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{{Infotaula fàrmac}}
<!-- Definition and medical uses -->
'''Levothyroxine''', also known as '''{{small|L}}-thyroxine''', is a manufactured form of the [[thyroid hormone]] [[thyroxine]] (T<sub>4</sub>).<ref name=AHFS2016/><ref name=King2010/> It is used to treat [[hypothyroidism|thyroid hormone deficiency]], including the severe form known as [[myxedema coma]].<ref name=AHFS2016/> It may also be used to treat and prevent certain types of [[thyroid tumors]].<ref name=AHFS2016/> It is not indicated for [[weight loss]].<ref name=AHFS2016/> Levothyroxine is taken by mouth or given by [[intravenously|injection into a vein]].<ref name=AHFS2016/> Maximum effect from a specific dose can take up to six weeks to occur.<ref name=AHFS2016/>
<!-- Side effects and mechanism -->
Side effects from excessive doses include weight loss, trouble tolerating heat, sweating, anxiety, trouble sleeping, [[tremor]], and [[fast heart rate]].<ref name=AHFS2016/> Use is not recommended in people who have had a recent [[myocardial infarction|heart attack]].<ref name=AHFS2016/> Use during [[pregnancy]] has been found to be safe.<ref name=AHFS2016/> It is recommended that dosing be based on regular measurements of [[thyroid-stimulating hormone]] (TSH) and T<sub>4</sub> levels in the blood.<ref name=AHFS2016/> Much of the effect of levothyroxine is following its conversion to [[triiodothyronine]] (T<sub>3</sub>).<ref name=AHFS2016>{{cite web|title=Levothyroxine Sodium|url=https://www.drugs.com/monograph/levothyroxine-sodium.html|publisher=The American Society of Health-System Pharmacists|access-date=8 December 2016|url-status=live|archive-url=https://web.archive.org/web/20161221013136/https://www.drugs.com/monograph/levothyroxine-sodium.html|archive-date=21 December 2016|df=dmy-all}}</ref>
<!-- Society and culture -->
Levothyroxine was first made in 1927.<ref name=King2010>{{cite book|last1=King|first1=Tekoa L.|last2=Brucker|first2=Mary C.|title=Pharmacology for Women's Health|date=2010|publisher=Jones & Bartlett Publishers|isbn=9781449658007|page=544|url=https://books.google.com/books?id=E9qVyrNPsBkC&pg=PA544|language=en|url-status=live|archive-url=https://web.archive.org/web/20170910234123/https://books.google.com/books?id=E9qVyrNPsBkC&pg=PA544|archive-date=10 September 2017|df=dmy-all}}</ref> It is on the [[World Health Organization's List of Essential Medicines]], which lists the most effective and safe medicines needed in a [[health system]].<ref name=WHO19th>{{cite web|title=WHO Model List of Essential Medicines (19th List)|url=http://www.who.int/medicines/publications/essentialmedicines/EML_2015_FINAL_amended_NOV2015.pdf?ua=1|work=World Health Organization|access-date=8 December 2016|date=April 2015|url-status=live|archive-url=https://web.archive.org/web/20161213052708/http://www.who.int/medicines/publications/essentialmedicines/EML_2015_FINAL_amended_NOV2015.pdf?ua=1|archive-date=13 December 2016|df=dmy-all}}</ref> Levothyroxine is available as a [[generic medication]].<ref name=AHFS2016/> The wholesale cost in the [[developing world]] is about {{US$}}0.58 to {{US$}}12.28 per month.<ref name=ERC2014>{{cite web|title=Levothyroxine |url=http://mshpriceguide.org/en/single-drug-information/?DMFId=466&searchYear=2014|website=International Drug Price Indicator Guide|access-date=8 December 2016}}</ref> In the United States, a typical month of treatment costs less than {{US$}}25.<ref name=Ric2015>{{cite book|last1=Hamilton|first1=Richart|title=Tarascon Pocket Pharmacopoeia 2015 Deluxe Lab-Coat Edition|date=2015|publisher=Jones & Bartlett Learning|isbn=9781284057560|page=223}}</ref> Levothyroxine was the most commonly prescribed medication in the United States as of 2016, with more than 114 million prescriptions.<ref name=top300>{{cite web |title=The Top 300 of 2019 |url=https://clincalc.com/DrugStats/Top300Drugs.aspx |website=clincalc.com |accessdate=22 December 2018}}</ref>
== Medical use ==
