I Am Concerned. My Doctor Said My Thyroid Is Low

I am concerned. My doctor said my thyroid is low and has me on medicine but I went off it because I exercise and eat healthy and I am still gaining. I understand muscle weighs more but this is not right and I have been roller blading 4 miles 5 days a week at a good speed and get about 1400 calories because that is what a guy at complete nutrition said I needed according to their analyzing machine.
I'm so stuck and desperate to lose weight.