This prescription drug (sold under brand names such as Levothroid®, Levo-T®, Levoxyl®, Synthroid®, Tirosint® and Unithroid®) treats hypothyroidism. That's a condition where your thyroid gland doesn't make enough thyroid hormone. We also use this drug during treatment for thyroid cancer. Levothyroxine belongs to a class of drugs we call "hormones."