Levothyroxine, sold under brand names like Unithroid & Synthroid, treats this gland when it's underactive
| LIVESTRONG.COM [PAR] Aug 11, 2011 ... L-thyroxine, also known as levothyroxine, is a synthetic T4 thyroid ... and it's sold 
under the brand names Sythroid, Levoxyl, Unithroid or ... common drug used to 
treat hypothyroidism -- an underactive thyroid.