Systematic (IUPAC) name
Clinical data
Trade names Euthyrox among others, Synthroid, Thyrax
AHFS/Drugs.com monograph
MedlinePlus a682461
Pregnancy cat. A(US)
Legal status Prescription only
Routes Oral, Intravenous
Pharmacokinetic data
Bioavailability ~100%
Metabolism Mainly in liver, kidneys, brain and muscles
Half-life ca. 7 days (in hyperthyroidism 3-4 days, in hypothyroidism 9-10 days)
Excretion Through feces and urine
CAS number 51-48-9 YesY
ATC code H03AA01
PubChem CID 5819
DrugBank APRD00235
ChemSpider 5614 YesY
ChEBI CHEBI:18332 YesY
Synonyms O-(4-hydroxy-3,5-diiodophenyl)-3,5-diiodo-L-tyrosine
Chemical data
Formula C15H11I4NO4 
Mol. mass 776.87
SMILES eMolecules & PubChem
 N(what is this?)  (verify)

Levothyroxine, also L-thyroxine, synthetic T4, or 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine (thyroid hormone), used as a hormone replacement for patients with thyroid problems. The natural hormone is chemically in the chiral L-form, as is the pharmaceutical agent. Dextrothyroxine (D-thyroxine) briefly saw research as an anticholesterol agent but was pulled due to cardiac side-effects.


Medical uses

Levothyroxine is typically used to treat hypothyroidism.[1] It may also be used to treat goiter via its ability to lower thyroid-stimulating hormone (TSH), a hormone that is considered goiter-inducing.[2][3]

Adverse effects

Dosing must be carefully controlled to achieve TSH levels within the normal reference range. Long-term suppression of TSH values below normal values will frequently cause cardiac side-effects and contribute to decreases in bone mineral density (high TSH levels are also well known to contribute to osteoporosis).[4]

Patients prescribed too high a dose of levothyroxine may experience effects that mimic hyperthyroidism.[5] Overdose can result in heart palpitations, abdominal pain, nausea, anxiousness, confusion, agitation, insomnia, weight loss, and increased appetite.[6] 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 may require beta-blockers for increased sympathomimetic activity.[5]

The effects of overdosing appear 6 hours to 11 days after ingestion.[6]


There are also foods and other substances that can interfere with absorption of thyroxine replacement. People ought to avoid taking calcium and iron supplements within 4 hours,[7] as well as soy products within 3 hours of the medication, as these can reduce absorption of the drug. Grapefruit juice may delay the absorption of levothyroxine, but it is not believed to have a significant effect on bioavailability.[8] Other substances that reduce absorption are aluminium and magnesium containing antacids, simethicone or sucralfate, cholestyramine, colestipol, Kayexalate. A study of eight women suggested that coffee may interfere with the intestinal absorption of levothyroxine, though at a level less than eating bran.[9] Different substances cause other adverse effects that may be severe. Ketamine may cause hypertension and tachycardia and tricyclic and tetracyclic antidepressants increase its toxicity. On the other hand lithium can cause hyperthyroidism (but most often hypothyroidism) by affecting iodine metabolism of the thyroid itself and thus inhibits synthetic levothyroxine as well.


Generic Levothyroxine, 25 MCG Oral Tablet

Dosages vary according to the age groups and the individual condition of the patient, body weight and compliance to the medication and diet. Monitoring of the patient's condition and adjustment of the dosage is periodical and necessary. Levothyroxine is taken on an empty stomach approximately half an hour to an hour before meals.[5]

Brand names

Common brand names include Thyrax, Euthyrox, Levaxin, L-thyroxine, Eltroxin and Thyrax Duotab in Europe; Thyrox in South Asia; Eutirox, Tirosint, Levoxyl and Synthroid in North America. There are also numerous generic versions.


