What Is the 1mg Overnight Dexamethasone Suppression Test?
The 1mg overnight dexamethasone suppression test (DST) is a simple outpatient screening test that checks whether your body is producing too much cortisol — a condition called hypercortisolism or Cushing's syndrome.
Dexamethasone is a synthetic steroid that, in a healthy person, signals the brain to temporarily "turn off" cortisol production overnight. If cortisol does not suppress as expected, it may indicate the body's normal feedback system isn't working — which can point to excess cortisol from the adrenal glands, a pituitary tumor, or another source.
💡 A simple way to think about it: Dexamethasone acts like a "pause button" for cortisol. A healthy body hears the signal and pauses. In Cushing's syndrome, the pause button doesn't work — cortisol stays elevated even after taking the pill.
Why Did My Doctor Order This Test?
Your endocrinologist may have ordered this test for one or more of the following reasons:
- You have an adrenal nodule or mass found incidentally on imaging (an "adrenal incidentaloma")
- You have symptoms possibly suggesting Cushing's syndrome — such as unexplained belly weight gain, a fatty hump between the shoulders, easy bruising, purple stretch marks, muscle weakness, or blood sugar problems
- You have difficult-to-control high blood pressure or diabetes
- Routine screening as part of an adrenal workup
- To rule out subclinical Cushing's — excess cortisol that hasn't yet caused obvious symptoms
How to Prepare — Step-by-Step Instructions
This test requires minimal preparation, but following the timing exactly is essential for accurate results.
📋 Your Test Protocol — At a Glance
- Before test day: Pick up your 1mg dexamethasone tablet from the pharmacy
- Day before: Eat and drink normally. Avoid alcohol.
- 11:00 PM exactly: Take the 1mg dexamethasone tablet with a glass of water
- Overnight: Sleep normally — no special restrictions
- Next morning, 8:00–9:00 AM: Go to the lab for a single blood draw (serum cortisol)
✅ Do:
- Take the tablet at exactly 11:00 PM — even one hour off can affect results
- Get your blood drawn between 8:00 AM and 9:00 AM the following morning — do not delay past 9:00 AM
- Drink water freely before and after the blood draw
- Tell your doctor about every medication, supplement, spray, cream, and herbal product you use
❌ Do Not:
- Do not take the dexamethasone at a different time
- Do not take other steroids (prednisone, hydrocortisone) unless medically required and your doctor is aware
- Do not delay your morning blood draw past 9:00 AM
- Do not drink alcohol the evening before the test
- Do not start new over-the-counter medications or supplements before the test without checking first
Medications & Substances That Can Interfere
Certain medications can cause a false positive (cortisol looks high when it isn't) or a false negative (cortisol looks suppressed when there is actually a problem). Always disclose everything you take.
| Medication / Substance | Effect on Test | Common Examples |
|---|---|---|
| Estrogen-containing medications | May cause false positive | Birth control pills, oral hormone replacement therapy |
| Enzyme-inducing medications | Speed up dexamethasone breakdown → false positive | Rifampin, phenytoin (Dilantin), carbamazepine (Tegretol), phenobarbital |
| Other corticosteroids | May suppress cortisol → false negative | Prednisone, hydrocortisone, inhaled steroids (Advair, Symbicort), nasal sprays (Flonase), topical steroid creams |
| Heavy alcohol use | Can cause false positive — "pseudo-Cushing's" | Avoid alcohol night before; chronic heavy use must be disclosed |
| Certain antidepressants / antipsychotics | May affect the HPA axis and results | Discuss all psychiatric medications with your provider |
⚠️ Easy to forget: Nasal steroid sprays (Flonase, Nasonex), inhaled steroids (Advair, Symbicort, Flovent), and topical steroid creams are absorbed into the body even though they seem "local." Always mention these at your visit.
Understanding Your Results
Your endocrinologist will interpret your results alongside your full clinical picture. Here is a general guide to what the numbers mean:
| Morning Cortisol (8–9 AM) | Interpretation | What Happens Next |
|---|---|---|
| ≤ 1.8 mcg/dL | Normal suppression — Cushing's very unlikely | Usually no further cortisol testing needed |
| 1.9 – 5.0 mcg/dL | Borderline / indeterminate | Additional testing often ordered (24-hr urine, salivary cortisol) |
| > 5.0 mcg/dL | Failure to suppress — abnormal | Further evaluation for Cushing's syndrome indicated |
An abnormal result does not automatically mean you have Cushing's syndrome. Stress, depression, obesity, alcohol use, and interfering medications can all cause falsely elevated cortisol. Your endocrinologist will carefully evaluate results alongside your symptoms and history before drawing any conclusions.
📞 Results timeline: Most cortisol results are available within 1–2 business days. Your care team will contact you to review results and discuss any needed follow-up. You can also check through the patient portal.
Frequently Asked Questions
Can I eat before my morning blood draw?
A light snack and water are fine. Avoid a heavy meal right before the draw. The most important thing is to not delay your blood draw past 9:00 AM — cortisol rises naturally throughout the morning, so timing matters for accurate interpretation.
What if I forgot to take the dexamethasone at exactly 11:00 PM?
If you took it within 30–60 minutes of the scheduled time, call our office and let us know the exact time — the test may still be valid. If you forgot entirely or took it significantly early or late, contact us before going to the lab. We will likely need to reschedule to avoid an inaccurate result.
Will dexamethasone affect my sleep?
Some patients notice mild insomnia, a slight jittery feeling, or increased energy — these are common, temporary steroid effects. Most people sleep normally. Note any unusual symptoms and mention them at your follow-up appointment.
I'm on birth control pills. Do I need to stop them?
Estrogen in birth control pills raises cortisol-binding proteins, which can cause a falsely elevated cortisol result. Your endocrinologist will advise you — sometimes they ask you to hold the pill for 6 weeks before testing; other times they factor this into interpreting results. Do not stop any medication on your own without being told to do so.
I use a nasal steroid spray for allergies every day. Does that matter?
Yes, it can. Intranasal corticosteroids like fluticasone (Flonase) are absorbed to a small degree and may suppress cortisol, potentially creating a false negative. Always mention every spray, cream, and inhaler you use — even ones that seem minor.
My result came back abnormal. Does that mean I have Cushing's syndrome?
Not necessarily. An abnormal DST result is a starting point for evaluation, not a final diagnosis. Your endocrinologist will likely order additional confirmatory tests — such as a 24-hour urine free cortisol collection or late-night salivary cortisol — before reaching any conclusion. Many patients with an abnormal screening result do not have true Cushing's syndrome after full evaluation.
Is this the same as a "low-dose dexamethasone suppression test"?
The 1mg overnight DST is a form of low-dose suppression testing. There is also a formal 2-day low-dose test (0.5mg every 6 hours for 48 hours) used in specific clinical situations. The 1mg overnight test is the most common first-line outpatient screening tool and is what most patients receive first.
Can I drive myself to the lab the next morning?
Yes. Dexamethasone taken the previous night is fully cleared by morning and will not impair your ability to drive.
What happens if I do have Cushing's syndrome?
If confirmatory testing supports a diagnosis, your endocrinologist will work to identify the source of excess cortisol — usually with additional blood tests and imaging such as MRI of the pituitary or CT of the adrenal glands. Treatment depends on the underlying cause and may include surgery, medication, or radiation. Your care team will guide you through every step.