I had my mirena out about 3 mths ago,after having it nearly 5 years,I also suffered heart palpitations(SVT),I am on Metoprolol to control them but, I still get them bad so I started taking Magnesium to see if that helps and it is a bit better,I had my thyroids removed about 2 years ago,just after the op I got my palpitations and couldnt stop them so they wired me up and I had a heart rate of 172,the next was the worst I had to have the injection called Impending Doom to slow my heart rate and was in intensive care for 3 days,My palpitations got worse while on Mirena,luckily I have learnt how to stop them myself Most of the time.
Its an electrical problem in your heart that causes it,you can have an op to fix it ,if they can find the exact spot where the trouble is.Try the magnesium.
Good Luck.