The overnight dexamethasone suppression test is a method used to help diagnose Cushing syndrome. Cushing syndrome is a condition caused by prolonged high levels of cortisol in the body. Cortisol is a hormone produced by the adrenal glands that affects various body functions such as blood pressure and memory. Dexamethasone is a synthetic version of cortisol, and the idea behind the test is that after taking dexamethasone, the body's cortisol level should drop. The test is usually done overnight, where the patient takes dexamethasone between 11 p.m. and midnight the night before, and a blood sample is taken between 8 a.m. and 9 a.m. the next morning to measure cortisol levels. If the cortisol level is below 1.8 mcg/dL in the low-dose test, it indicates no Cushing syndrome; if it is higher, it suggests the presence of Cushing syndrome. The high-dose test is used to determine the specific cause of Cushing syndrome, and if cortisol levels drop by more than 50%, it likely indicates a pituitary tumor.