Dexamethasone Suppression Test

A dexamethasone suppression test assesses how the body's stress-hormone system responds to a signal that should normally turn cortisol production down. It is a dynamic test used to investigate whether the body is making too much cortisol.

What the test assesses

Cortisol is a hormone from the adrenal glands, and its production is normally controlled by a feedback loop often called the hypothalamic-pituitary-adrenal axis. The brain releases signals that prompt the pituitary gland to release adrenocorticotropic hormone (ACTH), which in turn tells the adrenal glands to make cortisol. When cortisol rises, it normally feeds back to switch off these signals, keeping the system in balance. Dexamethasone is a synthetic steroid that mimics cortisol's feedback signal. In a healthy system, giving it should cause the body to lower its own cortisol output, a response called suppression.

The test takes advantage of this loop. By giving dexamethasone and then measuring cortisol, clinicians can see whether the feedback system responds normally. A failure to suppress suggests the loop is not working as it should, and that observation is the heart of the test.

Why a dynamic test rather than a single cortisol

Cortisol naturally rises and falls across the day and climbs with stress and illness, so a single random cortisol level can be hard to interpret. By introducing a controlled feedback signal and then watching the response, the dexamethasone suppression test asks a sharper question than a one-off measurement: not simply how high cortisol is at a moment, but whether the system that should restrain cortisol still works.

Why a clinician might order it

This test is mainly used when there is concern about Cushing's syndrome, a condition of excess cortisol. It helps answer whether cortisol production is appropriately turned down by feedback or whether it remains high despite the suppressing signal. Different versions of the test, using different amounts and timing, can also help locate where in the loop a problem lies once excess cortisol is confirmed.

Excess cortisol can be associated with a range of non-specific features, and the difficulty is that those features overlap with common conditions. A test that probes the feedback loop directly helps a clinician decide whether the suspicion is supported enough to pursue, which is why the dexamethasone suppression test is often one of the first steps when cortisol excess is being considered rather than a test reserved for the end of an evaluation.

What the test does not do

The dexamethasone suppression test asks whether the feedback loop restrains cortisol; it does not measure cortisol output across a full day, and it does not directly image or locate any underlying cause. It also is not used to evaluate the opposite problem of too little cortisol. Understanding what a given test is built to answer helps explain why clinicians combine several different cortisol assessments rather than relying on one.

How the test works

The test follows a set protocol. Dexamethasone is taken by mouth at a specified time, and a blood sample for cortisol is drawn afterward, typically the following morning, to see whether the dexamethasone suppressed cortisol as expected. The specific protocol is chosen based on the clinical question, and the timing of the dose and the draw is important to a valid result.

Common forms of the test

Several versions exist. An overnight form uses a single evening step followed by a morning cortisol draw and is often used as an initial screen. Longer forms gather measurements over more than one day. Higher-step versions are used in different settings to help distinguish among causes once cortisol excess has been established. The choice among these is a clinical decision, and this page does not describe amounts or schedules.

How to prepare

Preparation centers on timing and on medications. The dexamethasone must be taken exactly as scheduled, and the blood draw must occur at the planned time, because the test depends on the relationship between the two.

What can affect results

The dexamethasone suppression test is sensitive to interference. Medications that change how the liver processes steroids can speed up or slow down the clearance of dexamethasone, shifting how much suppression occurs for reasons unrelated to the adrenal glands. Estrogen-containing preparations can raise the protein that carries cortisol in the blood and affect measured levels. Acute illness, major stress, poor or irregular sleep, and some other conditions can also blunt suppression. Because of all this, an unexpected result is frequently repeated or paired with other tests rather than treated as final.

How results are generally interpreted

Results are read by comparing the cortisol level after dexamethasone against the response expected when feedback is working normally.

• Cortisol suppresses as expected: the feedback loop is responding normally, which makes excess cortisol from a problem in the loop less likely.
• Cortisol fails to suppress: the system did not respond to the suppressing signal, which can occur in Cushing's syndrome and usually prompts further testing.
• Borderline or unclear results: may be affected by medications, illness, or timing and often lead to repeat or additional testing.

Illustrative interpretation patterns

The patterns below are illustrative only; exact thresholds vary by laboratory, assay, and the specific protocol used. Always rely on the interpretation that accompanies your own test and a clinician's assessment.

A screening step, not a final answer

An abnormal dexamethasone suppression result is a signal to look further, not a diagnosis. Because cortisol physiology is easily perturbed and the test has many sources of interference, clinicians generally confirm an unexpected finding with repeat testing and with complementary assessments before drawing conclusions. The reverse is also true: a normal-looking result is interpreted in the context of the clinical suspicion, since no single test is perfect. This measured, multi-step approach is standard for evaluating cortisol excess.

Related tests

This test is often used with other cortisol assessments. Some look at cortisol output over a full day, while a stimulation approach probes the opposite question of whether the adrenal glands can raise cortisol in response to a stimulating signal. ACTH may be measured to help locate where in the loop a problem lies. See the blood tests index for related adrenal tests, the hormones index for background on cortisol and ACTH, and the conditions index for Cushing's syndrome.