Cortisol is a steroid hormone produced and released by the adrenal glands, which are situated just above the kidneys. This hormone, sometimes called a glucocorticoid, plays a broad role in the body, affecting nearly every tissue and organ system. Its primary functions involve regulating metabolism, managing the body’s response to stress, and modulating immune activity. Cortisol also contributes to maintaining blood pressure and helps the body utilize glucose for energy.
Cortisol’s Daily Rhythm and the Morning Peak
The significance of a morning cortisol measurement is directly linked to the hormone’s predictable 24-hour cycle, known as the diurnal variation or circadian rhythm. Cortisol levels are regulated by the Hypothalamic-Pituitary-Adrenal (HPA) axis, a complex communication system between the brain and the adrenal glands. The HPA axis ensures that cortisol levels peak in the early morning and gradually decline to their lowest point around midnight.
The sharp increase shortly after waking is a distinct event called the Cortisol Awakening Response (CAR). This response is superimposed on the general morning rise, often leading to a peak in concentration approximately 30 to 60 minutes after a person wakes up. The CAR is a strong neuroendocrine signal that prepares the body for the anticipated demands of the upcoming day.
This morning peak mobilizes energy resources, promotes alertness, and helps the body transition from sleep to activity. Because the morning measurement captures this highest physiological concentration, it is the standard time used in clinical testing to assess the general health and function of the HPA axis. Loss of this expected diurnal pattern is often one of the first indicators of adrenal dysfunction.
Standard Reference Ranges for Morning Cortisol
Interpreting a morning cortisol level requires understanding that no single number defines “normal,” as reference ranges vary widely depending on the laboratory, the assay technology used, and the type of sample collected. For a standard serum (blood) cortisol test drawn between 6:00 AM and 8:00 AM, a typical reference range for adults falls between 10 to 20 micrograms per deciliter (\(\mu\)g/dL). Other common ranges are cited as 5 to 25 \(\mu\)g/dL for an 8:00 AM collection.
A morning serum cortisol value below 5 \(\mu\)g/dL strongly suggests adrenal insufficiency, while a value above 15 \(\mu\)g/dL indicates sufficient adrenal function. Values falling in the indeterminate zone, such as between 5 and 10 \(\mu\)g/dL, often necessitate further dynamic testing to confirm a diagnosis. These numerical guides are for serum total cortisol, which measures both the bound and unbound hormone.
Cortisol circulates in the blood mostly bound to Cortisol-Binding Globulin (CBG), with only 3 to 5% circulating as biologically active free cortisol. While total cortisol is the most common lab measurement, free cortisol measurements offer a more accurate picture of the active hormone available to tissues. These are often obtained through saliva or 24-hour urine collection. Saliva testing is useful for assessing the daily curve, as its morning range is much lower, typically 10.2 to 27.3 nanograms per milliliter (ng/mL).
Common Causes of Abnormal Morning Levels
When morning cortisol levels fall outside the expected reference range, it indicates a disruption in the HPA axis, suggesting conditions of either hormone excess or deficiency.
High Cortisol
Persistently elevated morning cortisol levels, known as hypercortisolism, are associated with Cushing’s Syndrome. This condition results from the body producing too much cortisol, often due to tumors in the pituitary or adrenal glands, or from taking high doses of corticosteroid medication. Symptoms linked to chronic high cortisol include unexplained weight gain, especially around the face and trunk, muscle weakness, easy bruising, and high blood pressure.
Low Cortisol
Conversely, consistently suppressed morning cortisol levels, known as hypocortisolism, suggest Adrenal Insufficiency. Primary Adrenal Insufficiency (Addison’s Disease) occurs when the adrenal glands are damaged. Secondary or tertiary insufficiency results from problems in the pituitary gland or hypothalamus, which fail to signal the adrenals. Low morning cortisol is associated with symptoms such as overwhelming fatigue, unintended weight loss, loss of appetite, and low blood pressure. A single morning test is only a screening tool, and any abnormal result requires a thorough evaluation by a physician to establish a definitive diagnosis.