Cortisol Test

Your morning cortisol level is one of the most informative single measurements in endocrinology. It tells you whether your body is producing enough of its primary stress hormone, or whether it is producing far too much. Either extreme carries serious consequences, and both are diagnosable with well-established tests you can pursue directly.

Cortisol is produced by the outer layer of your adrenal glands, two small structures that sit on top of your kidneys. The hormone touches nearly every system in your body: it regulates blood pressure, blood sugar, immune activity, metabolism, and your ability to respond to physical and psychological stress. Getting the level right matters enormously.

What the Test Actually Measures

A cortisol blood test measures the total amount of cortisol circulating in your blood at the moment of the draw. Because cortisol follows a strong daily rhythm, when you get the blood drawn is as important as the number itself. Your body naturally produces the most cortisol in the early morning, just before and shortly after waking. Levels then fall gradually through the day and reach their lowest point overnight during sleep.

This daily rhythm is driven by your sleep-wake cycle, not by clock time. If you work night shifts or have severely disrupted sleep, your morning value may not reflect true peak output, which can complicate interpretation. Median cortisol falls by roughly 30 nmol/L (about 1 µg/dL) per hour between 7:00 AM and noon, which is why the standard diagnostic window is a blood draw between 8:00 and 9:00 AM.

Reference ranges also depend on the laboratory method used. One study using a standard immunoassay approach found estimated morning reference intervals of 5.7 to 25.6 µg/dL for samples collected between 8:00 and 10:00 AM, with the highest concentrations appearing in the earliest part of that window and in younger adults aged 18 to 35.

How the Body Controls Cortisol Production

Your brain and adrenal glands communicate through a feedback loop called the stress hormone control axis, formally known as the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus signals the pituitary gland, the pituitary releases a messenger hormone called corticotropin (abbreviated ACTH), and ACTH instructs the adrenals to produce cortisol. When cortisol rises, it signals back to the brain to slow production. This loop keeps levels in a healthy range.

When this system breaks down, it can fail in two directions. Your adrenal glands can underperform, producing too little cortisol regardless of how loudly the brain signals. Or the system can become dysregulated in the other direction, producing cortisol continuously even when the body should be resting. Understanding which failure mode is present determines everything about how you respond.

Reading Your Result: Adrenal Insufficiency

Adrenal insufficiency is the medical term for a state where your adrenal glands cannot produce enough cortisol to meet your body's needs. It is more common than most people realize. The most frequent cause is not a disease of the adrenal glands themselves but rather a consequence of taking prescription steroid medications, which suppress the brain's signaling pathway over time. Other causes include damage to the pituitary gland from a tumor, surgery, or radiation, as well as autoimmune destruction of the adrenal glands themselves.

The European Society of Endocrinology and Endocrine Society have established the following cutpoints for interpreting an early morning cortisol result drawn near 8:00 AM.

If your morning cortisol falls below 5 µg/dL, that result alone carries significant clinical weight. If it falls in the 5 to 10 µg/dL range, the number does not resolve the question on its own and a stimulation test, described below, becomes the standard path forward.

Symptoms that most commonly prompt this evaluation include fatigue in 50 to 95% of cases, loss of appetite and weight loss in 43 to 73%, nausea and abdominal discomfort in 20 to 62%, and low blood pressure on standing in 68%. One important and underrecognized trigger is unexplained low sodium in the blood, a finding present in 84% of people with undiagnosed adrenal insufficiency in one patient survey. Elevated potassium was present in 34% of the same group.

Distinguishing the Type of Insufficiency

A low cortisol alone does not tell you where the problem originates. Pairing the cortisol result with a measurement of ACTH and DHEAS (a weaker adrenal hormone whose level reflects the overall output of the adrenal glands) allows a much more precise diagnosis.

If you are currently taking or tapering off any steroid medication, including inhaled or topical formulations, this pattern is worth discussing with a clinician before reducing your dose further. Glucocorticoid-induced adrenal insufficiency is the most common form encountered, and HPA axis testing is specifically recommended for people who have received doses above physiological replacement levels.

For results in the intermediate range, the standard confirmation test is the ACTH stimulation test, sometimes called the cosyntropin test. In this test, a synthetic version of ACTH is injected and cortisol is measured at 30 and 60 minutes afterward. A peak cortisol that fails to reach the threshold at 30 minutes may still reach it by 60 minutes, which is why both timepoints are measured.

Reading Your Result: Cortisol Excess

The other major reason to test cortisol is to investigate whether your body is producing too much of it, a condition called Cushing syndrome. Chronically elevated cortisol damages multiple organ systems over time, contributing to weight gain concentrated around the abdomen and face, high blood pressure, bone thinning, impaired wound healing, easy bruising, and metabolic disruption.

No single symptom is unique to cortisol excess, but certain features are more discriminating than others. Purple stretch marks wider than 1 centimeter, easy bruising, facial flushing, and proximal muscle weakness (meaning difficulty rising from a chair or lifting your arms above your head) are the findings that most reliably distinguish true Cushing syndrome from other conditions that can look similar.

A standard morning cortisol is not the primary screening tool for excess. Three first-line tests are recommended for suspected hypercortisolism, each with distinct performance characteristics.

Because cortisol fluctuates day to day, two to three measurements are recommended before confirming or excluding Cushing syndrome. One abnormal result is not sufficient. Screening for cortisol excess is particularly recommended if you have osteoporosis at a young age, metabolic syndrome that is not responding to standard interventions, or multiple features from the discriminating symptom list above.

What Moves This Biomarker

Several factors shift cortisol levels in ways that have nothing to do with actual adrenal or pituitary disease. Understanding these is essential before interpreting any result.

Certain proteins in the blood carry cortisol through circulation, and changes in those carrier proteins alter what the test detects without changing how much cortisol is actually active in your tissues. Pregnancy and oral estrogen medications increase the level of a carrier protein called cortisol-binding globulin (CBG), which raises total measured cortisol and can produce a falsely elevated result. Liver disease (cirrhosis), kidney disease causing protein loss (nephrotic syndrome), and critical illness reduce CBG and albumin levels, producing falsely low results.

Disruptions to your sleep-wake cycle, including night shift work, jet lag, and severe insomnia, alter the normal daily cortisol rhythm and make morning values unreliable as a diagnostic baseline.

Certain medications alter the metabolism of dexamethasone (used in the suppression test) through the liver's drug-processing enzyme system. Drugs that accelerate dexamethasone breakdown, including phenobarbital and carbamazepine, can cause a false-positive suppression test result, appearing to show Cushing syndrome when none is present. Drugs that slow dexamethasone metabolism, including fluoxetine and diltiazem, can produce the opposite effect. High-dose biotin supplementation can interfere with some immunoassay platforms and should be disclosed before testing.

Questions This Biomarker Can Answer

• Is my fatigue, low blood pressure, and poor stress tolerance consistent with my adrenal glands not producing enough cortisol?
• Is my weight gain around the face and abdomen, easy bruising, and high blood pressure consistent with my body overproducing cortisol?
• Is the problem with my cortisol originating in my adrenal glands themselves, or is it coming from a signaling failure higher up in my brain?
• Has my long-term use of steroid medication suppressed my adrenal gland function to a degree that requires monitoring before I reduce my dose?
• Is my unexplained low blood sodium a sign that my adrenal glands are failing to produce enough cortisol?