Is your TSH
genuinely low? If yes, then you need to evaluate the reason for it and take appropriate treatment from an Endocrinologist
. If you are gaining weight then that will also be looked into by the the Endocrinologist as low TSH and wight gain is a rare-though not an impossible combination.
Please recheck your TSH and FT4 from an NABL certified lab and then take the reports to Endocrinologist for further help. Take care.