Thyroid Help

I have been on synthroid since last April. My TSH levels first tested at around 15. So fo course I was put on synthroid, starting out slowly and finally ending at 100mcg. Now it's probably been about 3 months or so since the last blood draw and it came back with my TSH in the normal range. I recently had bloodwork done, still on the 100 mcg, and got a letter from my PCP today saying my levels were at 16.1. HOW can this happen??? In shock Kim