How does one cope with an over-active Thyroid?

I just completed my annual physical and blood tests. I was very surprised to find out from my doctor that I had a TSH of 0.009. He called me to tell me he had ordered Methimazole at my pharmacy and for me to start taking it. This is scary stuff for me. Since this past December I have been dieting and lost over 40 lbs. and feeling wonderful. Exercising daily and lifting weights. My last TSH a year ago was 1.630 and normal. How could something like this happen in just 12 months?