I was diagnosed with an Underactive Thyroid years ago and am taking Thyroxine. Its always up and down but for a while now its been stable. My doctor has told me he needs to keep an eye on it now that im pregnant so i had a blood test last Friday and will get my results next week. Im convinced its gone Overactive as my heart is pounding all the time and im constantly shivering, although coldness is with Underactive? Ive been reading up on Google and the answers are quite frightening, some saying you can have heart failure! Are Under or Overactive dangerous? What are the chances of my thyroid changing from one to the other? Im so scared!