Adrenocorticotropic hormone (ACTH) deficiency, sometimes referred to as secondary adrenal insufficiency, is a rare (affecting fewer than 1 in 100,000) and potentially life-threatening form of adrenocortical failure in which there is partial or complete lack of ACTH production and secretion by the anterior pituitary gland. ACTH acts to stimulate release of cortisol from the adrenal cortex during both the diurnal rhythm and exposure to stressors. Onset may occur throughout the life span.