What are the causes of Cushings syndrome?
Cushing’s syndrome (hypercortisolism) is ACTH dependent in 80% to 85% of cases, leading to excessive cortisol production by the adrenal glands and adrenal hyperplasia. Eighty percent to 85% of such cases are caused by a hyperfunctional pituitary adenoma (in which case the hypercortisolism is known as Cushing’s disease), and 10% to 15% of such cases are caused by ectopic (nonpituitary) ACTH-secreting tumors (including small cell lung carcinoma, bronchial carcinoid tumor, thymoma, pancreatic neuroendocrine tumor, and medullary thyroid carcinoma).
ACTH-independent Cushing’s syndrome occurs in 15% to 20% of cases, and is most often secondary to a hyperfunctional adrenal adenoma, followed by a hyperfunctional ACC, and least commonly by adrenal hyperplasia. Exogenous administration of glucocorticoids is an additional cause of ACTH-independent Cushing’s syndrome.