Dexamethasone Suppression Test


Screen/diagnose Cushing’s syndrome.


The test is done in the outpatient setting.

  1. The patient takes dexamethasone 1 mg at bedtime (around 11 pm).
  2. Cortisol level is measured at 8 to 9 am the morning after taking dexamethasone.


Cortisol level after dexamethasone:

  • < 50 nmol/L – rules out adrenal insufficiency (Sn 100%; Sp 91.9%). [Ceccato 2015]
  • 50-140 nmol/L – borderline, possible subclinical adrenal Cushing’s syndrome.
  • > 140 nmol/L – abnormal test, consistent with Cushing’s syndrome (Sn 86.8%; Sp 96.7%). [Ceccato 2015]


  • Drugs that increase cortisol binding globulin (CBG) may lead to a false-positive result. Examples include estrogen containing oral contraceptive pills and mitotate. Pregnancy will also lead to elevated CBG due to high estrogen levels.
  • Dexamethasone metabolism is accelerated by CYP 3A4 inducers leading to false-positive results and inhibited by CYP 3A4 inhibitors leading to false-negative results. Some examples:
Inducers, rapid dexamethasone metabolismInhibitors, slow dexamethasone metabolism
Phenobarbital, Phenytoin, Carbamazepine, PrimidoneAprepitant/fosaprepitant
Rifampin Rifapentine EthosuximideItraconazole

See this list of a CYP inducers and inhibitors.


  • 24-hour urinary free cortisol level
  • Midnight salivary cortisol levels


  • Ceccato F, Barbot M, Zilio M, Frigo AC, Albiger N, Camozzi V, Antonelli G, Plebani M, Mantero F, Boscaro M, Scaroni C. Screening Tests for Cushing’s Syndrome: Urinary Free Cortisol Role Measured by LC-MS/MS. J Clin Endocrinol Metab. 2015;100(10):3856.