Feeling low or down does not always mean that you have a Major Depression. Sometimes having low energy or feeling down can be caused by an acute (short-term) infection such as a cold or flu. Mood disorders can also be caused by chronic (long-term) medical conditions. Together, mood disorders due to known physiological or medical causes account for as many as 5 to 15% of all treated cases. Unfortunately, many mood disorders due to medical causes go unrecognized until after standard therapies for depression have failed.
Depressive symptoms can be related to and even caused by other medical problems, psychiatric illnesses, substance abuse, or a combination of factors. The overlap between depression and other mental and physical illnesses further complicates the difficult task of trying to find out exactly what is going on with a particular individual who is complaining of depressive symptoms.
To establish whether someone truly suffers from Major Depression, other potential causes of depressed mood must be ruled out (eliminated as possibilities). Some of the other disorders that must be ruled out include General Medical Conditions (like heart disease, endocrine problems and/or recent surgeries), Anxiety Disorders, Substance Abuse, Seasonal Affective Disorder, Borderline Personality Disorder, Premenstrual Dysphoric Disorder, and Schizoaffective Disorder. For example, a person presenting with depression may be suffering from a hormone imbalance or show depressive symptoms related to Borderline Personality Disorder. Appropriate diagnosis, then, requires monitoring of symptomatic people for a period of time to determine whether their depression is best categorized as Major Depression or some other depressive diagnosis. During this monitoring/rule out period, a clinician will need complete laboratory and psychological analyses, and knowledge of the patients' histories and backgrounds in order to make accurate diagnoses.
Mood Disorder Due to a General Medical Condition
Depression can be caused by general medical conditions that affect the body's regulatory systems, or chronic illnesses that cause ongoing pain. In order to meet the DSM diagnosis of Mood Disorder Due to a General Medical Condition, there must be evidence from a person's medical history, physical examination, and laboratory findings that depressive symptoms are a direct consequence of a medical disorder. This diagnosis is particularly important because depression can complicate a person's recovery from a medical condition, and/or increase the risk of that person attempting and completing suicide.
Endocrine and reproductive system disorders are common sources of depressive symptoms. The thyroid, an endocrine gland located in the neck, produces thyroid hormone. People with low levels of thyroid hormone (a condition called hypothyroidism) often experience fatigue, weight gain, irritability, memory loss, and low mood. Treating hypothyroidism by supplementing or replacing thyroid hormone with the medication Synthroid often alleviates depression. Cushing's syndrome, a hormonal disorder caused by prolonged exposure of the body's tissues to high levels of the hormone cortisol, can also cause depressive symptoms.
Reproductive hormones can also influence mood. Since these hormones affect mood primarily in women, the culprit is thought to be the ovary, which secretes estrogens and testosterone. In fact, the role of estrogen in mood disorders is the current focus of many scientific studies. Although the relationship between sex hormones and mood conditions is well documented (See our following discussion concerning PMDD), exactly how sex hormones affect mood is unclear. Additional research is necessary to uncover the link between reproductive hormones and depression.
The number of other medical conditions that have been implicated as causes of depression is staggering. HIV/AIDS, diabetes, arthritis, strokes; and neurological disorders such as Parkinson's disease, Huntington's disease, multiple sclerosis, and Alzheimer's disease can all cause depressive symptoms. About 25-40% of people with neurological conditions will develop depression at some point. Chronic illnesses that are not neurological in nature such as metabolic conditions (e.g. vitamin B12 deficiency), autoimmune conditions (e.g., lupus and rheumatoid arthritis), viral or other infections (hepatitis, mononucleosis, herpes), and certain cancers (e.g., pancreatic) can also cause a person to feel depressed. Day-to-day living with these complicated and sometimes painful diseases can be quite demanding as a person deals with medical appointments, medication management, and modifying daily activities to accommodate fatigue and/or pain. For example, people with severe arthritis may become depressed simply by having to endure their continuing struggle to complete routine tasks such as getting dressed.