The dexamethasone suppression test (DST) assess adrenal gland function by measuring how cortisol levels change in response to oral doses or an injection of dexamethasone.
It’s typical use is to diagnose Cushing’s syndrome.
Dexamethasone is an exogenous steroid that provides negative feedback to the pituitary gland to suppress the secretion of adrenocorticotropic hormone (ACTH).
It binds to glucocorticoid receptors in the anterior pituitary gland, resulting in regulatory modulation.
Overnight DST: An oral dose of dexamethasone is given between 11pm and midnight, and the cortisol level is measured at 8 – 9am the next morning.
Two-day DST: This involves giving an oral dose of dexamethasone at six-hourly intervals for 2 days, with the cortisol level measured 6 hours after the final dose is given.
Low-dose and high-dose variations of the test exist: Low (usually 1–2 mg) and high (8 mg) doses of dexamethasone.
A low dose of dexamethasone can suppress cortisol in individuals with no pathology in endogenous cortisol production.
A high dose of dexamethasone exerts negative feedback on pituitary neoplastic ACTH-producing cells (Cushing’s disease), but not on ectopic ACTH-producing cells or adrenal adenoma.
A normal DST results in a decrease in cortisol levels upon administration of low-dose dexamethasone.
Results with Cushing’s disease involves no changes in cortisol on low-dose dexamethasone, but inhibition of cortisol on high-dose dexamethasone.
If the cortisol levels are unchanged by low- and high-dose dexamethasone, then other causes of Cushing’s syndrome must be considered.