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.

It has already been established that long term secretion of cortisol (both the cause and symptom of depression) will decrease the volume of the brain. For example, international flight attendants that don't get proper time off over a two week period will show a reduction in their hypothalamus volume. -- (if remember correctly, the volume loss is not associated with becoming dumber, but maybe just short term working memory decrease from stress)