My 8yr old had an xray in er for injury and md mentioned her bones were thin.she takes mvi (multivitamin) and drinks plenty of milk. what could cause thin bones?

See below: Multiple reasons can be the cause of thin bones including diet, activity, genetics, geographic location (eg. Northern vs southern with longer days and more sunlight), amount of sun exposure, etc. I would not rely on one x-ray done in an er to make a general diagnosis. See you pediatrician for further evaluation and work up if needed.