Overnight Dexamethasone Suppression Test
The overnight dexamethasone suppression test screens for Cushing’s syndrome with a single 1 mg tablet taken at night and a morning cortisol measurement; it is sensitive but produces false positives, so it is used selectively.
Plain English
Patient-Friendly Explanation
The overnight dexamethasone suppression test is a simple way of checking whether the body is producing too much of the hormone cortisol. Cortisol is made by the adrenal glands under instructions from a control centre in the brain, and normally its level falls overnight. Dexamethasone is a synthetic copy of cortisol that should switch that control centre off. The test is done by taking one milligram of dexamethasone between eleven at night and midnight, then having a blood sample taken between eight and nine the next morning to measure cortisol. If the control system is working normally, the morning cortisol will be very low, usually below fifty nanomoles per litre. If it stays high, the body may be making cortisol on its own, which is the pattern of Cushing’s syndrome. The test matters in a weight clinic because Cushing’s syndrome is one of the few hormonal conditions that genuinely causes weight gain, particularly around the abdomen, along with easy bruising, purple stretch marks, muscle weakness and a rounded face. It is uncommon, which shapes how the test should be used. At the standard cut-off the test correctly identifies almost everyone who has the condition, but it also flags some people who do not, because obesity itself, depression, alcohol use, oral contraceptives, some medicines and acute illness can all keep morning cortisol high. In a study of just over a thousand patients with obesity, about one in eighty had a positive test but only about one in five hundred turned out to have Cushing’s syndrome, and the authors recommended testing people with obesity who also have diabetes, high blood pressure or abnormal cholesterol rather than everyone with obesity. For this reason the test is used selectively, and a positive result is never a diagnosis on its own: it leads to repeat testing with a twenty-four hour urine cortisol collection, a late-night saliva cortisol, or a two-day version of the same test. Practical details matter, too. The tablet has to be swallowed - a surprising proportion of abnormal results come from not taking it, or from medicines that speed its clearance - and measuring the dexamethasone level in the same blood sample can show whether the test was valid.
Medical
Clinical Definition
The 1 mg overnight dexamethasone suppression test (ODST) is performed by giving 1 mg of dexamethasone orally between 2300 and 2400 h and measuring serum cortisol between 0800 and 0900 h the following morning. Adequate suppression is generally defined as a serum cortisol below 50 nmol/L (1.8 micrograms/dL), a threshold derived from healthy-subject data and adopted by the Endocrine Society in its 2008 guideline; specificity at this cut-off is only 60-80% because obesity, alcohol use, psychiatric illness, advanced age, oral contraceptives, oestrogen therapy, CYP3A4 inducers such as carbamazepine, phenytoin or rifampicin and acute illness all raise morning cortisol without true hypercortisolism. Obesity-specific validation in 86 patients with body mass index 30 to 53 kg/m2 using a stricter cut-off of 80 nmol/L reported a false-positive rate of 2.3%, close to the approximately 1% seen in non-obese populations, and concluded that the ODST remains a valid screen in obesity. A 2024 study of 1,008 patients with obesity found that 1.2% had a positive 1 mg ODST and that Cushing’s syndrome was proven in only 0.2%, with 100% sensitivity and 99% specificity, and concluded that screening is reasonable in the metabolically unhealthy phenotype - obesity with diabetes, hypertension, dyslipidaemia or coronary disease - rather than in all patients with obesity. Reported prevalence of Cushing’s syndrome in obesity cohorts ranges from 0.2% to about 5%, with the higher figures from selected referral populations. A positive test is followed by confirmatory testing: 24-hour urinary cortisol (sensitivity 95-100% and specificity around 98% when the collection is complete), late-night salivary cortisol, or the 2-day 2 mg dexamethasone suppression test, followed by plasma ACTH and imaging when hypercortisolism is confirmed. Simultaneous measurement of serum dexamethasone identifies invalid tests; in a series of 1,300 tests, 11.2% were invalid, mostly because the dexamethasone was not taken, was taken with glucocorticoids, or was cleared faster by CYP3A4-inducing drugs.
Key Insight
Why It Matters
Cushing’s syndrome is rare, but missing it means missing a treatable cause of weight gain, diabetes and high blood pressure, so selective screening is worthwhile. Just as important is not over-diagnosing: at the standard cut-off many people with obesity have an abnormal result without having the disease, and an isolated positive test can cause unnecessary anxiety and further investigation. In this clinic the test is offered when the history and examination suggest it - central weight gain with easy bruising, proximal muscle weakness, purple striae, or obesity with poorly controlled diabetes and hypertension at a young age - and a positive result is always confirmed by a second, different test.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: October 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
