pineneedles, make sure your thyroid panel included TSH, free T4 and free T3 plus thyroid antibodies. It's relatively easy to get all that tested privately in the UK via affordable at-home tests. The test which is harder to get, is the rt3 test. The only place I've found where you can order it yourself, is the Blue Horizons Platinum thyroid test (google). It includes a bunch of other stuff but the only reason I get this particular test, is the rt3. You have to do a venous draw for it (I go to an aesthetics clinic for that, as it's nurse-run and she offers blood draw services) and it takes 3 weeks for the results to come back. Plus it is expensive, about £140.
I do think it's worth doing rT3 once...
And yes, thyroid and estrogen interact because high estrogen can create more TBG (thyroid binding globulin) which is like SHBG but for thyroid hormones - it binds up your thyroid hormones so you may then feel hypo.
I have also been through the whole cardio thing. Because the tremors were so strong I thought they were palpitations at first. I bought this little Kardia device which connects with my iPhone and does reliable ECGs - it was what my local hospital were using but I cut out the waiting list by buying my own for £100. I did ECGs at night, in the middle of all this, and emailed them to the cardiac nurse, who said they were always normal. My pulse was fast at 95bpm at night, but the actual heart rhythm was normal.
I also had an echocardiogram, which was also normal. I realised at this point that the tremors were not from my heart. Tingling and buzzing are known symptoms caused by a range of things, from iron deficiency to B12 deficiency - but as always it's best to take the most obvious thing... and if you've just started HRT an then you get this, it's most likely due to hormones. THe problem is how to get rid of it. For me, going quite high with the estrogen and stablising there has really helped although it's now causing flooding during periods so I am very slowly coming down on the gel. I hope I don't get these symptoms back or it's not going to be clear what to do. (Don't really want a Mirena.)