How is Cushing's Syndrome Diagnosed? Cushing's Syndrome is diagnosed with Low Dose Dexamethasone Suppression Test. Cushing's Syndrome is caused when there is excess cortisol, a glucocorticoid hormone secreted by adrenal cortex. Why Random cortisol levels cannot be used to diagnose Cushing's Syndrome? One cannot simply measure random plasma cortisol to make a diagnosis of excess cortisol hormone in the body because cortisol levels fluctuate throughout the day, and can also be altered by other factors like illness and stress. Thus, the diagnosis of Cushing's Syndrome is made with Overnight Dexamethasone Suppression Test. What is overnight low dose dexamethasone test? It is a good outpatient test in which low dose ( 1 milligrams) of Dexamethasone is given at midnight and serum cortisol levels are measured next morning ( at 8 AM) . In normal individuals the midnight dose of dexamethasone suppresses the cortisol levels to <50 nmol/L
In the first step the diagnosis of Cushing's Syndrome is established with either low dose dexamethasone suppression test or 24 hour urinary free cortisol test. Then in second step, various localization tests are performed to pinpoint the exact cause of Cushing's Disease. The diagnosis of Cushing's Syndrome is made if there is NO suppression of morning plasma cortisol levels after a midnight low dose of dexamethasone. Alternatively, raised levels of 24 hour urinary free levels of cortisol is used. Then to localize the cause of Cushing's Syndrome , first step is to measure plasma ACTH levels. If plasma ACTH level is undetectable then the source of increased cortisol level is likely an adrenal tumor/ hyperplasia. The feedback inhibition due to increased cortisol levels make ACTH levels undetectable. Next step is to go for imaging studies - Adrenal CT/MRI. If no mass is found, go for adrenal vein sampling. If ACTH l