ATLANTA, Georgia (CNN) -- Driving a taxi for a living can be exhausting. Just ask 62-year-old Stephen Belcher of Atlanta, Georgia.
Stephen Belcher says he used to work 12 hours a day, seven days a week.
"I worked seven days a week, 12 hours a day normally," Belcher said. "I just thought I was tired and maybe if I got a good night's sleep I'd feel better."
Like millions of Americans, Belcher was really suffering from fatigue. It's a common complaint among an estimated 20 to 30 percent of patients who see their primary care physician.
"Most of the time you know why you are tired and the tiredness goes away after adequate sleep or rest," said Dr. Susmita Parashar, who specializes in internal medicine at the Emory School of Medicine in Atlanta. "Fatigue on the other hand, is a persistent daily lack of energy that impairs your ability to function normally."
Belcher described it as feeling burned out. "I would go to the grocery store and park my car. By the time I would go to the front door I would have to sit down and rest before I actually started shopping."
Parashar called fatigue an important marker that patients and doctors shouldn't ignore. Health Minute: Watch more on tired vs. fatigue »
"The list of causes of fatigue is quite long," Parashar said. "It includes anemia, underactive thyroid, diabetes, depression, sleep apnea, insomnia, chronic pain, liver, kidney and heart disease and in some cases cancer."
Belcher knew something was wrong. After months of putting it off, he finally went to the emergency room. He was found to have myriad ailments, from sleep apnea to emphysema to a heart blockage.
If Belcher hadn't gone to the hospital, Parashar said, his combination of conditions could have been fatal. "He really saved his life."
Before you start panicking about your own complaints, Parashar wants patients to know that fatigue is a relatively non-specific symptom. That means it can show up in a variety of forms in different patients.
"It's telling a doctor that a patient's body is not able to keep up with normal activities, so it's important to determine the cause," Parashar said.
Parashar starts tackling the problem by taking a detailed medical history, asking patients about work, sleep, exercise, alcohol intake, stress and their living environment. Then she does a detailed physical exam.
Depending on her findings she might order blood tests to look for anemia, an underactive thyroid or diabetes. X-rays or an electrocardiogram might be used in the investigation. She also does a depression screening for some patients.
As in Belcher's case, the comprehensive evaluation doesn't always reveal a single cause.
Parashar advises not to wait more than a week or two to see a doctor about fatigue. "You might want to see if a good night's sleep and improving your diet will help, but if it's something that is really bothering you and you can't do your normal activities, call your doctor."
When it comes to preventing fatigue, Parashar offered similar advice. "Take care of your body, get adequate sleep, good nutrition, regular exercise and avoid alcohol."
Some experts also recommend stress reduction techniques such as yoga, meditation and breathing exercises.
Belcher cut back on his work and started swimming a couple of times a week. With proper treatment and medication for his ailments, he reported he's doing much better these days.
"This was a big wakeup call for me," Belcher said. "Now I'm feeling great." E-mail to a friend