Levothyroxine is typically used to treat [[hypothyroidism]],<ref name="pmid18662921">{{cite journal | vauthors = Vaidya B, Pearce SH | title = Management of hypothyroidism in adults | journal = BMJ | volume = 337 | pages = a801 | date = July 2008 | pmid = 18662921 | doi = 10.1136/bmj.a801 }}</ref> and is the treatment of choice for people with hypothyroidism,<ref name=Roberts>{{cite journal |doi=10.1016/S0140-6736(04)15696-1 |title=Hypothyroidism |journal=The Lancet |volume=363 |issue=9411 |pages=793 |year=2004 |last1=Roberts |first1=Caroline GP |last2=Ladenson |first2=Paul W }}</ref> who often require lifelong thyroid hormone therapy.<ref name=Gaitonde>{{cite journal | vauthors = Gaitonde DY, Rowley KD, Sweeney LB | title = Hypothyroidism: an update | journal = American Family Physician | volume = 86 | issue = 3 | pages = 244–51 | date = August 2012 | pmid = 22962987 | url = http://www.aafp.org/link_out?pmid=22962987 }}</ref> Doses of levothyroxine that normalize serum TSH may not normalize LDL cholesterol and total cholesterol.<ref>{{cite journal |vauthors=McAninch EA, Rajan KB, Miller CH, Bianco AC |title=Systemic Thyroid Hormone Status During Levothyroxine Therapy In Hypothyroidism: A Systematic Review and Meta-Analysis |journal=J. Clin. Endocrinol. Metab. |volume= |issue= |pages= |date=August 2018 |pmid=30124904 |doi=10.1210/jc.2018-01361 |pmc=6226605}}</ref>
It may also be used to treat [[goiter]] via its ability to lower [[thyroid-stimulating hormone]] (TSH), a hormone that is considered goiter-inducing.<ref name="pmid16507633">{{cite journal | vauthors = Svensson J, Ericsson UB, Nilsson P, Olsson C, Jonsson B, Lindberg B, Ivarsson SA | title = Levothyroxine treatment reduces thyroid size in children and adolescents with chronic autoimmune thyroiditis | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 91 | issue = 5 | pages = 1729–34 | date = May 2006 | pmid = 16507633 | doi = 10.1210/jc.2005-2400 | url = http://jcem.endojournals.org/cgi/pmidlookup?view=long&pmid=16507633 }}</ref><ref name=Dietlein_2007>{{cite journal | vauthors = Dietlein M, Wegscheider K, Vaupel R, Schmidt M, Schicha H | title = Management of multinodular goiter in Germany (Papillon 2005): do the approaches of thyroid specialists and primary care practitioners differ? | journal = Nuklearmedizin. Nuclear Medicine | volume = 46 | issue = 3 | pages = 65–75 | year = 2007 | pmid = 17549317 | doi = 10.1160/nukmed-0068 }}</ref> Levothyroxine is also used as interventional therapy in people with nodular thyroid disease or thyroid cancer to suppress thyroid-stimulating hormone (TSH) secretion.<ref name=Mandel>{{cite journal | vauthors = Mandel SJ, Brent GA, Larsen PR | title = Levothyroxine therapy in patients with thyroid disease | journal = Annals of Internal Medicine | volume = 119 | issue = 6 | pages = 492–502 | date = September 1993 | pmid = 8357116 | doi = 10.7326/0003-4819-119-6-199309150-00009 }}</ref> A subset of people with hypothyroidism treated with an appropriate dose of levothyroxine will describe continuing symptoms despite TSH levels in the normal range.<ref name="Gaitonde" /> In these people, further laboratory and clinical evaluation is warranted as they may have another cause for their symptoms.<ref name="Gaitonde" /> Furthermore, it is important to review their medications and possible dietary supplements as several medications can affect thyroid hormone levels.<ref name="Gaitonde" />
Levothyroxine is also used to treat subclinical hypothyroidism which is defined by an elevated TSH level and a normal-range free T<sub>4</sub> level without symptoms.<ref name="Gaitonde" /> Such people may be asymptomatic<ref name="Gaitonde" /> and whether they should be treated is controversial.<ref name="Roberts" /> One benefit of treating this population with levothyroxine therapy is preventing development of hypothyroidism.<ref name="Roberts" /> As such, it is recommended that treatment should be taken into account for patients with initial TSH levels above 10 mIU/L, people with elevated thyroid peroxidase antibody titers, people with symptoms of hypothyroidism and TSH levels between 5–10 mIU/L, and women who are pregnant or want to become pregnant.<ref name="Roberts" /> Oral dosing for patients with subclinical hypothyroidism is 1 μg/kg/day.<ref name="LexiComp" />
