Dexamethasone suppression test is a test to understand the function of the hypothalamic-pituitary-adrenal axis. It is used to diagnose cortisolism and identify the cause of Cushing’s syndrome. The clinical significance of dexamethasone test is different. The clinical significance of small dose dexamethasone test is to know whether there is cortisolism. The clinical significance of high-dose dexamethasone test is to identify the cause of Cushing’s syndrome; if the 24h urinary free cortisol and 17-hydroxycorticosteroid content can be suppressed by more than 50%, then the diagnosis of pituitary Cushing’s disease; if not suppressed, then adrenal adenomas, cortical carcinoma, or ectopic corticotrophic hormone tumors. Therefore, the dexamethasone suppression test should be performed in a hospital outpatient clinic.