Can anyone tell me about abnormal and under-functioning thyroid?

After my 15 year old daughter had blood work done we were told she has an abnormal and under-functioning thyroid. The nurse who called with the results didn’t give much information other than to say the doctor wanted to start my daughter on medication but once she started she would not be able to stop.
I’m not big on medication so we decided to talk with a specialist first.
Can anyone share their personal experiences with thyroid problems?
Is there any side effects to this medication?
Any natural treatment?