Hi there! I'm a newbie also and as you can see am up in the middle of the night, as I'm sure are a lot of others.

I have the same problem with the waking up in the middle of the night and the racing heart. I also have thyroid problems and am on levothyroxine...have been for years since I was diagnosed with underactive thyroid by my primary care physician. I first started waking up between 1-3 a.m. last year I think. I discussed this with my ob/gyn and with other symptoms I have he prescribed a progesterone cream for me to use daily except during my period as well as a dose of Dong Quai. Just to let you know, after reading a lot of articles on a lot of different sites, there seems to be a consensus that the waking up around that time with anxiety and racing heart are symptoms of perimenopause. While the progesterone cream helped, I'm still not sleeping through each and every night, but I can tell you that I am having fewer episodes. I ended up in the ER as you did at one time and went through the whole heart workup since heart problems do run in my family. However, I have the added problem of intermittent atrial fibrillation, which, according to the cardiologist, can be caused/exacerbated by hormones. Oh joy. *insert disgusted face here* lol
Anyway, just wanted you to know that you're not alone. And while you seem to be on the lower end of the perimenopause age scale, I myself am 45, by no means does it make it any easier...especially since you are probably running after your 4-yr. old a lot of the time and must be exhausted at the end of the day keeping up. lol I'll post more as I learn more about what helps. I know that diet and exercise Do make a difference and I have to applaud you for being so pro-active about supplements and looking into alternative reading material. I've found a lot of articles on here Very helpful and have even recommended this site to another friend of mine who is just starting to have some of the same symptoms.
I can tell you one thing funny though. We just got our power back last Fri. after being without for 2 weeks. Now you'd think with all the stress of that and going through Ike that things would be worse symptom-wise. Yes, I did wake up some during the night many times. It was a little on the warm side with no air or air circulation at those times the temps were in the 80s, but I never noticed waking up with the racing heart then. I only woke up because I was hot. I don't have problems, that I know of, with hot flashes yet. *knocks wood* In fact, I have the opposite problem. I wake up freezing and can't get warm. I thought I was going crazy to begin with, but apparently this too is yet another symptom of perimenopause. Leave it to me to be in the minority and get the opposite of hot flashes. lol
Please do post if you find other things that help, but in the meantime know this: you're not alone. So when you wake up with one of these "episodes" (I prefer that term to attack, lol)...just think that there's probably someone else somewhere, maybe not even too far from you, experiencing the very same thing. I checked into the chatroom on the off chance that someone might be there because of that very thing, but no such luck. Doesn't mean someone won't be there, but it might help to chat with someone. I find that playing an online game or doing research helps to calm me.
Keep up the pro-active approach and please share anything that works for you. I know I'm always happy to find new things which might help to discuss with my health professional! Take care and hang in there, Cas