Levothyroxine, also known as L-thyroxine, is a manufactured form of the thyroid hormone thyroxine (T4). It is used to treat thyroid hormone deficiency (hypothyroidism), including Hashimoto’s disease and a severe form known as myxedema coma. It may also be used to treat and prevent certain types of thyroid tumors. It is not indicated for weight loss. Levothyroxine is taken by mouth or given by intravenous injection. Maximum effect from a specific dose can take up to six weeks to occur. Levothyroxine was first made in 1927. It is on the World Health Organization’s List of Essential Medicines. Levothyroxine is available as a generic medication. In 2019, it was the second most commonly prescribed medication in the United States, with more than 102 million prescriptions.