(DSTs) tests based on the principle that appropriate doses of dexamethasone can suppress the output of cortisol from the adrenal glands in the normal state and that this ability is reduced or lost in Cushing’s syndrome. In the overnight DST 1 mg of dexamethasone is administered at midnight and the serum cortisol level is measured at 9.00 am the next morning. Failure to suppress cortisol output may indicate Cushing’s syndrome but also occurs in patients with obesity and depressive illness. In the low-dose DST (0.5 mg dexamethasone every 6 hours for 48 hours), cortisol suppression occurs in patients with obesity and depression but not in those with Cushing’s syndrome. In the high-dose DST (2 mg dexamethasone every 6 hours for 48 hours), cortisol is suppressed in patients with Cushing’s disease (in which excess amounts of ACTH are secreted by the pituitary gland) but not in those with Cushing’s syndrome due to other causes. Although the low- and high-dose tests are unreliable, all three tests should be performed to aid the diagnosis of Cushing’s syndrome.
Subjects: Medicine and Health.