Due to another medical condition
"Mood disorder due to a general medical condition" is used to describe manic or depressive episodes which occur secondary to a medical condition. There are many medical conditions that can trigger mood episodes, including neurological disorders (e.g. dementias), metabolic disorders (e.g. electrolyte disturbances), gastrointestinal diseases (e.g. cirrhosis), endocrine disease (e.g. thyroid abnormalities), cardiovascular disease (e.g. heart attack), pulmonary disease (e.g. chronic obstructive pulmonary disease), cancer, and autoimmune diseases (e.g. multiple sclerosis). Not otherwise specified
Mood disorder not otherwise specified (MD-NOS) is a mood disorder that is impairing but does not fit in with any of the other officially specified diagnoses. In the DSM-IV MD-NOS is described as "any mood disorder that does not meet the criteria for a specific disorder." MD-NOS is not used as a clinical description but as a statistical concept for filing purposes. The diagnosis of MD-NOS does not exist in the DSM-5, however the diagnoses of unspecified depressive disorder and unspecified bipolar disorder are in the DSM-5.Most cases of MD-NOS represent hybrids between mood and anxiety disorders, such as mixed anxiety-depressive disorder or atypical depression. An example of an instance of MD-NOS is being in minor depression frequently during various intervals, such as once every month or once in three days. There is a risk for MD-NOS not to get noticed, and for that reason not to get treated. Causes
Meta-analyses show that high scores on the personality domain neuroticism are a strong predictor for the development of mood disorders.
It is argued that humans have retained the instinct to experience low mood during the winter months, even if the availability of food is no longer determined by the weather.Much of what is known about the genetic influence of clinical depression is based upon research that has been done with identical twins. Identical twins have exactly the same genetic code. It has been found that when one identical twin becomes depressed the other will also develop clinical depression approximately 76% of the time. When identical twins are raised apart from each other, they will both become depressed about 67% of the time. Because both twins become depressed at such a high rate, the implication is that there is a strong genetic influence. If it happened that when one twin becomes clinically depressed the other always develops depression, then clinical depression would likely be entirely genetic.Bipolar disorder is also considered a mood disorder and it is hypothesized that it might be caused by mitochondrial dysfunction. Sex differences
Mood disorders, specifically stress-related mood disorders such as anxiety and depression, have been shown to have differing rates of diagnosis based on sex. In the United States, women are two times more likely than men to be diagnosed with a stress-related mood disorder. Underlying these sex differences, studies have shown a dysregulation of stress-responsive neuroendocrine function causing an increase in the likelihood of developing these affective disorders. Overactivation of the hypothalamic-pituitary-adrenal (HPA) axis could provide potential insight into how these sex differences arise.