Pain in the arms , with burning sensation taking synthyroid for hypothyroidism
, could be polymyositis
or PMR polymyalgia rheumatica
, with headache fatigue and lethargy
I would like you to get ESR + CK level done , though the treatment of both the diseases is steroids.
Synthyroid dose could be low , cause it also causes polyneuropathy
or myopathy, increasing the dose can help