PTU vs MMI in Pregnancy

I M.B.B.S. 3rd Year Students at Pacific Medical College & Hospital, Udaipur, Rajasthan, during my studies I read about Antithyroid Drugs like Methimazole (MMI) and Propylthiouracil (PTU) which are used in Pregnancy so please give me details that what are the contents of this drugs and how they are useful?