I have been on thyroid medicine for 25 years. Changes are slow to manifest. Weight gain is a given, I used to be 200 lbs before my thyroid was destroyed (doctor's orders) by a radioactive pill. It was about a month of gaining weight, then gradually, over the next month or so, the pills kicked in. I stopped gaining, balancing at 260, where I've stayed for many years.
Recently, because of my age I guess, the doctor changed my level down to 175 micrograms. It all started In 1982 at 300 micrograms staying at that level until 1997 when they changed me to 0.200 Mg's or 200 micrograms. This because I was losing weight "yeah' and my heart was skipping with occasional atrial fib "no yeah."
Something important I have learned, which they did not tell me, was to take Synthroid (levothyroxine) two hours before any food. May the Lord be with you!
Here is a good link:
http://www.synthroid.com/