10 year old son has enlarged thyroid?

My son has been experiencing dizzy spells alot latley and sometimes has troubles walking. The doctor noticed his thyroid was enlarged and sent us for bloodwork which we did today! Does anyone know why my 10 year old would have this enlarged thyroid? Please...........also 3 years ago his white blood cells were a little elevated but doctor wasnt worried and he hasnt been checked since! PLEASE NOTE THIS CHILD IS NEVER SICK!