It is also used to treat myxedema coma, which is a severe form of hypothyroidism characterized by mental status changes and hypothermia.<ref name="Gaitonde" /> As it is a medical emergency with a high mortality rate, it should be treated in the intensive care unit<ref name="Gaitonde" /> with thyroid hormone replacement and aggressive management of individual organ system complications.<ref name="Roberts" />
[[File:007815180lg Generic Levothyroxine.jpg|thumb|Generic levothyroxine, 25 μg oral tablet]]
Dosages vary according to the age groups and the individual condition of the person, body weight and compliance to the medication and diet. Other predictors of the required dosage are [[sex]], [[Body mass index|BMI]], [[Sum activity of peripheral deiodinases|deiodinase activity (SPINA-GD)]] and [[etiology]] of [[hypothyroidism]].<ref name=pmid26335522>{{cite journal | vauthors = Midgley JE, Larisch R, Dietrich JW, Hoermann R | title = Variation in the biochemical response to <small>L</small>-thyroxine therapy and relationship with peripheral thyroid hormone conversion efficiency | journal = Endocrine Connections | volume = 4 | issue = 4 | pages = 196–205 | date = December 2015 | pmid = 26335522 | doi = 10.1530/EC-15-0056 | pmc=4557078}}</ref> Annual or semiannual clinical evaluations and TSH monitoring are appropriate after dosing has been established.<ref>{{EMedicine|article|122393|Hypothyroidism|treatment}}</ref> Levothyroxine is taken on an empty stomach approximately half an hour to an hour before meals.<ref name="RxList" /> As such, thyroid replacement therapy is usually taken 30 minutes prior to eating in the morning.<ref name="Gaitonde" /> For patients with trouble taking levothyroxine in the morning, bedtime dosing is effective as well.<ref name="Gaitonde" /> A recent study published in JAMA showed greater efficacy of levothyroxine when taken at bedtime.<ref>{{Cite news | title = Effects of Evening vs Morning Levothyroxine Intake: A Randomized Double-blind Crossover Trial | url = http://archinte.jamanetwork.com/article.aspx?articleid=776486 | url-status = live | archive-url = https://web.archive.org/web/20150906163348/http://archinte.jamanetwork.com/article.aspx?articleid=776486 | archive-date = 6 September 2015 | df = dmy-all }}</ref>
Poor [[Compliance (medicine)|compliance]] in taking the medicine is the most common cause of elevated TSH levels in people receiving appropriate doses of levothyroxine.<ref name="Gaitonde" />
=== Elderly ===
For older people (over 50 years old) and people with known or suspected [[Coronary artery disease|ischemic heart disease]], levothyroxine therapy should not be initiated at the full replacement dose.<ref name=Novothyrox>{{Cite web | title = Novothyrox (levothyroxine sodium tablets, USP) | url = http://www.accessdata.fda.gov/drugsatfda_docs/label/2009/021292s002lbl.pdf | access-date = 20 April 2014 | url-status = live | archive-url = https://web.archive.org/web/20130307010006/http://www.accessdata.fda.gov/drugsatfda_docs/label/2009/021292s002lbl.pdf | archive-date = 7 March 2013 | df = dmy-all }}</ref> Since thyroid hormone increases the heart's oxygen demand by increasing heart rate and contractility, starting at higher doses may cause an [[acute coronary syndrome]] or an [[arrhythmia|abnormal heart rhythm]].<ref name="Gaitonde" />
=== Pregnancy and breastfeeding ===
According to the U.S. [[Food and Drug Administration]] pregnancy categories, levothyroxine has been assigned Pregnancy Category A.<ref name="Novothyrox" /> Given that no increased risk of congenital abnormalities have been demonstrated in pregnant women taking levothyroxine, therapy should be continued during pregnancy.<ref name="Novothyrox" /> 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.<ref name="Novothyrox" />
Thyroid hormone requirements increase during and last throughout pregnancy.<ref name="Gaitonde" /> As such, it is recommended that pregnant women increase to nine doses of levothyroxine each week, rather than the usual seven, as soon as their pregnancy is confirmed.<ref name="Gaitonde" /> Repeat thyroid function tests should be done five weeks after the dosage is increased.<ref name="Gaitonde" />
