Synthroid is the brand name used by Abbott Laboratories for levothyroxine sodium (T4, a synthetic thyroid hormone) product. Levothyroxine is the basic T4 thyroid hormone responsible for metabolism. Patients who suffer from hypothyroidism, either for endogenous disease or after radioactive iodine ablation or thyroid resection, require exogenous thyroid hormone, and Synthroid is the most prescribed brand of T4.

It should be noted that Synthroid was not the first T4 produced for patients, and there are generic versions approved by the Food and Drug Administration, but Synthroid represented a significant advance on other brands and formulations in terms of uniformity of production and regularity of therapeutic action. Therefore, many endocrinologists and other prescribers specify a specific thyroid hormone brand because thyroid hormone has a narrow therapeutic index. In the United States, Synthroid is the most prescribed drug. Despite its frequent use, however, it is far from being a so-called "blockbuster drug" ($1 billion USD or more in sales annually).