TSH levels of 6.74 and normal T4 and T3 indicate that you have subclinical hypothyroidism. S.C.hypothyroidsm doesn't warrant treatment unless TSH level is above 10 ,if you have any symptoms related to hypothyroidism ,and if one is planning for pregnancy.Weight gaining is one of the symptoms related to hypothyroidism , however you have mentioned that it could be due to sedentary lifestyle.You can start life style and diet modification, exercise regularly for 30 minutes a day.Repeat the
thyroid function tests after 6-8weeks keeping in mind that along with TSH ,free T4 should be measured ,not total T4.