No doctor would ever be so foolish to treat a lab test. Lab test is just a piece of information (sometimes rather crucial though).

What brought the kid to clinical attention? How have her health and development been generally?

You need a complete history which will guide further physical examination and then only can you order tests. Once the results are there, you can base a clinical decision on all the information you have at hand.

The thyroid hormone reference levels vary slightly in paediatric age groups.

Did the doctor not tell you why he or she was considering thyroxine?