Causes of Cushing's Syndrome are as follows:   Looking at causes of Cushing Syndrome on basis of plasma ACTH levels, later helps in understanding the approach to investigations performed in diagnosis of Cushing's disease .   First use of exogenous steroids should be ruled out. Iatrogenic use of steroids is common cause of Cushing's Syndrome. Exogenous steroids cause feedback inhibition of ACTH secretion.   If plasma ACTH level is undetectable and there is no exogenous use of steroids then the source of increased cortisol level is likely an adrenal adenoma/ cancer / hyperplasia . The feedback inhibition due to increased cortisol levels make ACTH levels undetectable.   If ACTH levels are increased then it means the cause of high cortisol levels is high ACTH levels. This increased ACTH is due to either pituitary cause or ectopic ACTH secretion .   What are causes of ectopic ACTH secr...