From The Pill to the IUD

Debora
Hey guys, 
So I went to the doctor last Friday and since I have hypothyroidism and have to take hormone every morning he suggested that instead of getting the extra hormone from the pills I should get an IUD. JUst got a call from the doctor's office and I'll be getting it this Friday. I am kind of skeptical because I heard that a lot of woman still does get pregnant even with the IUD. Any tips suggestions or advice that I should know about it?