Hypothyroidism is common in any age group. Medicine needs to be started as
TSH is very high. Keep checking TSH regularly 3monthly to assess the dose of eltroxin wheathr needs to increase r keep same . Before starting eltroxin if you want to counter check TSH can repeat from some good laboratory in your area. If it's same from other lab also your child needs treatment regularly so that he will grow normally physical and mental.
Thyroid hormone is very important for overall growth of child. Allopathy is the right therapy.
Next Steps
keep following with your doctor,if you want can get opinion of Pediatric endocrinologist in your area.