Yes, I actually have experienced exactly what you are describing (although the tests on my thyroid showed no problems).
I was diagnosed with PSVTs (paroximal supraventricular tachycardia) at the age of 34 when I decided to give blood and all of the technicians in the Red Cross donor van thing couldn't get a regular heartbeat. Funny thing is that I was used to my heart beat being like that, it had been that way for YEARS...I think since I was a teen, at least.
I ended up going to the ER to get it checked out because they told me it was NOT normal...and holy cow, I was admitted for 5 days! It took time for them to run tests and get it regulated by medication.
Long story short, after 10 different medications, MANY holter monitors and 3 hospital admissions over 1 year, I finally opted to have the radio frequency catheter ablation procedure. It ended up that they were able to cauterize 4-5 different 'trouble spots' in my heart, but 1 spot was too close to the sinus node and they left it alone (because if they missed and burnt my SN, I would have ended up with a pacemaker forever, which I did not want if I could avoid it)
So now, I have periods where I will go into PSVTs, but not very often...nothing like pre-ablation. And I am on no medications.
If you have ANY questions at all, please send me a pm or ask here, I will check back. It is scary when you are first diagnosed because it's your heart!!! But, it can be managed with medication for the most part and as you doctor said, it is usually benign (mine is). The only thing is that each medicine has a different set of side effects. And sometimes what works now, will not work later. It's a real balancing act.
Also, I highly recommend your seeing an Electrophysiologist (EP), if possible. Especially if medications have a hard time 'curing' you. A cardiologist specializes in the plumbing of the heart...an EP is a cardiologist who specializes in the electrical system of the heart. You need an electrician, LOL.
I am sure you have tons of questions and I could go on and on...just let me know what you are interested in knowing. Good luck, it's not that bad!