Why is my T4 level high when my diagnosis is Hypothyroidism?

I havd been Hypothyroid for 11 years. I was stable on a dose of 125 mcg of Synthroid for about 6 or 7 years, then my dose needed to be decreased to 112 mcg. My does of 112 mcg was stable for about 4 or 5 years, now all of the sudden I am once again in a hyperthyroid state. My TSH level was 0.01 uIU/mL, my T4 was 1.72 ng/dL, and my White Blood Cell Count was 4,100 (may/may not be related). My doctor lowered my dose to 100 mcg. I think that I was under a great deal fo stress both times when the dosages needed to be lowered. When I asked why my dose needed to be lowered my doctor said that I must be making my thyroid hormone now than I was previously. Is this possible, or could their be another answer? I am not totally satisfied with my doctor's answer.