Cortisol (Free) in Serum or Plasma

Your doctor may order this test if you have symptoms that could be linked to cortisol levels that are too high or too low. High cortisol can cause weight gain, a rounded face, high blood pressure, high blood sugar, muscle weakness, and thinning skin, while low cortisol may lead to fatigue, unexplained weight loss, dizziness, and salt cravings. The test may also be used to follow up on abnormal findings from other lab work or to monitor treatment for adrenal or pituitary conditions.

Your doctor may recommend this test to evaluate adrenal gland function, check for conditions like Cushing’s syndrome or adrenal insufficiency, or assess how your body responds to stress or medications that affect cortisol levels.

Before the test, your healthcare provider will explain the procedure and may give you specific instructions, because cortisol levels naturally rise and fall throughout the day and are usually highest in the early morning. You may be asked to schedule the blood draw for a particular time, avoid eating or drinking anything except water for a set period, or temporarily pause certain medications, such as corticosteroids, estrogen supplements, or birth control pills, if it is safe to do so. Tell your provider about all medicines, vitamins, and herbal products you take, and mention any unusual stress, recent illness, or pregnancy, since these factors can affect your results.

No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the blood draw if cortisol levels are being measured at specific times (e.g., morning or evening). Avoid caffeine or stressing before the test if instructed.

If your sample is collected with a blood draw, a nurse or phlebotomist will wrap an elastic band around your upper arm, clean the site, usually the inside of your elbow, with an antiseptic, and insert a small needle into a vein to collect blood into one or more tubes. You may feel a brief sting or pinch when the needle goes in, and the collection usually takes only a few minutes. Some cortisol tests instead use a saliva sample that you collect at home at a specific time, or a 24-hour urine collection, in which you save all of your urine in a special container over a full day.

A healthcare provider will draw blood from a vein in your arm. The sample will be sent to a lab for analysis using a sensitive immunoassay method to detect cortisol levels.

Cortisol results are interpreted based on the reference range provided by the laboratory, the time of day the sample was taken, and your overall health, so a single value should always be reviewed by your doctor. A result that is higher or lower than expected does not automatically mean you have a serious condition, because stress, illness, pregnancy, certain medications, and poor sleep can all temporarily change cortisol levels. Your provider may repeat the test, order additional tests such as an ACTH level or a dexamethasone suppression test, or refer you to an endocrinologist to determine the cause and decide on the right treatment plan.

Results are compared to general reference ranges, but individual ranges can vary by lab. High or low levels may signal underlying health concerns that require further evaluation by your doctor.