Dexamethasone Suppression Test

Excerpt

The dexamethasone suppression test (DST) is used in the evaluation of endogenous Cushing syndrome (CS) by assessing for the lack of suppression of the hypothalamic-pituitary-adrenal (HPA) axis by exogenous corticosteroids. The first use of dexamethasone for assessing CS was in 1960 by Liddle. He developed a test based on the lack of suppression of endogenous cortisol production in CS versus the physiologic suppression in nonaffected individuals achieved by dexamethasone.

Dexamethasone is a potent synthetic corticosteroid (dexamethasone 0.75 mg = prednisone 5 mg = methylprednisolone 4 mg = hydrocortisone 20 mg) with high affinity for the glucocorticoid receptors and long duration of action (biological half-life 36 to 54 hours; plasma half-life 4 to 5 hours). It possesses minimal mineralocorticoid activity, and unlike other glucocorticoids, it does not interfere with cortisol measurement in the plasma, urine, or saliva. These qualities make dexamethasone the preferred steroid for assessing the HPA axis.

The dexamethasone suppression test is also used to investigate mild autonomous cortisol excess (MACE) in patients with incidentalomas. For the diagnosis of CS and also MACE, a serum cortisol concentration of 1.8 µg/dL (50 nmol/L) is the widely recommended cut-off that increases the diagnostic sensitivity of the test to approximately 95%. Serum cortisol concentrations <1.8 µg/dL suggest adequate HPA axis suppression post dexamethasone and exclude CS.