What is the most common cause of Cushing's syndrome?

• A. Iatrogenic long-term corticosteroid therapy
• B. Pituitary adenoma
• C. Adrenal adenoma
• D. Ectopic ACTH-producing tumor

Answer: (A)

Exogenous corticosteroid therapy is the most common cause of Cushing syndrome. When systemic steroids are used long-term, they deliver glucocorticoid effects from outside the body, producing the familiar features of hypercortisolism. This external source suppresses the hypothalamic-pituitary-adrenal axis, so endogenous ACTH falls and the adrenal glands become quiet. That combination—high circulating corticosteroids from outside with low internal ACTH—defines iatrogenic Cushing’s. Other causes, like a pituitary adenoma or ectopic ACTH–producing tumors, generate cortisol endogenously and are much less common. An adrenal adenoma can also cause cortisol excess but is rarer, and in that situation ACTH is typically low due to feedback.