A serious and common mood disorder, depression affects 121 million people in the world, according to the World Health Organization (WHO). The organization adds that depression is the second cause of global burden of disease in 15- to 44-year-olds. This mood disorder can range from mild to severe and can result in suicidal thoughts and behaviors. Depression can be caused by multiple factors, such as genetics, life events, neurotransmitter levels, changes in hormones and drug use.
Genetics
The National Institutes of Health (NIH) states that depression tends to run in families, which can be due to genes and/or learned behaviors. For example, if a patient grows up seeing one or more parents depressed, she may reproduce the behaviors, thinking of them as "normal." If a person has a genetic predisposition for depression, she will be more likely to develop depression when faced with another cause of depression. In addition, women are twice as likely to be diagnosed with depression.
Life Events
Certain events during life can trigger depression. The Mayo Clinic notes that a death of a loved one or high levels of stress can be reasons for depression. During the teenage years, a breakup or disappointment in school also can result in depression. Traumatic events during childhood, such as abuse or neglect, can cause changes to the brain, making a person more susceptible to depression.
Neurotransmitter Levels
Certain neurotransmitters--such as serotonin, dopamine and norepinephrine--are linked to mood. When there are low levels of these neurotransmitters, it results in depression. As a result, depression can be treated with antidepressants, which increase the levels of one or more neurotransmitters in the brain.
Changes in Hormones
The Mayo Clinic states that changes in hormone levels can affect mood. For example, menopause, when women experience changes in hormone levels including a decrease in estrogen, can result in depression symptoms. Thyroid problems also can be a reason for depression. The NIH notes that hypothyroidism, in which the thyroid does not produce enough hormones, can cause depression.
Drug Use
Drugs use is another reason for the onset of depression. Alcohol, sedatives and benzodiazepines are depressants, meaning they decrease the function of the central nervous system. When abused, these drugs can result in depression. If a patient already has depressive symptoms, using alcohol and drugs can worsen those symptoms.
