Try www.tpa-uk.org.uk you will get a lot of information there,. I am having trouble with hypothyroidism and palpitations myself.
You need to say what medication you are taking and how much.
I'm not sure anyone will be able to help with your question. Have you checked out the thyroid forum? If you provide more information, someone may be able to give you more specific answers. I did find this article that may be helpful. http://www.mythroid.com/heartdisease.html