Levothyroxine

Levothyroxine
Clinical data
Trade names	Synthroid, Levoxyl, Thyrax, others
Other names	3,5,3′,5′-Tetraiodo-L-thyronine
AHFS/Drugs.com	Monograph
MedlinePlus	a682461
License data	US DailyMed: Levothyroxine;
Pregnancy; category	AU: A;
Routes of; administration	Oral, intravenous
ATC code	H03AA01 (WHO) ;
Legal status
Legal status	AU: S4 (Prescription only); CA: ℞-only; US: WARNINGRx only;
Pharmacokinetic data
Bioavailability	40-80%
Metabolism	Mainly in liver, kidneys, brain and muscles
Elimination half-life	ca. 7 days (in hyperthyroidism 3–4 days, in hypothyroidism 9–10 days)
Excretion	Feces and urine
Identifiers
	IUPAC name (S)-2-Amino-3-[4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodophenyl]propanoic acid;
CAS Number	51-48-9 6106-07-6 (levothyroxine sodium hydrate);
PubChem CID	5819;
DrugBank	DB00451;
ChemSpider	5614;
UNII	Q51BO43MG4;
KEGG	D08125; as salt: D01010;
ChEBI	CHEBI:18332;
ChEMBL	ChEMBL1624;
CompTox Dashboard (EPA)	DTXSID8023214 ;
ECHA InfoCard	100.000.093
Chemical and physical data
Formula	C15H11I4NO4
Molar mass	776.874 g·mol−1
3D model (JSmol)	Interactive image;
Melting point	231 to 233 °C (448 to 451 °F)
Solubility in water	Slightly soluble (0.105 mg·mL−1 at 25 °C) mg/mL (20 °C)
	SMILES NC(Cc1cc(I)c(Oc2cc(I)c(O)c(I)c2)c(I)c1)C(O)=O;
	InChI InChI=1S/C15H11I4NO4/c16-8-4-7(5-9(17)13(8)21)24-14-10(18)1-6(2-11(14)19)3-12(20)15(22)23/h1-2,4-5,12,21H, 3,20H2, (H, 22,23)/t12-/m0/s1 ; Key:XUIIKFGFIJCVMT-LBPRGKRZSA-N;

Levothyroxine, also known as L-thyroxine, is a synthetic form of the thyroid hormone thyroxine (T₄). It is used to treat thyroid hormone deficiency (hypothyroidism), including 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 orally (by mouth) or given by intravenous injection. Levothyroxine has a half-life of 7.5 days when taken daily, so about six weeks is required for it to reach a steady level in the blood.

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 Organization's List of Essential Medicines. Levothyroxine is available as a generic medication. In 2021, it was the third most commonly prescribed medication in the United States, with more than 89 million prescriptions.

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