Suggest Opinion And Dosage Of Thyroid Medicine During Pregnancy

Hi , I am 20 weeks pregnant, my TSH level is 4.72 My physician tried to put me on Synthroid - 25 mcg ( but I took its generic version called Levothyroxine ) but due to some kind of Tachy cardia problem before, I could not continue this medication as it effected adversely on my heart beats and my heart beats race up to 3 times the normal and it took almost a week to get stable or a little normal... also wanted to mention that I took only 1/2 a tablet of 25 mcg of this medicine, which reacted instantly, so my doc asked me to stop it and to see a Cardio, all came normal when they did ultra sound of my heart and other check ups. Now my Physician asked me to try Thyronorm which I used to take in India prior my pregnancy , but she would like to know if this is safe to take during pregnancy , also please advice how much should be the right amount of dosage in my case.