Hezzer, it could be that their bodies just require more replacement thyroid hormone than yours (their thyroid was more out of whack). When I was fist dxed as hypo, by TSH was well over 100, but 25 mcg of Synthroid set my levels straight. Not that I am thyroid-less I am on 212 mcg.
Also, check out
this link on the Synthroid site, it may help you save a little dough on Synthroid, there are also coupons!
Cynthia