The most common cause of Cushing's syndrome is:

The most common cause of Cushing's syndrome is iatrogenic corticosteroid use, due to the widespread clinical use of glucocorticoids for inflammatory and autoimmune conditions. This exogenous steroid intake leads to suppression of the hypothalamic-pituitary-adrenal (HPA) axis, causing symptoms of cortisol excess. Unlike endogenous causes, this form is reversible upon discontinuation or tapering of steroids. Recognizing iatrogenic Cushing's is crucial, as management primarily involves adjusting steroid therapy to minimize complications. Surgical intervention is not typically required in these cases since the cause is drug-induced rather than structural.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.