Is TSH the only thyroid related test that's being done? Is your doctor not testing Free T4 and Free T3? Those are the actual thyroid hormones and are far more important than TSH, which is a pituitary hormone and is an indicator, at best. Disease or dosage should never be determined, based solely on TSH, alone. If this is what your doctor is doing, you should ask her/him to test Free T4 and Free T3, along with TSH. If s/he refuses, you should find a different doctor asap.
Weight gain/inability to lose is one of the most prevalent symptoms of hypothyroidism.
If you do have results for Free T4 and Free T3, please post them, along with their corresponding reference ranges.