Cortisol is a steroid hormone produced by the adrenal glands, which sit atop the kidneys. It serves as the body’s primary mechanism for responding to stress, playing a role in mobilizing energy stores, regulating metabolism, and modulating the immune system’s response to inflammation. Cortisol is part of a complex feedback system designed to maintain stability, and its levels fluctuate naturally throughout a person’s life. Understanding “normal” cortisol requires knowing its function, measurement methods, and the many variables that cause its levels to shift.
The Role of Cortisol and How It Is Measured
Cortisol production is managed by the Hypothalamic-Pituitary-Adrenal (HPA) axis, a chain of communication between the brain and the adrenal glands. When the body encounters a stressor, the hypothalamus signals the pituitary gland, which then releases adrenocorticotropic hormone (ACTH). ACTH travels to the adrenal cortex to stimulate cortisol release. This response provides immediate energy and suppresses non-essential processes like digestion and immune activity, helping the body manage a “fight or flight” situation.
When measuring cortisol in the bloodstream, the standard unit of measure in the United States is micrograms per deciliter (\(\mu\text{g}/\text{dL}\)). This unit quantifies the mass of the hormone present in a fixed volume of blood serum. Cortisol testing can also be performed using saliva or urine samples, with each type providing a different perspective on the hormone’s activity. Serum testing measures both the bound and unbound (total) cortisol, whereas 24-hour urine or salivary tests often reflect the free, biologically active portion.
Understanding the Diurnal Variation
Cortisol levels are not static but follow a robust and predictable 24-hour cycle known as the diurnal or circadian rhythm. This rhythm is directly tied to the body’s sleep-wake cycle and is regulated by a central pacemaker in the brain. Levels begin to rise in the late stages of sleep, resulting in a sharp surge upon waking, known as the Cortisol Awakening Response (CAR).
The peak concentration generally occurs in the early morning, typically around 8:00 AM. Following this peak, cortisol levels gradually decline throughout the day and evening. The lowest point, or trough, is reached late at night, usually around midnight. Because of this significant daily fluctuation, a single cortisol measurement is meaningless without knowing the precise time the sample was collected.
Normal Cortisol Ranges Across the Lifespan (\(\mu\text{g}/\text{dL}\))
Cortisol ranges vary significantly across a person’s life, reflecting changes in adrenal function and the body’s overall needs. These figures are generalized guidelines, and laboratory reference ranges can differ based on the specific testing method used. Therefore, a test result must always be interpreted using the specific reference range provided by the performing laboratory.
In infancy, cortisol levels show high variability; total serum cortisol in newborns can range broadly from 1 to 24 \(\mu\text{g}/\text{dL}\). As children grow, their diurnal rhythm matures, often resembling the adult pattern by age six. For children and adolescents (up to age 16), morning serum cortisol levels generally fall in the range of 3 to 21 \(\mu\text{g}/\text{dL}\).
The standard adult reference range defines the morning peak (8:00 AM) and the afternoon trough (4:00 PM). An adult’s morning serum cortisol level often ranges from 5 to 23 \(\mu\text{g}/\text{dL}\). In the late afternoon, this range is expected to drop to approximately 3 to 13 \(\mu\text{g}/\text{dL}\).
In older adults, the overall pattern of cortisol secretion often changes. While morning levels may remain within the young adult range, the diurnal curve can become less pronounced, or “flattened.” This means the late-night trough may be slightly higher than in younger adults, suggesting a change in the HPA axis feedback loop with age.
Factors That Shift Cortisol Levels
Beyond the natural daily and age-related fluctuations, several chronic factors can cause sustained shifts in cortisol levels. Endocrine disorders represent the most significant pathological causes of abnormal cortisol production.
Endocrine Disorders
Cushing’s Syndrome, for instance, results from prolonged exposure to excessively high cortisol levels, leading to a loss of the normal diurnal rhythm. Conversely, Addison’s Disease, or primary adrenal insufficiency, is characterized by the adrenal glands failing to produce sufficient cortisol, resulting in chronically low levels.
Medications
The use of certain medications, particularly synthetic glucocorticoids like prednisone, can profoundly affect natural cortisol production. These external steroids mimic cortisol and suppress the HPA axis, causing the body’s own adrenal glands to reduce or cease making the hormone. Abrupt cessation of these medications can result in dangerously low cortisol levels due to this suppression.
Stress and Pregnancy
Chronic psychological or physical stress also leads to sustained elevations in the hormone, increasing the overall cortisol output throughout the day. However, it is important to distinguish this from the temporary spike that occurs during an acute stressful event.
Furthermore, pregnancy represents a state of naturally elevated total cortisol, which is due to the increase in estrogen stimulating the production of Corticosteroid-Binding Globulin (CBG). This increased binding protein means that while total cortisol levels rise significantly, the amount of free, active cortisol may not be as dramatically altered.