Most labs call suppression “normal” when the morning cortisol after dexamethasone is under 1.8 µg/dL (50 nmol/L), with some labs using higher cutoffs.
Cortisol suppression testing is one of those lab results that looks simple until you see the fine print. One report says “normal,” another says “borderline,” and suddenly you’re stuck asking: normal by which cutoff, for which version of the test, and measured in what units?
This article clears up what “normal values” usually mean for dexamethasone suppression testing, why different labs print different ranges, and how to read your number without guessing.
What This Test Is Checking
Dexamethasone suppression testing checks whether your body can turn cortisol down when it receives a signal that it has enough steroid on board. Dexamethasone is a synthetic glucocorticoid. In many people, it tells the pituitary to lower ACTH, which leads the adrenal glands to lower cortisol output.
The test is used when a clinician is checking for cortisol excess (often discussed under Cushing syndrome) or when evaluating adrenal nodules that might produce extra cortisol. The most common screening version is the 1 mg overnight test.
Why “Normal” Is Not One Number
Two big reasons explain the range you’ll see across reports:
- Different test formats. “Suppression test” can mean the overnight 1 mg test, a 2-day low-dose test, or a high-dose test used for certain follow-up questions.
- Different lab methods and cutoffs. Cortisol assays vary (immunoassay vs. LC-MS/MS), and labs choose cutoffs to balance missed cases vs. false alarms. Some labs still use older, higher thresholds in certain settings.
Most Common Normal Cutoff For The 1 mg Overnight Test
The value that shows up again and again in modern guidance is a morning cortisol under 1.8 µg/dL (which is 50 nmol/L) after taking 1 mg dexamethasone the night before. Many clinics describe that as a normal suppressive response for the low-dose overnight test.
You’ll also see a second, looser threshold used in some workflows. Cortisol results between 1.8 and 5 µg/dL are sometimes treated as a gray zone where follow-up testing or a closer look at interfering factors makes sense. Some sources still describe under 5 µg/dL as “normal” for certain contexts, but the tighter 1.8 cutoff is widely used to reduce missed cortisol excess.
If your report lists a different “normal range,” it does not automatically mean the lab is wrong. It means the lab chose a different decision point based on its method and local practice.
Units Trip People Up
Cortisol may be reported as µg/dL or nmol/L. The same cutoff can look different on paper:
- 1.8 µg/dL equals 50 nmol/L.
- 5 µg/dL equals 138 nmol/L.
If you compare two reports, confirm that the units match before you compare the numbers.
Cortisol Suppression Test Normal Values With Real-World Cutoffs
Here’s a practical way to interpret the common versions of dexamethasone suppression testing. Use your lab’s reference interval first, then use this as a plain-language map of what many clinics mean by “normal,” “borderline,” and “not suppressed.”
Table 1: Common DST Formats And Typical Interpretation Cutoffs
| Test Format | How It’s Done | Typical “Suppressed/Normal” Target |
|---|---|---|
| 1 mg overnight DST | 1 mg dexamethasone at night; cortisol drawn around 8 a.m. | < 1.8 µg/dL (50 nmol/L) is commonly used |
| 1 mg overnight DST (older threshold) | Same dosing and timing | < 5 µg/dL (138 nmol/L) may appear on some lab ranges |
| 2-day low-dose DST (2 mg/48 h) | 0.5 mg every 6 hours for 48 hours; cortisol assessed after dosing | Cortisol suppressed, often using the 1.8 µg/dL (50 nmol/L) decision point |
| High-dose overnight DST (8 mg) | Higher dose dexamethasone overnight; cortisol checked next morning | Some protocols look for a large drop from baseline (often >50%), not a single absolute value |
| High-dose 2-day DST | Higher-dose schedule over 2 days in some protocols | Interpretation depends on protocol and clinical question |
| Late-night salivary cortisol (not DST) | Saliva sample near bedtime on one or more nights | Uses lab-specific salivary cutoffs, often with repeat sampling |
| 24-hour urine free cortisol (not DST) | Urine collected for 24 hours, sometimes repeated | Compared to the assay’s normal range; elevation above range drives concern |
| Serum dexamethasone level check (add-on) | Dexamethasone measured with the cortisol draw in some labs | Helps confirm you absorbed the dose when cortisol is borderline |
The takeaway: “normal” for the common overnight test usually means suppression below 1.8 µg/dL (50 nmol/L). Beyond that, the meaning of the number depends on the exact protocol and why the test was ordered.
How To Read Your Result Without Overreacting
Start with three steps that prevent most mix-ups:
- Confirm the test type. Was it the 1 mg overnight test, a 2-day test, or a high-dose follow-up?
- Confirm the units. µg/dL and nmol/L can make the same cutoff look different.
- Match the cutoff to your lab method. Your report’s range is tied to the assay used in that lab.
A Simple Interpretation Pattern People Use
Many clinicians think in “buckets,” especially with the overnight 1 mg test:
- Suppressed. Cortisol under 1.8 µg/dL (50 nmol/L) is often treated as a normal suppressive response.
- Borderline zone. Cortisol between 1.8 and 5 µg/dL may lead to repeat testing, a second screening test, or a check for interference.
