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Your symptom: Your heart is racing for no apparent reason.

A sped-up heartbeat could stem from hyperthyroidism, a treatable condition that affects more than one in 100 Americans and tells your ticker to beat too fast. If you feel your heart beating super-fast for just a brief period without warning, you may have a heart condition called supraventricular tachycardia. Although it affects about one in every 300-500 people, the most common kind of SVT, atrioventricular nodal reentrant tachycardia, occurs more frequently in younger women than in other demographics.

Doctors aren't sure what causes some people to have SVT; while for some people, stress, lack of sleep or physical activity may trigger an episode, for others, there's no obvious cause. "If your heart rate is over 100 when you're just sitting around, or you can feel your heart pounding and you feel funny and lightheaded, that's worth talking to a doctor," says Ann Celi, MD, MPH, an internist at Brigham and Women's Hospital in Boston. Call your doc up ASAP (yes, even if you feel fine once the episode passes), because it can indicate that you're at a much higher risk for heart failure. She'll help you figure out what's up.