Late-onset isolated ACTH deficiency is a rare, acquired, pituitary hormone deficiency characterized by secondary adrenal insufficiency, with normal secretion of anterior pituitary hormones, except for ACTH. Patients present with weakness, fatigue, weight loss, anorexia, vomiting/nausea, hypoglycemia, and abnormally low serum ACTH and cortisol levels. Association with autoimmune disease such as Hashimoto's thyroiditis has been described.