Levothyroxine

Systematic (IUPAC) name
3,5,3',5'-Tetraiodo-L-thyronine
Identifiers
CAS number	51-48-9
ATC code	H03AA01
PubChem	853
DrugBank	APRD00235
Chemical data
Formula	C15H11I4NO4
Mol. mass	798.86 (anhydrous)
SMILES	eMolecules & PubChem
Synonyms	O-(4-hydroxy-3,5-diiodophenyl)-3,5-diiodo-L-tyrosine
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
Therapeutic considerations
Pregnancy cat.	A(US)
Legal status	℞ Prescription only
Routes	Oral, Intravenous
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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). The natural hormone is chemically in the 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.

The EU has recently standardized the use of the International Nonproprietary Name "levothyroxine" for the drug. Common brand names include Thyrax, Euthyrox, Levaxin, L-thyroxine and Eltroxin in Europe; Thyrox in South Asia; Eutirox, Levoxyl and Synthroid in North America. There are also numerous generic versions.

Contents 1 Uses 2 Dosage for humans and precautions 3 Dosage for dogs with hypothyroidism 4 Adverse effects 5 Marketing and approvals in the United States 6 References 7 External links

[edit] Uses

This medicine is a hormone replacement usually given to patients with thyroid problems, specifically, hypothyroidism.^[1]

It also has been given to people who have goitre or an enlarged thyroid gland.^[2]

[edit] Dosage for humans and precautions

Dosages vary according to the age groups and the individual condition of the patient, body weight and compliance to the medication and diet. Maximum dosage may reach 400 mcg per day but that is rare. Monitoring of the patients 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 to maximize its absorption, often with water to ease swallowing and help the tablet dissolve for absorption.

There are also foods and other substances that can interfere with absorption of thyroxine replacement. Patients under treatment ought to avoid taking calcium and iron supplements within 4 hours,^[3] as well as soy products within 3 hours of the medication, as these can reduce absorption of the drug. Other substances that reduce absorption are aluminium and magnesium containing antacids, simethicone or sucralfate, cholestyramine, colestipol, Kayexalate. Recently, in a study of 8 women, it has been discovered that coffee may interfere with the intestinal absorption of levothyroxine, though at a level less than eating bran.^[4] 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.

[edit] Dosage for dogs with hypothyroidism

The dosage for dogs with hypothyroidism is usually a twice daily dose of between 10-40 mcg per kg of body weight. Usually at the beginning of the treatment, the lowest dose is used and 4 weeks later the blood values are checked to see if the dog has responded. The dosage is increased as necessary until normal levels are achieved.

[edit] Adverse effects

Synthetic levothyroxine can cause adverse effects such as palpitations, nervousness, headache, insomnia, swelling of the legs and ankles, weight loss, and increased appetite. 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.

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

[edit] Marketing and approvals in the United States

Synthroid is the most prescribed brand of T4 in the United States. Synthroid was marketed in 1955, but was not FDA approved at that time as it was "generally regarded safe".^[5] In the 1990s, in response to debate as to whether Synthroid was more effective than other levothyroxine preparations, (which ended up concluding that there was little difference between Synthroid and generic brands) all levothyroxine preparations were required to undergo the formal FDA approval process. Synthroid was approved by the FDA on 24 July 2002.^[6]

[edit] References

[edit] External links

• Detailed Euthyrox (Levothroid/Levothyroxine) Consumer Information: Uses, Precautions, Side Effects
• Overdose Some useful information on thyroid hormones, on a webpage dealing with chronic or acute overdoses of such hormones.
• Information on Levothyroxine at Drugs.com
• Elaborate article about the use of thyroid drugs Written by an MD
• What is the "Best" Thyroid Drug? Is it Synthroid, Unithroid, Armour, Thyrolar, or Something Else? Article by health activist and patient advocate Mary Shomon

v • d • e Thyroid therapy (H03) Thyroid hormones Levothyroxine sodium# · Liothyronine sodium · Tiratricol · Thyroid gland preparations Antithyroid preparations Sodium-iodide symporter inhibitor Perchlorate (Potassium perchlorate) · Pertechnetate (Sodium pertechnetate) Thyroid peroxidase inhibitors (thioamide) Thiouracils: Methylthiouracil · Propylthiouracil# · Benzylthiouracil Sulfur-containing imidazole derivatives: Carbimazole · Thiamazole Block conversion of T4 to T3 Ipodate sodium · Propylthiouracil# Other Diiodotyrosine · Dibromotyrosine #WHO-EM. ‡Withdrawn from market. CLINICAL TRIALS: †Phase III. §Never to phase III