Possible factors include the following: Greater exposure to or heightened response to daily stresses Higher levels of monoamine oxidase (the enzyme that degrades neurotransmitters considered important for mood) Higher rates of thyroid dysfunction Endocrine changes that occur with menstruation and at menopause In peripartum-onset depression, symptoms develop during pregnancy or within 4 weeks after delivery (postpartum depression); endocrine changes have been implicated, but the specific cause is unknown.