I was on synthroid for 2 years, but I wasn't very good at taking my medicine- then while I took it for 2 months straight and I conceived. But it all depends on where your levels are at. When I went to the doctor and had them test me for pregnancy and my thyroid my level was 12. (With my first pregnancy) this pregnancy I believe it's because you're more fertile afterwards
I lost my last baby due to issues from the hyperthyroidism, but we were able to conceive within 4 months and I haven't had any problems this pregnancy.