levothyroxine dosages

hi y`all, my levothyroxine dosage has varied in the past year from as low as 100 up to its present level of 300 ! which ive been on for over 6 mths.
i feel lethargic, only have to look at food to put weight on, i came "home" from living/working in the USA where i had an energetic job doing maintenance plumbing, up all hours, on call, 7 day weeks etc, grabbing a variety of KFC, Mc Donalds, Burger King, Pizza hut, "all you can eat" etc wherein i didnt seem to weigh much differently from one week to the next, i came home and was diagnosed as hyperthyroid, running hot especially at night, intermittently re-diagnosed as hypothyroid and becoming heavier, various medications were tried, monthly bloodworks done, and all the time i`m gaining weight. 2 yrs ago i had radio-iodine treatment and now off the back of approx 3 monthly bloodworks my levothyroxine dosage has gone past what my doctor once described as the highest dose she ever remembered admistering (250 microgrammes) up to 300. I dont do chocolate, eat less and less but givem i`m now 23 stones, up from 16.5 stones before being diagnosed, any food consumption now seems to be viewed as gluttony ! i wish i`d never been diagnosed in the 1st place and taken my chances. any comments welcome.