Hello. I m Dr. Christensen. Your palpitations (the fluttering in your chest), breathlessness and headaches could be symptoms of an abnormal heart rhythm, such as atrial fibrillation. While some of your medications could certainly contribute to a rhythm abnormality, these disorders can occur spontaneously, too (that is, they just pop up out of the blue ). If you haven t seen your doctor recently, I would suggest that you do so as soon as possible. He/she should check your TSH and T4 levels to make sure your Synthroid dosage is correct. Too much Synthroid can trigger abnormal heart rhythms. Your potassium level should also be checked, as hydrochlorothiazide can wash out potassium, Atacand can cause you to retain potassium, and either a low or high potassium level can trigger rhythm disturbances. A few additional lab tests will rule out other causes of abnormal rhythms, such as anemia. Your doctor will probably want to get an ECG to see if your heart is functioning normally; if your ECG is normal but your doctor is concerned that your heart is intermittently going out of rhythm, you may need to wear a small cardiac monitor for several days as you go about your usual routine. I hope that answers your question. Good luck!