Everyone goes through an occasional bout of the blues, but for some people, these spells of sadness are actually depression, a serious mental illness that can cause tremendous pain and suffering. People with depression often stop enjoying events and activities that once were pleasurable. For some, just getting through each day can become a challenge. In severe cases, suicide can be the devastating consequence.

According to the National Institute of Mental Health, almost 19 million people in the United States suffer from a depressive illness in a given year — almost 10 percent of the population. Women are affected twice as often as men. Depression is also the leading cause of disability in the world, according to the World Health Organization.

It's easy to mistake hypothyroidism for depression. After all, depression can be a symptom of thyroid disease, usually hypothyroidism. And both conditions can bring on fatigue, memory problems, and trouble sleeping. They can also cause weight gain and inexplicable aches and pains. Take Mary Ann, who experienced severe depression before she was diagnosed with hypothyroidism:

Mary Ann and her husband had been married for only a year when she sank into a deep funk. She cried all the time and never felt happy about anything in her life. Her depression was so severe that she considered leaving her husband, thinking she had married the wrong man and that maybe she was better off single. Fortunately, a doctor's visit quickly led to a diagnosis of hypothyroidism. Today, Mary Ann has three children and is happily married.

Many signs and symptoms of depression are subtle and easily blamed on stress, fatigue, and a hectic lifestyle. But as Mary Ann's story shows, depression may also suggest that you have an underactive thyroid.

Some experts argue that everyone with depression should be tested for thyroid disease. But many well-meaning doctors will simply prescribe an antidepressant without considering that a thyroid disorder may be at the root. If you suspect that your depression is linked to your thyroid, insist on a thyroid test, especially if you have a family or personal history of thyroid or autoimmune disease.