I have thyroid issues also (no thyroid so take medication to replace what I dont have)

Yes over and under active thyroid can cause birth defects and misscarriage. I think from memory that the problems are associated with brain development.

I am currently over replaced on my thyroid medication- I take more than I need to prevent my cancer from returning. I am also nearly 25 weeks pregnanat and to date every test indicates my baby is healthy and normal.

On another forum a girl has just been diagnosed at 5 week gestation with hyperthyroid and told by her GP that so long as they get her levels under control by 15 weeks the baby wont be affected (makes sense as the placenta doesnt function properly til that time). I dont know how true that is though

Hope that helps