| Systematic (IUPAC) name | |
|---|---|
| 3,5,3',5'-Tetraiodo-L-thyronine | |
| 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 |
| Identifiers | |
| CAS number | 51-48-9 |
| ATC code | H03AA01 |
| PubChem | CID 5819 |
| DrugBank | APRD00235 |
| ChemSpider | 5614 |
| UNII | Q51BO43MG4 |
| ChEBI | CHEBI:18332 |
| ChEMBL | CHEMBL1624 |
| Synonyms | O-(4-hydroxy-3,5-diiodophenyl)-3,5-diiodo-L-tyrosine |
| Chemical data | |
| Formula | C15H11I4NO4 |
| Mol. mass | 776.87 |
| SMILES | eMolecules & PubChem |
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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.
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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]
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.
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]
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.
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