In Addison's disease, the most diagnostic test is -

The best method used for diagnosis of adrenal insufficiency is ACTH stimulation test. Addison's disease is produced due to deficiency of serum cortisol, but the measurement of these hormones is not the best way to diagnose addison's disease. Addison's disease is best diagnosed by ACTH stimulation test. The ACTH stimulation test detects the adrenal deficiency at very early stage of addisons disease when serum cortisol levels are normal. During the early phase of addison's disease, the serum cortisol levels are normal, but the adrenal reserve is decreased. During these phases the adrenal deficiency is precipitated by stress. ACTH stimulation can expose the cortisol deficiency even at these stages  "Upon ACTH stimulation the increase in cortisol level is less than normal." ACTH stimulation test is performed by assessing cortisol response upon administering 250 IV of consyntopin  i.v. or i.m. The criteria for normal response is serum cortisol level >500 nmol/l (>18 µg/d1). If the response is subnormal addison's disease is diagnosed. To further differentiate between primary and secondary addison's disease, serum aldosterone level is measured from the same serum sample. Serum aldosterone Primary addison's disease    → Decreased Secondary addison's disease    →     Normal