Well, first off, congratulations!
As for the rest of it... it sounds like you have the appropriate medical team following you. Maybe it would make you feel better to get a second opinion from a different endocrinologist? At least then you'd know if they were on the same page or if someone else has some different ideas as to what's going on with your thyroid levels.
On the positive side, you'll probably get lots of extra pictures of your baby since they'll want to follow growth patterns closely.