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What are the best tests to screen for cushing syndrome?
Plasma cortisol levels >15 μg/dL in the afternoon or evening (in an unstressed patient) are suggestive of hypercortisolism; British endocrinologists prefer a midnight cortisol level, which can be measured noninvasively in saliva.
Urinary free cortisol values >100 μg/day are abnormal, and values >400 μg/day (more than four times the upper limit of the reference range) are suggestive of Cushing syndrome.
Many clinicians use the overnight dexamethasone suppression test (which is more convenient than the classical “low-dose” test) as a screen for Cushing syndrome. Morning (8 am ) plasma cortisol levels >5 μg/dL are suggestive of Cushing syndrome; patients with levels >1.8 μg/dL are candidates for further testing.
The below table summarizes the usual test results for patients with Cushing syndrome.
Summary of Typical Test Results in the Evaluation of Cushing Syndrome
| PATHOLOGY | PLASMA/URINARY CORTISOL LEVELS | CORTICOTROPIN LEVEL | CORTISOL LEVEL AFTER LOW-DOSE DEXAMETHASONE SUPPRESSION TEST | CORTISOL LEVEL AFTER HIGH-DOSE DEXAMETHASONE SUPPRESSION TEST |
|---|---|---|---|---|
| Pituitary microadenoma (Cushing disease) | Elevated | Elevated | Not suppressed a | Suppressed b |
| Ectopic source of corticotropin | Elevated | Elevated | Not suppressed | Not suppressed |
| Ectopic source of corticotropin-releasing hormone | Elevated | Elevated | Not suppressed | Variable, but usually suppressed |
| Adrenal tumor or hyperplasia | Elevated | Suppressed | Not suppressed | Not suppressed |
| Pseudo-Cushing syndrome | Normal or slightly elevated, often with deranged circadian variation | Variable, but often suppressed | Usually suppressed; formerly used as an “objective” indicator of severity of depression | Suppressed |
a Suppressed: <1.8 μg/dL. b Suppressed: <50% of baseline.
