I too started menstrating at 10. I dont know if I was the first in my class because I was very shy and didnt talk much so never asked. My aunt started when she was nine. I also developed large breasts early on and when boys noticed I started wearing very baggy clothes. It wasnt until I was 21 that I started wearing girlier clothes. I too have a thyroid problem (graves disease) but am not sure that had anything to do with the menstrating "early". I put early in quotes because I am not sure 10 is early. I agree that you both should have a talk with her pediatrician. I also think you should have a talk with her about dealing with bullies and teasing. Children can find any difference to tease a child about. Whether it is glasses, weight or breasts.