Cortisol is a steroid hormone produced and released by the adrenal glands, which are located on top of the kidneys. It plays a part in numerous physiological processes, including the regulation of metabolism, the body’s immune response, and blood pressure. Often labeled the “stress hormone,” its production increases in response to physical or emotional stress, preparing the body for a “fight or flight” scenario.
The Diurnal Rhythm of Cortisol
Cortisol secretion is not constant; it follows a 24-hour cycle known as a diurnal rhythm, governed by the brain’s hypothalamus. Cortisol levels are at their highest point in the early morning, around 30 to 40 minutes after waking. This morning surge, or cortisol awakening response (CAR), helps promote wakefulness and provides energy for the day.
As the day progresses, the concentration of cortisol in the body gradually declines, reaching its lowest point around midnight or during the initial hours of sleep. Temporary spikes can occur during the day in response to stressful events, but the overall pattern of high morning levels and low nighttime levels remains consistent in a healthy individual.
This predictable pattern is why the timing of a cortisol test is important for accurate interpretation. A measurement taken in the morning will be much higher than one taken in the late afternoon. Disruptions to this rhythm, such as those caused by working night shifts or chronic stress, can affect the body’s internal clock and overall cortisol output.
Cortisol Testing Methods
Because cortisol levels fluctuate, several methods are used to measure them, with the most common being blood, saliva, and urine tests. The choice of test depends on the information a healthcare provider needs about a person’s adrenal function.
A blood test measures serum cortisol, providing a snapshot of the hormone’s concentration at a single moment. To assess the daily rhythm, blood samples are often drawn twice: once in the morning (around 8 AM) and again in the afternoon (around 4 PM). Blood tests measure both free cortisol (the active form) and cortisol bound to proteins.
Saliva tests also measure cortisol at a specific point and are non-invasive, making it easier to collect multiple samples. This method measures free cortisol. A 24-hour urine test measures the total amount of cortisol excreted over a full day, which gives an average of cortisol production and smooths out the daily fluctuations.
Reference Ranges by Age Group
Normal cortisol levels vary by the time of day, the test used, and a person’s age. The specific values considered “normal” can differ between laboratories, so these reference ranges are a guide.
For adults and older adults, a morning (8 AM) blood serum cortisol level is between 5 and 23 micrograms per deciliter (mcg/dL). By the afternoon (4 PM), this level drops to a range of 3 to 13 mcg/dL. For children between 1 and 16, the morning range is 3 to 21 mcg/dL, while the afternoon range is 3 to 10 mcg/dL. Newborns have a reference range of 1 to 24 mcg/dL.
Morning salivary cortisol levels (7 AM-9 AM) are between 100-750 nanograms per deciliter (ng/dL), falling to less than 401 ng/dL in the afternoon and under 100 ng/dL around midnight. For 24-hour urine tests, the normal range for an adult is less than 100 mcg/24h. Adolescents have a range of 5-55 mcg/24h, and children have a range of 2-27 mcg/24h. Pubertal status can also affect morning cortisol, with post-pubertal individuals showing higher morning levels than pre-pubertal children.
Factors Influencing Cortisol Levels
Beyond the diurnal cycle and age, several factors can cause changes in cortisol levels that should be considered when interpreting test results. Acute physical or emotional stress is a primary driver of increased cortisol secretion. Events like surgery, hospitalization, or significant psychological distress can cause levels to rise.
Lifestyle factors also influence cortisol levels. The quality and duration of sleep are directly linked to the cortisol rhythm, and poor or irregular sleep patterns can disrupt the normal cycle. Diet and substances can also have an effect; for example, caffeine stimulates cortisol production, which can temporarily elevate levels.
Certain medications can interfere with cortisol measurements. Exogenous glucocorticoids, such as prednisone, can cross-react with testing assays, leading to falsely elevated results. Other drugs, including some anticonvulsants, may also affect normal cortisol variation. Metabolic factors like obesity and insulin sensitivity can also affect morning cortisol concentrations.