According to the U.S. Food and Drug Administration pregnancy categories, levothyroxine has been assigned category A. Given that no increased risk of congenital abnormalities has been demonstrated in pregnant women taking levothyroxine, therapy should be continued during pregnancy. Furthermore, therapy should be immediately administered to women diagnosed with hypothyroidism during pregnancy, as hypothyroidism is associated with a higher rate of complications, such as spontaneous abortion, preeclampsia, and premature birth.Thyroid hormone requirements increase during and last throughout pregnancy. As such,  pregnant women are recommended to increase to nine doses of levothyroxine each week, rather than the usual seven, as soon as their pregnancy is confirmed. Repeat thyroid function tests should be done five weeks after the dosage is increased.While a minimal amount of thyroid hormones is found in breast milk, the amount does not influence infant plasma thyroid levels. Furthermore, levothyroxine was not found to cause any adverse events to the infant or mother during breastfeeding. As adequate concentrations of thyroid hormone are required to maintain normal lactation,  appropriate levothyroxine doses should be administered during breastfeeding. Children
Levothyroxine is safe and effective for children with hypothyroidism; the goal of treatment for children with hypothyroidism is to reach and preserve normal intellectual and physical development. Contraindications
Levothyroxine is contraindicated in people with hypersensitivity to levothyroxine sodium or any component of the formulation, people with acute myocardial infarction, and people with thyrotoxicosis of any etiology.
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.