Dexamethasone suppression test
The dexamethasone suppression test is used to assess adrenal gland function by measuring how cortisol levels change in response to an injection of dexamethasone. It is typically used to diagnose Cushing's syndrome.
The DST was historically used for diagnosing depression, but by 1988 it was considered to be "at best, severely limited in its clinical ability" for this purpose.
Physiology
Dexamethasone is an exogenous steroid that provides negative feedback to the pituitary gland to suppress the secretion of adrenocorticotropic hormone. Specifically, dexamethasone binds to glucocorticoid receptors in the anterior pituitary gland, which lie outside the blood-brain barrier, resulting in regulatory modulation.Interpretation
Low-dose and high-dose variations of the test exist. The test is given at low and high doses of dexamethasone, and the levels of cortisol are measured to obtain the results.A low dose of dexamethasone suppresses cortisol in individuals with no pathology in endogenous cortisol production. A high dose of dexamethasone exerts negative feedback on pituitary neoplastic ACTH-producing cells, but not on ectopic ACTH-producing cells or adrenal adenoma.
Dose
A normal result is a decrease in cortisol levels upon administration of low-dose dexamethasone. Results indicative of Cushing's disease involve no change 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 with further work-up necessary.After the high-dose dexamethasone, it may be possible to make further interpretations.
| Cortisol | ACTH† | Interpretation |
| is not suppressed by low or high doses | Undetectable or low | Primary hypercortisolism is likely; Cushing's syndrome, not disease |
| is not suppressed by low doses, but is suppressed by high doses | Normal to elevated but not in hundreds | Cushing's disease should be considered because the pituitary still retains some feedback control. A pituitary MRI would be needed to confirm. |
| is not suppressed by high or low doses | Elevated in hundreds | Ectopic ACTH syndrome is likely. If an adrenal tumor is not apparent, a chest CT and abdominal CT is indicated to rule out a different tumor secreting ACTH. |
†ACTH as measured prior to dosing of dexamethasone
Equivocal results should be followed by a corticotropin-releasing hormone stimulation test, with inferior petrosal sinus sampling.