regulation. While the diurnal rhythm persists, it can become “flattened,” meaning the morning peak is less pronounced and the nighttime trough is higher. The mean daily serum cortisol level may increase slightly with advancing age, which has implications for cognitive function and health outcomes.
Testing and Measurement Methods
The method used to measure cortisol greatly affects the reference range and the type of information obtained. Serum, or blood, testing is frequently used for acute measurement and typically measures the total cortisol, including both active and inactive forms bound to proteins. A morning serum test assesses adrenal function, with normal results often falling between 5 and 25 mcg/dL.
Salivary cortisol testing is a non-invasive way to measure the biologically active, or “free,” cortisol that is not bound to proteins. This method is highly effective for monitoring the diurnal curve, especially for capturing the late-night trough at home. A common normal range for late-night salivary cortisol in adults is approximately 2.2 to 4.1 nanograms per milliliter (ng/mL).
A 24-hour urine collection provides a measure of the total unbound cortisol excreted over an entire day, offering a comprehensive picture of overall cortisol production. This method integrates the peaks and troughs of the diurnal cycle into a single value, reflecting the mean free cortisol exposure. For adults, a typical normal range for 24-hour urinary free cortisol is 10 to 55 micrograms per day (mcg/day).