What Causes Heart Palpitations When Suffering From Hashimoto S Thyroiditis?

I was recently diagnosed with Hashimoto s Thyroiditis. I do not smoke or drink or use drugs. I was recently prescribed .5 mcg of synthroid. In the meantime..for about a month now, i have been experiencing what I think is heart palpitations it was so bad in the beginning, that i walked into a clinic thinking I was surely having a heart attack or some kind of cardiac event. But this sensation never ceases....i go to sleep with it and i wake up with it..i m exhausted and usually winded very easily...which is hugely disappointing as I had to give up on taking my brazilian ju jitsu classes because i could no longer keep up with the training. I feel out of breath and i just feel like my heart is somehow skipping beats or something..i dont know....it came out of nowhere and it won t go away...i just took a Lorazepam, that they gave me during the last event....because they just chucked it up to being an anxiety attack...but i do not think this is an anxiety attack. i truly believe that something is wrong...I am so tired every day that i dont want to wake up...I m just so incredibly exhausted...I m getting winded just typing this....