ACTH dependent Cushing syndrome is caused by : ACTH producing pituitary adenoma Ectopic ACTH Bronchial or pancreatic carcinoid Small cell cancer of lung Medullary carcinoma of thyroids Pheochromocytoma ACTH Independent Cushing syndrome is caused by : Adrenocoical adenoma/ carcinoma Primary pigmented nodular adrenal disease Adrenal hyperplasia McCune Albright Syndrome Petrosal/Peripheral ACTH ratio > 2 indicates the presence of a pituitary adenoma-secreting ACTH. NOTE: Overall the most common cause of Cushing's syndrome is Iatrogenic steroids but if asked regarding ACTH dependent cause then answer should be given as Pituitary adenoma.