Oooooh, yeah, do I know why! If you are over 50, OR if you have been hypo for more than a few months (severely hypo), OR if you have a history of heart arrhythmias, the starting dose should be, as you stated, 12.5-25 mcg. I qualified for all three categories, and my (former) PCP started me on 88 mcg (had never been on thyroid meds before). Within a couple of weeks, my heart arrhythmia had gone wild. I backed off to 44 mcg. Another couple of weeks, same scenario. Backed off to 25 mcg.