What Is The Treatment For Hypothyroidism And Weight Gain?

I am recently diagnosed with hypothyroidism, I have low total T3 only, 72.3-73. I always remain under weight like 38-42 kg, my age is 27; female. There r so many other symptoms of hypothyroidism i have. after two months treatments of thyroxin of 50 mcg there is no remarkable change specially in my weight nor in my blood reports then my endocrinologist make my dose double in april, now from two month i am on 100mcg. Still no change specially in my weight. Please do advice something that will improve my underweight problem specially.