Table Confirmatory Serum Testing for Addison Disease Test Result Plasma ACTH High (≥ 50 pg / mL [> 11 pmol / L]) Serum cortisol Low (< 5 mcg / dL [< 138 nmol / L]) ACTH stimulation test Subnormal (ie, 30-minute cortisol should be < 15 - 18 mcg / dL [< 414 - 497 nmol / L], according to the assay) Prolonged (24-hour) ACTH stimulation test Cortisol should be subnormal at 1 hour and should not rise further at 24 hours ACTH = adrenocorticotropic hormone.