Cortisol is important for controlling blood pressure, immune function, metabolism and the body's stress response, according to the National Endocrine and Metabolic Diseases Information Service. Low levels of cortisol can be due to Addison's disease, which is characterized by the body's inability to produce sufficient cortisol; hypopituitarism, which is a failure of the pituitary gland to signal the adrenal glands to produce cortisol; or from glucocorticoid drugs, which suppress normal pituitary or adrenal gland function and are contained in pills, skin creams, eyedrops, inhalers, joint injections and chemotherapy, according to MedlinePlus.
The gradual destruction of the adrenal cortex by the body's own immune system causes most cases of Addison's disease, reports MedicineNet. Primal adrenal insufficiency results when 90 percent of the adrenal cortex has been destroyed, rendering the gland incapable of producing glucocorticoid (cortisol) and mineralocorticoid (aldosterone) hormones. Insufficient secretion of ACTH by the pituitary gland can also cause Addison's disease.