It is good she is on medication for her hypothyroidism and she is active with sports. Hypothyroidism does cause high weight gain due to the thyroid regulates our metabolism rate. Suggest the whole family eat the following so she does not feel different and everyone will be healthier: low fat meats (chicken, fish, loin of pork) or non-meat sources (soy products, nuts), low fat vegetarian combinations (rice & beans, pasta & beans, low fat cheese & pasta), low fat and substitute dairy products (1% milk, low fat/fat free yogurts, low fat soy milk/soy yogurt), use plant oils, non trans-fat margarines, and eat a lot of fruits and vegetables. Eat daily high fiber breads and cereals (oatmeal) and limit ‘junk’ foods. Portion control is important and here some general tips = ½ cup for all starches and vegetables; one piece of fruit or ½ cup of small fruits; 1 cup for low fat dairy products, and only 3oz of low fat meat or protein substitution a meal (only 6-7ozs total a day). She needs physical activity daily, and walking is the easiest form of exercise. Try to walk for 30 minutes or more daily. Limit time in front of the TV/DVD/computer or non-active-video games. There have been studies showing the longer the length of time a child sit in front of the TV, the greater the weight gain. Help cook simple healthy foods so she learns how to eat right. Make everything fun so you will enjoy living healthy. Hoped this helped you.
Try Red Light Green Light Eat Right. It is a book with recipes and ideas for child nutrition. It was written by a pediatrician who is a specialist in child weight issues. I just got it for my daughter. She is 5 foot 5 and weight 157 pounds. She loves it because she doesn't have to be hungry on it, it allows them to snack on veggies and fruit throughout the day, and encourages healthy eating through a system, not calorie counting or denial. Since your child has a medical reason for her weight, it will allow you to be sure she is not eating excess calories and is getting proper nutrition without feeling like the food police. AND the whole family can get on the program, so she won't feel like she is in jail.