I have been on Lannett labs levothyroxine all along but would love to switch as of price fixing thing they have going on with Mylan causing price to rise from $2 to $13! All I could find on bioequivalence between manufacturers is this excerpt from a 2006 article, but if your manufacturer is listed, maybe it will give you some idea if you will be getting slightly more or less. For sure, get labs six weeks after switch to see if any effect.
"For example, the Sandoz generic LT4 product is 12.5% more bioavailable than Synthroid® (Abbott Laboratories, Abbott Park, IL), but is 2.3% less bioavailable than Levoxyl® (King Pharmaceuticals, Bristol, TN). Synthroid® is 9% less bioavailable than the generic product from Mylan Laboratories, and 3% less bioavailable than LT4 Lannett (the generic version of Unithroid®, Jerome Stevens Pharmaceuticals, Bohemia, NY). "