HELP! hypothyroid, synthroid, gaining on lowcal diet

Hi, I was diagnosed with Hypothyroidism last month. i was put on 50mg synthroid.
before i was put on meds, if i ate about 1800 cal a day i was able to maintain my weight.
now that im on the meds, ive been averaging 1600 cals a day and gaining.

does anyone have experience with this? any ideas of what i can do to stop this weight gain in its tracks?