Hi Bethers,
I've looked up Synthroid on the internet and it is a class A drug and considered very safe during pregnancy. I'm assuming that your doctor is aware that you want to begin trying for another pregnancy again and knows that you are taking this medication. If not, I would just let him know so he is fully aware of your situation.
I personally don't have thyroid issues but there are a few around here that do. Maybe they can reassure you with their own personal experiences. I know I always found the experiences of others to be so much more comforting rather than just textbook numbers and jibberish!
Good luck to you!
Joanna