When my thyroid was out of whack, my heart did some funky things and I was quite worried. I had a whole bunch of tests done back then and heart was fine. I am on synthroid now and last I checked a month ago, my levels were fine. Now I am getting periods of shortness of breath, far worse when I lie down. Sometimes I wake up suddenly at night unable get air. Some dizziness and vertigo. I am extremely tired, extremely. I have gained weight, mostly in my abdomen. When I eat it makes me fee sick. I get chest pains, usually brief piercing pain. Sometimes it is more spead out pain, more restrictive. I can feel my heart, like there is extra pressure there and my heart beats can be eratic. So how can you tell the difference between heart problems and thyroid caused symptoms?