You have to check her BMI. If she is overweight or obese you will have to the reason. Apart from seeing a doctor or a nutritionist You can do a few things -make a food diary and exercise diary. -cut down on fats .Increase fiber like salads before meal, avoid fast foods and sweet drinks. This will help dietician to calculate calories which she is taking. Apart from thyroid she will need some more test to rule out and serious like hormonal causes of of high weight. For this a qualified doctor can only help you. I would suggest you to take help of a good pediatrician or a Pediatric hormone specialist in endocrinologist in your area.