Which antithyroid drug can be safely used in pregnancy
This question has been repeated several times before. Remember these important points. 
All antithyroid drugs can cross placenta so risk of fetal hypothyroidism and goitre is always there. Both carbimazole and propylthiouracil can be used during pregnancy, but propylthiouracil is the drug of choice because its greater protein binding allows low doses of propylthiouracil to be transferred across the placenta and therefore less chance of hypothyroidism. 
The lowest effective dose of propylthiouracil should be given and it is often possible to stop treatment in the last trimester since TSH Receptor antibodies tend to decline in pregnancy.
Radioactive iodine is C/I during pregnancy. It can destroy the fetal thyroid tissue resulting in cretinism Carbimazole and methimazole have been associated with rare cases of foetal aplasia cutis.
Propylthiouracil
I121
Methimazole
Carbimazole