Inclusion Criteria for healthy volunteers:  Male and female outpatients 18 years or older without prior diagnosis of primary or central adrenal insufficiency.  Inclusion Criteria for patients with known adrenal insufficiency:  Males and females 18 years or older with established diagnosis of primary or central adrenal insufficiency as previously documented in the electronic medical record by a failed cosyntropin stimulation test (peak cortisol level < 18 μg/dL) and current use of physiologic, replacement dose glucocorticoids.  Inclusion Criteria for patients with suspected adrenal insufficiency:  Males and females 18 years or older with suspected diagnosis of primary or central adrenal insufficiency by any cause based on clinical evaluation by a study team member.  Exclusion Criteria for all groups:  Moderate to severe liver impairment or abnormal albumin or cortisol binding globulin concentrations Renal impairment with eGFR < 60 mL/min/1.73m2 and/or diagnosis of nephrotic syndrome Pregnancy or nursing mothers Use of estrogen preparations Major depressive disorder, severe malnutrition, anorexia, chronic fatigue syndrome (disorders that alter HPA axis function) Use of any medications that induce hepatic cytochrome P-450 enzymes, e.g., barbiturates, phenytoin, rifampin, amino-glutethimide and mitotane Active medical treatment of Cushing's syndrome (status-post surgical treatment is acceptable) Use of oral or nasal steroid inhalers in the past two weeks (healthy volunteers and patients with known adrenal insufficiency only)* Regular use of hydrocortisone cream. Use of steroid injections in the past 6 months (healthy volunteers and patients with known adrenal insufficiency only) * Regular use of alcohol (healthy volunteers and patients with known adrenal insufficiency only)* Regular use of opioids (healthy volunteers and patients with known adrenal insufficiency only)* Regular use of suboxone (healthy volunteers and patients with known adrenal insufficiency only)* Regular use of megestrol acetate (healthy volunteers and patients with known adrenal insufficiency only)* Use of biotin in the past 72 hours Uncontrolled hypo- or hyperthyroidism