While a minimal amount of thyroid hormones are found in breast milk, the amount does not influence infant plasma thyroid levels.<ref name="LexiComp" /> Furthermore, levothyroxine was not found to cause any adverse events to the infant or mother during breastfeeding.<ref name="LexiComp" /> As adequate concentrations of thyroid hormone are required to maintain normal lactation, it is recommended that appropriate levothyroxine doses be administered during breastfeeding.<ref name="LexiComp" />
===
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.<ref name="Novothyrox" />
== 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.<ref name="LexiComp" /> Levothyroxine is also contraindicated for people with uncorrected adrenal insufficiency, as thyroid hormones may cause an acute adrenal crisis by increasing the metabolic clearance of glucocorticoids.<ref name="Novothyrox" /> For oral tablets, the inability to swallow capsules serves as an additional contraindication.<ref name="LexiComp" />
==Side effects==
Adverse events are generally caused by incorrect dosing. Long-term suppression of TSH values below normal values will frequently cause cardiac side-effects and contribute to decreases in bone mineral density (low TSH levels are also well known to contribute to [[osteoporosis]]).<ref name=Frilling_2004>{{cite journal | vauthors = Frilling A, Liu C, Weber F | title = Benign multinodular goiter | journal = Scandinavian Journal of Surgery | volume = 93 | issue = 4 | pages = 278–81 | year = 2004 | pmid = 15658668 | doi = 10.1177/145749690409300405 }}</ref>
Too high a dose of levothyroxine causes [[Hyperthyroidism#Signs and symptoms|hyperthyroidism]].<ref name="RxList">{{Cite web | title = Synthroid (Levothyroxine Sodium) Drug Information: Uses, Side Effects, Drug Interactions and Warnings | publisher = RxList | url = http://www.rxlist.com/synthroid-drug.htm | access-date = 18 July 2010 | url-status = live | archive-url = https://web.archive.org/web/20100511145303/http://www.rxlist.com/synthroid-drug.htm | archive-date = 11 May 2010 | df = dmy-all }}</ref> Overdose can result in heart [[palpitations]], abdominal pain, nausea, anxiousness, confusion, agitation, [[insomnia]], weight loss, and increased appetite.<ref name=Toxicity /> Allergic reactions to the drug are characterized by symptoms such as difficulty breathing, shortness of breath, or swelling of the face and tongue. Acute overdose may cause [[fever]], [[hypoglycemia]], [[heart failure]], [[coma]], and unrecognized [[adrenal insufficiency]].
Acute massive overdose may be life-threatening; treatment should be symptomatic and supportive. Massive overdose can be associated with increased [[Sympathetic nervous system|sympathetic]] activity and thus require treatment with [[beta-blockers]].<ref name="RxList" />
The effects of overdosing appear 6 hours to 11 days after ingestion.<ref name=Toxicity>{{Cite web | title = Toxicity, Thyroid Hormone | author = Irizarry, Lisandro | publisher = WebMd | url = http://www.emedicine.com/emerg/TOPIC800.HTM | date = 23 April 2010 | access-date = 31 October 2010 | url-status = live | archive-url = https://web.archive.org/web/20081014025341/http://www.emedicine.com/emerg/TOPIC800.HTM | archive-date = 14 October 2008 | df = dmy-all }}</ref>
== Interactions ==
There are foods and other substances that can interfere with absorption of [[thyroxine]]. Substances that reduce absorption are [[aluminium]] and [[magnesium]] containing [[antacids]], [[simethicone]], [[sucralfate]], [[cholestyramine]], [[colestipol]], and [[polystyrene sulfonate]]. [[Grapefruit]] juice may delay the absorption of levothyroxine, but based on a study of 10 healthy people aged 20–30 (8 men, 2 women) it may not have a significant effect on bioavailability in young adults.<ref name="pmid16120075">{{cite journal | vauthors = Lilja JJ, Laitinen K, Neuvonen PJ | title = Effects of grapefruit juice on the absorption of levothyroxine | journal = British Journal of Clinical Pharmacology | volume = 60 | issue = 3 | pages = 337–41 | date = September 2005 | pmid = 16120075 | pmc = 1884777 | doi = 10.1111/j.1365-2125.2005.02433.x }}</ref> A study of eight women suggested that coffee may interfere with the intestinal absorption of levothyroxine, though at a level less than eating bran.