Dexamethasone Suppression Test: Low-Dose vs High-Dose Explained

What Is a Dexamethasone Suppression Test?

A dexamethasone suppression test measures whether a steroid dose can lower your cortisol. Dexamethasone is a lab-made version of cortisol. When a healthy person takes it, the brain senses "enough cortisol is here" and tells the adrenal glands to stop making their own. Cortisol then falls.

Your body runs this on a feedback loop called the HPA axis — the link between the hypothalamus, the pituitary gland, and the adrenal glands. Dexamethasone acts on the pituitary to switch off the ACTH signal that drives cortisol. If cortisol stays high after the dose, the loop is not responding normally. That points toward a source of cortisol that ignores the brain's "stop" signal, which is the hallmark of cortisol imbalance seen in Cushing's syndrome.

The test is chosen because dexamethasone does not show up on a standard cortisol blood test. That lets the lab measure your own cortisol cleanly, without the drug interfering with the result.

Low-Dose DST Protocols (Overnight and 48-Hour)

The low-dose test screens for whether cortisol is too high in the first place. It comes in two forms.

Overnight 1 mg test (most common): You take 1 mg of dexamethasone by mouth at 11 PM. A blood draw the next morning between 8 and 9 AM measures cortisol. This is the quickest screen and the one most people get first.

Two-day (48-hour) 2 mg test: You take 0.5 mg every 6 hours for 2 days — 8 doses in total. Cortisol is measured 6 hours after the last dose. This version is more precise and is often used when the overnight test is unclear or when the overnight result may be unreliable.

Both low-dose versions answer the same question: does your cortisol suppress when it should? Neither one tells you the cause on its own.

High-Dose DST: Finding the Source

The high-dose test helps locate where excess cortisol is coming from. It is used only after a low-dose test and other labs already show that cortisol is high. The dose is 8 mg — either as a single overnight dose or 2 mg every 6 hours for 2 days.

The logic is based on how different causes respond to a strong steroid signal. A pituitary tumor (Cushing's disease) often keeps some response to feedback, so a high dose can partly suppress its cortisol. A cortisol-making adrenal tumor, or an ACTH-producing tumor somewhere else in the body (called ectopic ACTH), usually ignores the signal and does not suppress at all.

In modern practice, the high-dose DST is used less often than it once was. Imaging and a test called inferior petrosal sinus sampling now do much of this locating work more reliably. Your endocrinologist decides whether the high-dose test still adds value in your case.

How to Read Your DST Result

A normal result is a morning cortisol that drops below the suppression cutoff after dexamethasone. For the overnight 1 mg test, most labs use a cutoff of 1.8 mcg/dL (50 nmol/L). A cortisol under that value is called normal suppression — your feedback loop worked, and Cushing's syndrome is unlikely.

A cortisol that stays above 1.8 mcg/dL is called failure to suppress. It does not diagnose Cushing's by itself. It means further testing is needed — usually repeat testing plus a late-night salivary cortisol and a 24-hour urine cortisol. Doctors look for two or three abnormal tests before making a diagnosis, because no single cortisol test is perfect.

Reference cutoffs vary slightly by lab and assay, so always read your result against the range printed on your own report.

Why False Positives Happen

A failed suppression result is often a false positive rather than true Cushing's syndrome. Several common things raise cortisol or change how you clear dexamethasone, and each can push the result over the cutoff without any disease being present.

• Estrogen and birth control: Oral estrogen raises cortisol-binding globulin, which lifts total cortisol. Stopping estrogen for about 6 weeks before testing improves accuracy.
• Fast dexamethasone metabolism: Drugs like phenytoin, carbamazepine, rifampin, and some others speed up how your liver clears dexamethasone, so less reaches your pituitary.
• Missed or mistimed doses: Taking the pill at the wrong time is one of the most common reasons for an odd result.
• Stress, illness, alcohol, and depression: Each can raise cortisol on its own — sometimes called pseudo-Cushing's.
• Poor absorption: Vomiting or gut conditions can lower how much dexamethasone you absorb.

This is exactly why doctors confirm an abnormal DST with other tests before acting. For a broader view of what shifts cortisol readings, see our guide to cortisol levels and reference ranges.

When Is the DST Ordered?

Doctors order a DST when signs of high cortisol do not add up to an obvious cause. The test is a screen, not a routine check, so it is reserved for people with a real clinical reason.

Common triggers include unexplained weight gain around the middle with thin arms and legs, a rounded "moon" face, purple stretch marks, easy bruising, high blood pressure or blood sugar that is hard to control, and muscle weakness. It is also used to check an adrenal nodule found by chance on a scan, to make sure it is not quietly making extra cortisol.

If your main concern is everyday stress or fatigue rather than these specific features, a DST is usually not the right first test. A morning cortisol or a 4-point salivary cortisol curve fits that question better. Talk with your doctor about which cortisol test matches your symptoms.