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This condition occurs when your thyroid is hyperactive and speeds up metabolism.

How to spot it: Telltale signs include sudden weight loss, a fast and irregular heartbeat, excessive sweating, difficulty sleeping and more frequent bowel movements.

To lower your risk: Don't smoke; lighting up is associated with a greater risk of Graves' disease, the most common cause of hyperthyroidism in the U.S.

Common treatment: Radioiodine, which is given orally to shrink the thyroid and help get it back on track, is the first course of action. Make sure your doctor keeps tabs on your blood work—the substance will often cause you to underproduce T4, leaving you with hypothyroidism, which has fewer long-term complications.

Try this! Fill up on vitamin D. Deficiency of this vitamin has been linked to both hyper- and hypothyroidism. Fatty fish are the best food sources of D: One serving of salmon provides more than the recommended dietary intake for a whole day.

Mehmet Oz, MD, is the host of The Dr. Oz Show (weekdays; check local listings).