Adrenal insufficiency is a syndrome of cortisol deficiency and is categorized as primary, secondary, or glucocorticoid induced. Although primary and secondary adrenal insufficiency are rare, affecting less than 279 per 1 million individuals, glucocorticoid-induced adrenal insufficiency is common. Primary adrenal insufficiency, which involves deficiency of all adrenocortical hormones, is caused by autoimmune destruction, congenital adrenal hyperplasia, pharmacological inhibition (eg, high doses of azole antifungal therapy), infection (eg, tuberculosis, fungal infections), or surgical removal of adrenal cortical tissue. Secondary adrenal insufficiency is caused by disorders affecting the pituitary gland, such as tumors, hemorrhage, inflammatory or infiltrative conditions (eg, hypophysitis, sarcoidosis, hemochromatosis), surgery, radiation therapy, or medications that suppress corticotropin production, such as opioids. Glucocorticoid-induced adrenal insufficiency is caused by administration of supraphysiological doses of glucocorticoids. Patients with adrenal insufficiency typically present with nonspecific symptoms, including fatigue (50%-95%), nausea and vomiting (20%-62%), and anorexia and weight loss (43%-73%). Glucocorticoid-induced adrenal insufficiency should be suspected in patients who have recently tapered or discontinued a supraphysiological dose of glucocorticoids. Early-morning (approximately 8 am) measurements of serum cortisol, corticotropin, and dehydroepiandrosterone sulfate (DHEAS) are used to diagnose adrenal insufficiency. Primary adrenal insufficiency is typically characterized by low morning cortisol levels (<5 µg/dL), high corticotropin levels, and low DHEAS levels. Patients with secondary and glucocorticoid-induced adrenal insufficiency typically have low or intermediate morning cortisol levels (5-10 µg/dL) and low or low-normal corticotropin and DHEAS levels. Patients with intermediate early-morning cortisol levels should undergo repeat early-morning cortisol testing or corticotropin stimulation testing (measurement of cortisol before and 60 minutes after administration of cosyntropin, 250 µg). Treatment of adrenal insufficiency involves supplemental glucocorticoids (eg, hydrocortisone, 15-25 mg daily, or prednisone, 3-5 mg daily). Mineralocorticoids (eg, fludrocortisone, 0.05-0.3 mg daily) should be added for patients with primary adrenal insufficiency. Adrenal crisis, a syndrome that can cause hypotension and shock, hyponatremia, altered mental status, and death if untreated, can occur in patients with adrenal insufficiency who have inadequate glucocorticoid therapy, acute illness, and physical stress. Therefore, all patients with adrenal insufficiency should be instructed how to increase glucocorticoids during acute illness and prescribed injectable glucocorticoids (eg, hydrocortisone, 100 mg intramuscular injection) to prevent or treat adrenal crisis. Although primary and secondary adrenal insufficiency are rare, glucocorticoid-induced adrenal insufficiency is a common condition. Diagnosis of adrenal insufficiency involves early-morning measurement of cortisol, corticotropin, and DHEAS. All patients with adrenal insufficiency should be treated with glucocorticoids and instructed how to prevent and treat adrenal crisis.