24-Hour Urinary Free Cortisol Test

Key takeaways

• A 24-hour urinary free cortisol test measures unbound cortisol excreted in urine over a complete 24-hour period, which helps smooth out the natural daily ups and downs of cortisol release.
• It is most commonly ordered to help evaluate suspected cortisol excess (Cushing syndrome) and, in some cases, to assess low cortisol production.
• Accurate results depend on collecting all urine for the full 24 hours and storing it as instructed, since a missed or extra sample can invalidate the test.
• Many factors can affect results, including certain medications, pregnancy, kidney function, heavy physical stress, and some foods or supplements, so tell your clinician about everything you take.
• An abnormal result is not a diagnosis by itself; it is usually interpreted alongside symptoms, medical history, and other tests such as late-night salivary cortisol or blood ACTH measurements.

What the Test Measures and Why It Is Ordered

Cortisol is a steroid hormone produced by the adrenal glands and regulated by the pituitary gland through ACTH. Most cortisol in the bloodstream is bound to proteins such as cortisol-binding globulin and albumin; only the unbound, or free, portion is filtered by the kidneys and appears in urine. The 24-hour urinary free cortisol test captures that free fraction across a full day rather than at a single moment.

Because cortisol secretion follows a daily rhythm and rises with stress, illness, and other factors, a one-time measurement can be misleading. Clinicians typically order a 24-hour UFC when they suspect the body is making too much cortisol, a state known as Cushing syndrome, which can cause weight gain in the trunk, easy bruising, muscle weakness, and high blood pressure. It may also be used, in selected situations, when low cortisol production is being considered.

How to Collect the Sample Correctly

A 24-hour urine collection usually begins in the morning. You discard the first urine you pass at the start time, note that time, and then save every drop of urine for the next 24 hours into the container provided by the laboratory. The collection ends when you void at the same time the following morning, and that final sample is added to the container. Keeping the container refrigerated or cool, as instructed, helps preserve the sample.

The most common reason for an inaccurate result is an incomplete collection. Missing even one void, or including an extra one outside the window, can make cortisol levels look falsely low or high. Write down the start and end times, keep the container away from children and pets, and follow any instructions about avoiding certain medications, strenuous exercise, or specific foods before and during the collection. If you accidentally spill or miss part of the sample, tell the laboratory, because the test may need to be repeated.

Understanding High and Low Results

A higher-than-expected urinary free cortisol level suggests the body is producing or exposed to excess cortisol. Causes can include an adrenal tumor, a pituitary tumor that drives ACTH production, or an ectopic source of ACTH elsewhere in the body. However, elevated results can also occur with severe physical stress, illness, pregnancy, obesity, depression, alcohol use, and some medications. For this reason, an abnormal UFC is usually confirmed with additional testing before a diagnosis is made.

A lower-than-expected level may point toward reduced cortisol production, which can occur with adrenal insufficiency or pituitary conditions. Yet low results are not always meaningful on their own, and some laboratories report wide reference ranges. Because interpretation depends heavily on context, results should always be reviewed by a clinician who can consider symptoms, other hormone tests, and imaging when appropriate.

Factors That Can Affect Accuracy and Next Steps

Several medications can interfere with cortisol measurement or with cortisol production itself. These include certain steroids, some diuretics, and drugs that affect liver enzymes. Pregnancy increases cortisol-binding globulin and can raise measured levels, while kidney disease can reduce clearance and alter results. Some herbal supplements and very high intakes of certain foods may also matter, so a complete medication and supplement list is important before testing.

If a 24-hour UFC result is abnormal, clinicians often repeat the test or add a late-night salivary cortisol measurement, an overnight dexamethasone suppression test, or blood tests for ACTH and other hormones. Imaging of the adrenal or pituitary glands may follow if a tumor is suspected. No single test stands alone; the goal is to build a consistent picture using symptoms, examination findings, and several laboratory results.

Frequently Asked Questions

Sources

• MedlinePlus: Cortisol Test
• National Institute of Diabetes and Digestive and Kidney Diseases: Cushing's Syndrome
• National Institutes of Health: Adrenal Gland Disorders
• Lab Tests Online: Cortisol
• Centers for Disease Control and Prevention: Laboratory Quality and Testing