not necessary will be life-long on medication, there is a new technology can froce the thyroid gland to convert the thyroid hormones
that's work on humen, i am not sure for the dog. sould be similar ask the veterinary professional.
I don;t really think ur dog has that, only lost hair and tired, you can't say is hypothyroidism, have to be more symptoms, for exmaple..
The skin might be more darker than normal? dog doesn't produce enough tears to keep the cornea moist? or the hair coat is thin and dull, that easily falls out?
best way to diagnose this is Radioactive Iodine or TSH Test