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 CST (peak cortisol level < 18 μg/dL) or morning serum cortisol < 3 mcg/dL with an appropriate clinical circumstance (e.g. Sheehan's syndrome, pituitary surgery and/or irradiation, bilateral adrenalectomy, etc.) that strongly supports a true diagnosis of primary or central adrenal insufficiency, and current use of physiologic, replacement dose glucocorticoids. Inpatients with non-critical illness and outpatients are eligible 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. Inpatients with non-critical illness and outpatients are eligible

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 - Uncontrolled major depressive disorder or psychiatric diseases, severe malnutrition, eating disorders, 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, aminoglutethimide and mitotane - Active medical treatment of Cushing's syndrome (status-post surgical treatment is acceptable) - Uncontrolled hypo- or hyperthyroidism - Use of biotin within the past 72 hours - Regular alcohol and/or cannabis use - Be assessed by the investigators as unsuitable for participation in this study for any reason Additional Exclusion Criteria for healthy volunteers only: - Regular use of any oral glucocorticoid (e.g. hydrocortisone, prednisone, dexamethasone) within 6 weeks of the screening visit - Use of oral or nasal steroid (glucocorticoid) inhalers in the past 2 weeks - Regular use of any steroid creams, gels, ointments, or lotions - Use of steroid (glucocorticoid) injections in the past 6 months (healthy volunteers) - Regular use of opioids - Regular use of suboxone - Regular use of megestrol acetate