Hypothyroidism, or an underactive thyroid, is treated with thyroid medication. Levothyroxine (Synthroid, Levoxyl, Levothroid, etc), a synthetic thyroid hormone, is most commonly used. Levothyroxine is T4 which is the long-acting storage form of thyroid hormone. This medication replaces the hormone from your thyroid.
More information can be found at the American Thyroid Association (ATA) website http://www.thyroid.org/wp-content/uploads/patients/brochures/Hypo_brochure.pdf Endocrinediseases.org: Thyroid: hypothyroidism - treatment