HI, I Am Symptomatic Of Low Thyroid - I Have

HI, I am symptomatic of low thyroid - I have no thyroid, my TSH is low, but my T3 is low - the endocrinologist says it doesn't matter what my t3 is once my thyroid is gone and wants to cut my synthroid back. I have a foggy brain and am so tired I can't stay awake all day. I have stopped working out as it tires me so terribly. I am scared I will not be able to function with my medicine cut back.