It does not matter about your thyroid in the first trimester..your baby does not start using or taking the hormones from you until after the 12 week when his/her thyroid starts to function!! As for the breech position i dont think that causes a problem with your babys health but if he or she doesnt turn and cant be turned then it poses a problem to you have a natural delivery..meaning you could end up with a section! Good luck!
You still have four weeks and there is plenty of time for baby to turn around. Breech causes no health problems and the only problem it might pose is during delivery. Most docs will attempt to turn baby if they don't turn by themselves, if that don't work, then they will talk about a c-section. Good luck and I hope you have a wonderful delivery. :)