<ref name="Benvenga">{{cite journal | vauthors = Benvenga S, Bartolone L, Pappalardo MA, Russo A, Lapa D, Giorgianni G, Saraceno G, Trimarchi F | title = Altered intestinal absorption of <small>L</small>-thyroxine caused by coffee | journal = Thyroid | volume = 18 | issue = 3 | pages = 293–301 | date = March 2008 | pmid = 18341376 | doi = 10.1089/thy.2007.0222 | publisher = Mary Ann Liebert }}</ref> Certain other substances can cause adverse effects that may be severe. Combination of levothyroxine with [[ketamine]] may cause [[hypertension]] and [[tachycardia]];<ref name="Austria-Codex">{{Cite book | title = Austria-Codex | edition = 62nd | veditors = Jasek W | year = 2007 | language = German | publisher = Österreichischer Apothekerverlag | location = Vienna | pages = 8133–4 | isbn = 978-3-85200-181-4}}</ref> and [[Tricyclic antidepressant|tricyclic]] and [[Tetracyclic antidepressant|tetracyclic]] [[antidepressants]] increase its toxicity. On the other hand, [[Lithium (medication)|lithium]] can cause hyperthyroidism (but most often hypothyroidism) by affecting [[iodine]] metabolism of the thyroid itself and thus inhibits synthetic levothyroxine as well.<ref>{{cite journal | vauthors = Bocchetta A, Loviselli A | title = Lithium treatment and thyroid abnormalities | journal = Clinical Practice and Epidemiology in Mental Health | volume = 2 | issue = 1 | pages = 23 | date = September 2006 | pmid = 16968542 | doi = 10.1186/1745-0179-2-23 | pmc = 1584230 }}</ref>
== Mechanism of action ==
Levothyroxine is a synthetic form of [[thyroxine]] (T<sub>4</sub>), an endogenous hormone secreted by the thyroid gland, which is converted to its active metabolite, {{small|L}}-triiodothyronine (T<sub>3</sub>).<ref name="Novothyrox" /> T<sub>4</sub> and T<sub>3</sub> bind to thyroid receptor proteins in the cell nucleus and cause metabolic effects through the control of DNA transcription and protein synthesis.<ref name="Novothyrox" /> Like its naturally secreted counterpart, levothyroxine is a [[Chirality (chemistry)|chiral]] compound in the {{small|L}}-form.
== Pharmacokinetics ==
Absorption of orally administered levothyroxine from the gastrointestinal tract ranges from 40 to 80%, with the majority of the drug absorbed from the jejunum and upper ileum.<ref name="Novothyrox" /> Levothyroxine absorption is increased by fasting and decreased in certain malabsorption syndromes, by certain foods, and with age. The bioavailability of the drug is decreased by dietary fiber.<ref name="Novothyrox" />
Greater than 99% of circulating thyroid hormones are bound to plasma proteins including thyroxine-binding globulin, [[transthyretin]] (previously called 'thyroxine-binding prealbumin'), and albumin.<ref name="LexiComp" /> Only free hormone is metabolically active.<ref name="LexiComp" />
The primary pathway of thyroid hormone metabolism is through sequential deiodination.<ref name="Novothyrox" /> The liver is the main site of T<sub>4</sub> deiodination, and along with the kidneys are responsible for about 80% of circulating T<sub>3</sub>.<ref name=Sherwood7thEd>{{Cite book | chapter = 19 The Peripheral Endocrine Glands | last = Sherwood | first = Lauralee | title = Human Physiology | year = 2010 | publisher = Brooks/Cole | page = 694 | isbn = 978-0-495-39184-5}}</ref> 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.<ref name="LexiComp" />
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.<ref name="LexiComp" /> Thyroid hormones are primarily eliminated by the kidneys (approximately 80%), with urinary excretion decreasing with age.<ref name="LexiComp" /> The remaining 20% of T<sub>4</sub> eliminated in the stool.<ref name="LexiComp" />
==History==
