Anyone have Hypothyroidism? Or any Doctors out there?

I was diagnosed with hypothyroidism in December 06. The Dr. prescribed me Synthroid 100mcg. I was fine when I went for a second blood test several months ago. Just recently I went to the Dr. to get another blood test. My TSH is now 2.40 She prescribed me 125 mcg. Then she said it was “low normal” witch makes no sense to me. What does that mean? Is normal 0.5? She also said I could stay with the 100 mcg. But why would I stay with 100 mcg if its low? So, is 2.40 too low? Should I up my dose? Why do Doctors have to make things so complicated. Someone help me out here that knows what they are talking about!! Thanks!