It may seem harmless to pop a multivitamin each day, but many dietary supplements still contain active ingredients that affect the body. They might also interact with prescription medications.

An overactive thyroid may affect more than the body's metabolism. It also appears to be associated with a small increased risk of developing a common heart arrhythmia called atrial fibrillation.

An overactive thyroid gland could put patients at higher risk of hospitalization for heart or blood vessel disease. This could be the case even if the gland was surgically removed.