As bb has said the doctor would be able to have her tested for thyroid as you are concerned.I also suggest you also look at other avenues, how is she doing at school, has she friends, have there been any changes in her life, a new sibling, a move , is other family around.Get her onto a good multiple food supplement ,anti oxidants Vit C and Vit B Complex.to help give her system a boost.,lastly talk to her, find out if she is worried, concerned about anything. good luck
You mentioned that she does not feel hungry, yet she is very active. You need to sit her down and explain how she needs to take care of her body. If she does not improve in this, you will have to take her to the doctors.
The doctor can do a thyroid test, but she has to eat healthy foods and healthy amounts.