Cortisol is established as a factor in brain volume. -- By "cause and symptom" I was paraphrasing it's permissive, suppressive, stimulatory, and preparative actions in association with the stress response (see Sapolsky, 2000). Cortisol has strong and well known association the acute and severe Major Depressive Disorder. While cortisol hypersecretion does not sufficiently explain the nature of depression, it is a contributing cause that continues to have maladaptive feedback, potentiate nueral pathways, and impair corticosteroid receptors throughout the depression furthering the severity.