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Why Is Depression Comorbid With Physical Illness?

The relationship between depression and physical illness is complex. Depression can be a physical consequence of an illness, an emotional reaction to the disabling effects of illness or a contributing cause of illness. To further complicate the situation, both depression and illness can be caused by yet other factors.

  1. Comorbidity

    • According to the U.S. Department of Health and Human Services, the rate of depression in the general population is estimated to be between 4.5 and 8 percent. Studies that examine the incidence of depression among the physically ill find rates ranging from 18 to 36 percent.

    Psychological Response to Illness

    • Depression can be a psychological response to illness. Physical pain and discomfort, loss of mobility, loss of ability to perform day-to-day functions normally and loss of income are just a few of the consequences of physical illness that trigger feelings of sadness, loss and depression.

    Illnesses Cause Depression

    • A wide range of illnesses share causal pathways with depression, and so directly cause depression. Diseases of the nervous system such as Parkinsons, dementia and multiple sclerosis; vascular diseases such as stroke and hypertension; endocrine disorders such as hypothryroidism, adrenal disorders and Cushing's syndrome; and metabolic illnesses such as diabetes, all affect brain and body chemistry to cause depression in those who are vulnerable.

    Depression Causes Illness

    • Depression has direct effects on health. Depression lowers immune system functioning. The immune system is the body's defense system, and compromise of immune-system functioning associated with depression has been implicated in disorders as diverse as arthritis, environmental allergies, the common cold, heart disease and cancer. Further, depression affects health behaviors, such as sleep, diet and exercise.

    Common Etiologies

    • Depression and illness can both be caused by certain common factors. For example, dietary deficiencies can contribute to both illness and depression. Likewise, hyperactivity, stress and a history of trauma can cause changes in the hypothalamic-pituitary-adrenal system, which in turn becomes a contributing cause to both depression and illnesses.

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