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- Main article: Clinical depression
- Hormonal factorsThe levels of hormones, the chemical messengers in the body that help regulate metabolism, have been linked to depression
- Postpartum depression (also known as postnatal depression) – Dr. Ruta M Nonacs writes that while many women experience some mood changes after giving birth, "10-15% of women experience a more disabling and persistent form of mood disturbance (eg, postpartum depression, postpartum psychosis)."  When it occurs, the onset typically is within three months after delivery, and it may last for several months. About two new mothers out of a thousand experience the more serious depressive disorder Postnatal Psychosis which includes hallucinations and/or delusions.
- Medical conditions – Certain illnesses, including cardiovascular disease, hepatitis, mononucleosis, hypothyroidism, and organic brain damage caused by degenerative conditions such as Parkinson disease or by traumatic blunt force injury may contribute to hormonal changes that may lead to depression, as may certain prescription drugs such as birth control pills and steroids. Gender dysphoria can also cause depression.
- ↑ Manev, R, Manev H (2004). 5-Lipoxygenase as a putative link between cardiovascular and psychiatric disorders. Critical Reviews in Neurobiology 16 (1�2): 181�6.