24-Hour Urine Cortisol Test

A 24-hour urine cortisol test measures the amount of cortisol the body excretes over a full day. By gathering urine across an entire day and night, it captures total cortisol output and smooths out the hour-to-hour swings that a single blood sample would show.

What the test measures

Cortisol is a hormone from the adrenal glands, and its level in the blood rises and falls in a daily rhythm, usually higher in the morning and lower at night. A single blood draw captures only one moment of that rhythm. The 24-hour urine test instead measures the free cortisol that passes into the urine over a whole day, giving an estimate of the body's total cortisol production rather than a snapshot. This makes it useful when the question is whether overall cortisol output is too high.

The measurement is often described as urinary free cortisol, because it reflects the unbound cortisol that the kidneys filter into the urine. Because it integrates the entire day, it is less affected by the timing of any single moment than a one-off blood level.

Why a clinician might order it

This test is commonly used in the evaluation of Cushing's syndrome, a condition of excess cortisol. Because it reflects total daily output, it can reveal sustained overproduction that a single measurement might miss. It is often used together with other cortisol assessments to build a fuller picture, and it may be repeated, since cortisol production can vary from day to day. A clinician chooses it when the goal is to gauge overall cortisol burden rather than a single point in the daily cycle.

How the test works

The test relies on a complete collection of all urine produced over a 24-hour period. Typically, the first urine of the morning is discarded to mark the start, and then every subsequent urination is collected into a provided container, including the first urine of the following morning to close the 24 hours. The total volume is recorded and a portion is analyzed for cortisol. Because the result depends on capturing the entire day's output, completeness of the collection is essential to a reliable result.

Why completeness matters so much

If any urination is missed, the collection no longer represents a full day, and the measured total can read falsely low. Conversely, collecting beyond the 24-hour window can overstate the output. Laboratories often provide a container and storage instructions, and they may measure creatinine in the same sample as a rough check on how complete the collection was. Following the start and stop times exactly is the single most important step in getting a usable result.

How to prepare

Preparation focuses on collecting correctly and storing the sample as instructed, often refrigerated. Following the timing precisely and not missing any urination keeps the result accurate. Several medications, including steroid medications, as well as stress and illness, can affect cortisol. A complete medication list helps the clinician interpret the result.

What can affect the result

• Collection completeness: a missed urination or an over-long window distorts the total.
• Steroid medications: these can raise or alter cortisol and its measurement.
• Stress and illness: physical stress can increase cortisol output.
• Very high fluid intake: large urine volumes can influence some assays.
• Storage and handling: samples often need refrigeration to stay reliable.

How it fits with other cortisol assessments

The 24-hour urine cortisol test is one of several ways to assess cortisol, and it is usually read as part of a set rather than alone. A blood cortisol level gives a snapshot at a chosen moment, useful for capturing the daily rhythm but limited to that instant. A late-night cortisol measurement looks specifically at whether the normal nighttime dip is preserved. Dynamic tests assess whether cortisol can be suppressed or stimulated on cue. Each angle answers a slightly different question, and the urine test contributes the perspective of total daily output. Because cortisol can vary from day to day and is sensitive to stress and illness, a clinician often combines approaches and may repeat the collection before reaching a conclusion.

Understanding what the test is good at also clarifies its limits. It is well suited to gauging overall cortisol burden but cannot, by itself, show the timing of cortisol release through the day or pinpoint where in the hormone system a problem might originate. Those questions call for other tests, which is why the urine collection is generally one piece of a wider evaluation.

How results are generally interpreted

Results reflect total cortisol excreted over the day and are read against laboratory reference ranges, always in clinical context.

• High urine cortisol can indicate excess cortisol production and, in the right clinical setting, supports further evaluation for Cushing's syndrome.
• Normal urine cortisol is reassuring but does not by itself rule out a cortisol problem, especially if output varies day to day, so the test may be repeated.
• Borderline results may reflect collection issues, medications, or stress and often lead to repeat or additional testing.

Illustrative interpretation patterns

The patterns below are illustrative only; exact reference ranges vary by laboratory, assay, and collection method. Always rely on the range printed on your own report and a clinician's interpretation.

Related tests

This test is often used alongside a blood cortisol measurement when excess cortisol is suspected, and it contrasts with the ACTH stimulation test, which evaluates whether cortisol can rise when stimulated. See the blood tests index for related adrenal tests, the hormones index for background on cortisol, and the conditions index for Cushing's syndrome. The guides index can help with reading the result.