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The gold standard for diagnosis is an 8 am serum cortisol test. According to the Cleveland Clinic, any reading below 15 ug/dL needs further investigation with an ACTH serum test and a cosyntropin stimulation test (also called an ACTH stimulation test.) A flowchart can be found here with a more detailed description: http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/endocrinology/diseases-of-the-adrenal-gland/