- Not suppressed. Cortisol clearly above the local cutoff raises the chance of cortisol excess and usually triggers follow-up testing rather than a final label from one number.
A single abnormal screening result rarely stands alone. Many workups use a repeat DST or a different screening test, since real life includes medication effects and day-to-day variation.
What Can Skew “Normal Values” On Paper
Even when you take dexamethasone exactly as instructed, a few common factors can shift the cortisol result. This matters most in that borderline zone where the decision changes based on small differences.
Medication And Hormone Effects
Some meds change how dexamethasone is broken down, which changes how strong the suppression signal is. Other meds change cortisol-binding proteins, which can raise measured total cortisol even if free cortisol is lower.
Estrogen-containing therapies (including some oral contraceptives) can raise cortisol-binding globulin and increase total serum cortisol readings. In some cases, clinicians pause estrogen therapy before testing or use a strategy that fits the clinical context.
Timing And Dose Issues
DST results depend on taking the dose at the correct time and doing the blood draw on schedule. A late dose, a missed dose, or a delayed morning draw can move the cortisol number.
Shift Work And Sleep Disruption
Cortisol follows a daily rhythm. A night-shift schedule or major sleep disruption can blur that rhythm and make morning levels less predictable. In those cases, clinicians may choose the screening test that best fits your schedule.
Illness And Physiologic Stress
Acute illness can raise cortisol and alter results. If you were sick, on steroids for another reason, or recently had surgery, your clinician may interpret the result differently or postpone testing.
Assay Differences
Different cortisol assays can produce different numbers from the same sample. That’s one reason the printed “normal range” can vary by lab. When results are borderline, some centers use more specific measurement methods or add a dexamethasone level to confirm adequate exposure.
Table 2: Common Reasons A DST Result Looks “Off”
| Potential Issue | What It Can Do | What Clinicians Often Do Next |
|---|---|---|
| Missed or late dexamethasone dose | Less suppression, cortisol reads higher | Repeat the test with clear timing instructions |
| Delayed morning blood draw | Value may not match the protocol cutoff | Repeat with an on-time draw |
| Estrogen therapy (including some OCPs) | Total cortisol can read higher via binding protein effects | Adjust testing approach or timing if appropriate |
| Drugs that speed dexamethasone breakdown | Dexamethasone level may be low, cortisol not suppressed | Review meds; consider dexamethasone level testing |
| Drugs that slow dexamethasone breakdown | Dexamethasone effect may be stronger than expected | Interpret with med list in hand |
| Recent steroid use (pills, inhalers, injections) | Can interfere with cortisol testing and interpretation | Pick a testing window that fits the medication history |
| Acute illness | Cortisol may rise, raising false alarms | Delay testing until recovery when feasible |
| Lab method differences | Different assays yield different numeric cutoffs | Use the lab’s reference and repeat within the same system |
What A “Normal” DST Result Usually Means
If your cortisol suppresses below the lab’s cutoff, it often makes clinically meaningful cortisol excess less likely. It does not rule out every adrenal or pituitary issue. It does mean the negative screen is usually reassuring in the setting it was ordered.
If your cortisol does not suppress, treat it as a signal for follow-up, not a final diagnosis from one lab draw. A clinician may repeat the DST, order late-night salivary cortisol, order a 24-hour urine free cortisol, or review medications and timing to see whether the number is reliable.
Borderline Results: What People Commonly Do Next
Borderline results often come down to one of two stories:
- True cortisol excess is present. The body does not suppress well even with adequate dexamethasone exposure.
- The test conditions were not clean. Medication effects, estrogen effects, timing slips, or illness push the number upward.
That’s why some centers add a dexamethasone level or measure free cortisol in selected cases, especially when total cortisol lands in the gray zone.
Practical Questions To Ask When You Get The Result
These questions keep the follow-up tight and focused:
- Which DST version did I have, and what time was the dose taken?
- What cortisol cutoff does this lab use for suppression?
- Are my units µg/dL or nmol/L?
- Do any of my current medications affect dexamethasone or cortisol measurement?
- Was I ill, sleep-deprived, or recovering from a procedure near the test date?
- What follow-up test fits my situation if the result is borderline?
If you’re reading your result at home, avoid labeling yourself from one number. The DST is a screening tool. Interpretation depends on context, timing, meds, and the lab method.
When To Seek Prompt Medical Review
If your test was ordered due to new symptoms, an adrenal mass, or difficult-to-control metabolic changes, follow up with the clinician who ordered the test. If you have severe symptoms or rapid changes, seek urgent care through local medical services.
References & Sources
- MedlinePlus (NIH).“Dexamethasone suppression test.”Explains why the test is done and how it evaluates cortisol suppression.
- Cleveland Clinic.“Dexamethasone Suppression Test.”Lists common low-dose overnight DST interpretation, including the <1.8 µg/dL suppression target.
- Endocrine Society (JCEM Guideline).“The Diagnosis of Cushing’s Syndrome.”Provides widely used screening thresholds, including cortisol >1.8 µg/dL after 1 mg DST as a criterion that suggests abnormal screening.
- European Society of Endocrinology (ESE).“ESE Clinical Update on Cushing’s Syndrome (Webinar Report).”Summarizes common DST approaches and notes serum cortisol below 50 nmol/L (1.8 µg/dL) as a normal response.