Cushing syndrome is overexposure of the body to cortisol. This is a rare diagnosis. It is different than Cushing disease (which implies a pituitary ACTH-secreting tumor)
Cushing syndrome can involve pituitary oversecretion or ectopic production of ACTH or corticotropin-releasing hormone. It can also occur due to an adrenal adenoma or carcinoma. Some people develop the condition because of exogenous corticosteroids
Diagnosis involves 2 steps:
Diagnosis and confirmation of hypersecretion of cortisol
Explore differential diagnosis and determine potential etiology
Those with Cushing syndrome exhibit oversecretion of cortisol that is not circadian in nature
Any of these screening tests can be performed: 24 hr urine cortisol, 1-mg overnight dexamethasone suppression test, serum or salivary midnight cortisol
Overnight dexamethasone suppression test is the simplest way of screening (give 1 mg of dexamethasone at 11 pm and measure plasma cortisol at 8 am).
A value of < 5 mg/dL rules out Cushing syndrome
After a positive overnight test, a high-dose dexamethasone suppression test is used to determine the etiology of Cushing syndrome

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