The American Cancer Society defines fatigue as "extreme tiredness that does not get better with rest." So if you're often fatigued, see your doctor. Leukemia, colon, or stomach cancer -- which can cause blood loss -- can result in fatigue.
"Fatigue can be a serious problem and it's easy to ignore," Runowicz says.
Your doctor will most likely do a physical exam and order blood tests to check your thyroid and rule out a thyroid condition, she says.