Thyroxine was first isolated in pure form in 1914 at the [[Mayo Clinic]] by [[Edward Calvin Kendall]] from extracts of hog thyroid glands.<ref>{{Cite journal | title = The isolation in crystalline form of the compound containing iodin, which occurs in the thyroid: Its chemical nature and physiologic activity | vauthors = Kendall EC | journal = J. Am. Med. Assoc. | year = 1915 | volume = 64 | issue = 25 | pages = 2042–2043 | doi = 10.1001/jama.1915.02570510018005| url = https://zenodo.org/record/1423411 }}</ref> The hormone was [[Organic synthesis|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 United States,<ref name=Kleinrock>{{Cite web | title = The Use of Medicines in the United States: Review of 2011 | last = Kleinrock | first = Michael | publisher = IMS Institute for Healthcare Informatics | url = http://www.environmentalhealthnews.org/ehs/news/2013/pdf-links/IHII_Medicines_in_U.S_Report_2011-1.pdf | access-date = 20 April 2014 | url-status = live | archive-url = https://web.archive.org/web/20131126174323/http://www.environmentalhealthnews.org/ehs/news/2013/pdf-links/IHII_Medicines_in_U.S_Report_2011-1.pdf | archive-date = 26 November 2013 | df = dmy-all }}</ref> with 23.8 million prescriptions filled each year.<ref name=Moore>{{Cite web | title = Monitoring FDA MedWatch Reports: Signals for Dabigatran and Metoclopramide | last = Moore | first = Thomas | work = QuarterWatch | publisher = Institute for Safe Medication Practices | url = http://www.ismp.org/quarterwatch/pdfs/2011Q1.pdf | access-date = 20 April 2014 | url-status = live | archive-url = https://web.archive.org/web/20130925064909/http://ismp.org/quarterwatch/pdfs/2011Q1.pdf | archive-date = 25 September 2013 | df = dmy-all }}</ref>
In 2016 it became the most commonly prescribed medication in the US, with more than 114 million prescriptions.<ref name=top300/>
===Available forms===
Levothyroxine for systemic administration is available as an oral tablet, an intramuscular injection, and as a solution for intravenous infusion.<ref name=LexiComp>{{Cite web | title = Levothyroxine (Lexi-Drugs) | work = LexiComp | url = http://online.lexi.com/lco/action/doc/retrieve/docid/patch_f/7172 | access-date = 20 April 2014 | url-status = live | archive-url = https://web.archive.org/web/20140929172946/http://online.lexi.com/lco/action/doc/retrieve/docid/patch_f/7172 | archive-date = 29 September 2014 | df = dmy-all }}</ref> Furthermore, levothyroxine is available as both brand-name and generic products.<ref name="Gaitonde" /> While the United States [[Food and Drug Administration]] (FDA) approved the use of generic levothyroxine for brand-name levothyroxine in 2004, the decision was met with disagreement by several medical associations.<ref name="Gaitonde" /> The [[American Association of Clinical Endocrinologists]] (AACE), the Endocrine Society, and the American Thyroid Association did not agree with the FDA that brand-name and generic formulations of levothyroxine were bioequivalent.<ref name="Gaitonde" /> As such, it was recommended that people be started and kept on either brand-name or generic levothyroxine formulations and not changed back and forth from one to the other.<ref name="Gaitonde" /> For people who do switch products, it is recommended that their TSH and free T<sub>4</sub> levels be tested after six weeks to check that they are within normal range.<ref name="Gaitonde" />
Common [[brand names]] include Eltroxin, Euthyrox, Eutirox, Letrox, Levaxin, Lévothyrox, Levoxyl, {{small|L}}-thyroxine, Thyrax, and Thyrax Duotab in Europe; Thyrox, Thyronorm in South Asia; Unithroid, Eutirox, Synthroid, and Tirosint in North and South America; and Thyrin and Thyrolar in Bangladesh. There are also numerous [[Generic drug|generic]] versions.
The related drug [[dextrothyroxine]] ({{small|D}}-thyroxine) was used in the past as a treatment for [[hypercholesterolemia]] (elevated cholesterol levels) but was withdrawn due to cardiac side effects.
== References ==
{{Reflist}}
== External links ==
* [http://medlibrary.org/drugs/thyroid/euthyrox.html Detailed Euthyrox (Levothroid/Levothyroxine) Consumer Information: Uses, Precautions, Side Effects]
* [http://druginfo.nlm.nih.gov/drugportal/dpdirect.jsp?name=Levothyroxine U.S. National Library of Medicine: Drug Information Portal – Levothyroxine]
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