ACTH stimulation is valuable in the differential diagnosis of adrenal insufficiency and hypersecretion. In Addisons disease, elevated levels are typical whereas low levels are observed when adrenal insufficiency is secondary to pituitary dysfunction. In Cushings syndrome, the ACTH levels can help to identify the cause of cortisol hypersecretion. ACTH levels are typically low when this is due to lesions or hyperplasia of the adrenal cortex & high when it is due to ectopic ACTH production or hypersecretion of ACTH by the pituitary.