Depression and illness: Chicken or egg?

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When depression strikes, doctors usually probe what's going on in the mind and brain first. But it's also important to check what's going on in the body, since certain medical problems are linked to mood disturbances. In fact, medical illnesses — and medication side effects — may be behind nearly 10% to 15% of all cases of depression.

It's not uncommon for a physical illness to trigger depression. Up to half of heart attack survivors and those with cancer report feeling blue, and many are diagnosed with depression. Many people who have diabetes, Parkinson's, or other chronic conditions become depressed.

It works in the other direction, too. Depression can affect the course of a physical disease. Take heart disease—depression has been linked with slower recovery from a heart attack and an increased risk for future heart trouble.

Here's another chicken-or-egg example. Two common thyroid disorders are well known to affect mood. If the thyroid makes too much hormone (hyperthyroidism), manic symptoms can result. If the gland makes too little thyroid hormone (hypothyroidism), exhaustion and depression can appear. Treating thyroid disease can often relieve the mood problems.

The list doesn't stop there. Other medical conditions associated with mood disorders include certain neurological conditions (multiple sclerosis, Parkinson's disease, Alzheimer's), other hormonal imbalances, and some nutritional deficiencies, such as a lack of vitamin B12.

The take-home message is that if you have depression, or think you might, a thorough physical exam and careful medical history could help pinpoint a physical source of the problem—and the most appropriate treatment.

For more on diagnosing and finding the right treatment on the different types of depression, buy Understanding Depression, a Special Health Report from Harvard Medical School.