The bioavailability of the drug is decreased by dietary fiber.Greater than 99% of circulating thyroid hormones are bound to plasma proteins including thyroxine-binding globulin, transthyretin (previously called thyroxine-binding prealbumin), and albumin. Only free hormone is metabolically active.The primary pathway of thyroid hormone metabolism is through sequential deiodination. The liver is the main site of T4 deiodination, and along with the kidneys, are responsible for about 80% of circulating T3. In addition to deiodination, thyroid hormones are also excreted through the kidneys and metabolized through conjugation and glucuronidation and excreted directly into the bile and the gut, where they undergo enterohepatic recirculation.Half-life elimination is 6–7 days for people with normal lab results; 9–10 days for people with hypothyroidism; 3–4 days for people with hyperthyroidism. Thyroid hormones are primarily eliminated by the kidneys (about 80%), with urinary excretion decreasing with age. The remaining 20% of T4 is eliminated in the stool. History
Thyroxine was first isolated in pure form in 1914, at the Mayo Clinic by Edward Calvin Kendall from extracts of hog thyroid glands. The hormone was synthesized in 1927 by British chemists Charles Robert Harington and George Barger. Society and culture
Economics
As of 2011, levothyroxine was the second-most commonly prescribed medication in the U.S., with 23.8 million prescriptions filled each year.In 2016, it became the most commonly prescribed medication in the U.S., with more than 114 million prescriptions. Available forms
Levothyroxine for systemic administration is available as an oral tablet, an intramuscular injection, and as a solution for intravenous infusion.
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 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.Side effects from excessive doses include weight loss, trouble tolerating heat, sweating, anxiety, trouble sleeping, tremor, and fast heart rate. Use is not recommended in people who have had a recent heart attack. Use during pregnancy has been found to be safe. Dosing should be based on regular measurements of thyroid-stimulating hormone (TSH) and T4 levels in the blood. Much of the effect of levothyroxine is following its conversion to triiodothyronine (T3).Levothyroxine was first made in 1927. It is on the World Health Organizations List of Essential Medicines. Levothyroxine is available as a generic medication. In 2020, it was the second most commonly prescribed medication in the United States, with more than 98 million prescriptions. Medical use
Levothyroxine is typically used to treat hypothyroidism, and is the treatment of choice for people with hypothyroidism who often require lifelong thyroid hormone therapy.It may also be used to treat goiter via its ability to lower thyroid-stimulating hormone (TSH), which is considered goiter-inducing.