Generic types of Synthroid have been found to be inconsistent in strength. If you search google for 'generic synthroid' you'll come up with a ton of information about it
Personally, after an awful incident with generic Wellbutrin XL, I won't take the generic form of any med I use on a daily basis. Lots of doctors and pharmacists will give you the schpiel that it's all approved by the FDA, and it's the same thing and all that, but those of us who have had bad experiences with generics know that's not always the case.