LevoThyroxine Sodium (Generic Synthroid) is used for treating low thyroid activity and treating or suppressing different types of goiters. It is also used with surgery and other medicines for managing certain types of thyroid cancer. Synthroid is a thyroid hormone. Thyroid hormones are produced and released by the thyroid gland and regulate the body's metabolism. When the thyroid gland is underactive (hypothyroidism) it does not produce enough thyroid hormone and metabolism slows.