  1. ^ Vaidya B, Pearce SH (2008). "Management of hypothyroidism in adults". BMJ (Clinical research ed.) 337: a801. doi:10.1136/bmj.a801. PMID 18662921. http://bmj.com/cgi/pmidlookup?view=long&pmid=18662921. 
  2. ^ Svensson J, Ericsson UB, Nilsson P, et al. (May 2006). "Levothyroxine treatment reduces thyroid size in children and adolescents with chronic autoimmune thyroiditis". The Journal of clinical endocrinology and metabolism 91 (5): 1729–34. doi:10.1210/jc.2005-2400. PMID 16507633. http://jcem.endojournals.org/cgi/pmidlookup?view=long&pmid=16507633. 
  3. ^ Dietlein, M; Wegscheider, K; Vaupel, R; Schmidt, M; Schicha, H (2007). "Management of multinodular goiter in Germany (Papillon 2005): do the approaches of thyroid specialists and primary care practitioners differ?". Nuklearmedizin. Nuclear medicine 46 (3): 65–75. PMID 17549317.  edit
  4. ^ Frilling, A; Liu, C; Weber, F (2004). "Benign multinodular goiter". Scandinavian journal of surgery : SJS : official organ for the Finnish Surgical Society and the Scandinavian Surgical Society 93 (4): 278–81. PMID 15658668.  edit
  5. ^ a b c "Synthroid (Levothyroxine Sodium) Drug Information: Uses, Side Effects, Drug Interactions and Warnings". RxList. http://www.rxlist.com/synthroid-drug.htm. Retrieved 2010-07-18. 
  6. ^ a b Irizarry, Lisandro (2010-04-23). "Toxicity, Thyroid Hormone". WebMd. http://www.emedicine.com/emerg/TOPIC800.HTM. Retrieved 2010-10-31. 
  7. ^ Ruth H. Michel, Patricia J. Neafsey, Laura Cox Dzurec: Self Medication Practices among Patients taking Levothyroxine. The Internet Journal of Advanced Nursing Practice. 2004. Volume 6 Number 2
  8. ^ Lilja JJ, Laitinen K, Neuvonen PJ (September 2005). "Effects of grapefruit juice on the absorption of levothyroxine". Br J Clin Pharmacol 60 (3): 337–41. doi:10.1111/j.1365-2125.2005.02433.x. PMC 1884777. PMID 16120075. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1884777. 
  9. ^ Benvenga, Salvatore; Bartolone, L.; Pappalardo, M.A.; Russo, A.; Lapa, D.; Giorgianni, G.; Saraceno, G.; Trimarchi, F. (March 2008). "Altered Intestinal Absorption of L-Thyroxine Caused by Coffee". Thyroid (New York: Mary Ann Liebert, Inc.) 18 (3): 293–301. doi:10.1089/thy.2007.0222. PMID 18341376. http://www.ncbi.nlm.nih.gov/pubmed/18341376?dopt=Citation. Retrieved 2009-05-16. 

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Look at other dictionaries:

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  • Levothyroxine — Lévothyroxine Lévothyroxine Général No CAS …   Wikipédia en Français

  • Levothyroxine sodium — A synthetic thyroid hormone used as a thyroid hormone replacement drug (brand names include Eltroxin, Levothroid, Levoxine, Levoxyl, Synthroid) used to treat an underactive thyroid gland (hypothyroidism). Because not all brands of levothyroxine… …   Medical dictionary

  • levothyroxine — noun The synthetic form of the thyroid hormone, thyroxine See Also: dextrothyroxine …   Wiktionary

  • levothyroxine — le·vo·thy·rox·ine or Brit lae·vo·thy·rox·ine .lē vō thī räk .sēn, sən n the levorotatory isomer of thyroxine that is administered in the form of its sodium salt C15H10I4NNaO4 in the treatment of hypothyroidism see levothroid, levoxyl, SYNTHROID …   Medical dictionary

  • levothyroxine sodium — see thyroxine …   The new mediacal dictionary

  • 51-48-9 — Lévothyroxine Lévothyroxine Général No CAS …   Wikipédia en Français

  • Desiccated thyroid extract — Combination of Levothyroxine Thyroid hormone Liothyronine Thyroid hormone Clinical data Pregnancy cat. A Legal status ℞ only …   Wikipedia

  • Liothyronine sodium — Systematic (IUPAC) name …   Wikipedia

  • Hypothyroidism — Infobox Disease Name = Hypothyroidism Caption = Thyroxine (T4) normally produced in 20:1 ratio to triiodothyronine (T3) DiseasesDB = 6558 ICD10 = ICD10|E|03|9|e|00 ICD9 = ICD9|244.9 ICDO = OMIM = MedlinePlus = eMedicineSubj = med eMedicineTopic …   Wikipedia

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