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. Synthroid was marketed in 1955, but was not FDA approved at that time as it was considered "generally regarded safe". In the 1990s, in response to various complaints, all levothyroxine preparations were required to undergo the formal FDA approval process. Synthroid was approved by the FDA on 2002-07-24 .

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). One of the biggest reasons for it not to be a "blockbuster drug" is its competition Cytomel. Especially, pre-contest diets of bodybuilders include Cytomel, a more popular thyroid drug rather than Synthroid due to its comparable effectiveness with a lot less quantities. Its use for bodybuilders usually consists of getting to the right body temperature for optimal fat burning .

Notes

• Company Report on Abbott
• Dr Mercola's site
• FDA approval
• Synthroid Profile