Underactive thyroid

My 3 year old daughter has just been diagnosed with an underactive thyroid (doctors have told me it extremely rare for a child her age to develop it) and I'm really confused and worried as I don't know much about it, Ive read a few things on the internet but I'm looking for real people's experience with it and hopefully reassurance. Is she going to develop another condition from it when she grows up? And affect her way of life? Be able to have kids? Or are her tablets going to make everything back to normal, the way a healthy thyroid would be? And be like there's nothing wrong. X