Levothyroxine.html



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	C15H10I4NNaO4
Mol. mass	798.86 (anhydrous)
SMILES	eMolecules & PubChem
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

levothyroxine, also known as 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 on 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.

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

Administration

It is recommended that levothyroxine be taken with an empty stomach approximately half an hour to an hour before meals to maximize its absorption. It is also recommended that the patient take the tablet with one glass of water to ease swallowing as well as to help the tablet dissolve for absorption.

Calcium and iron supplements (taken as a supplement with or without multivitamins) are known to suppress the absorption of levothyroxine and should be taken with a gap of at least four hours³. It is likely that zinc and magnesium would similarly affect absorption. Patients that started levothyroxine while taking calcium supplements may require re-titration of the levothyroxine.

Dosage, precautions and side effects

Dosages vary according 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.

There are also foods and other substances that can interfere with absorption of thyroxine replacement. Avoid taking calcium and iron supplements within 4 hours of the medication and avoid taking soy products within 3 hours of the medication as these can reduce absorption of the medication. Other substances that reduce absorption are aluminium and magnesium containing antacids, simethicone or sucralfate, Cholestyramine, colestipol, Kayexalate. Other 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.

Synthetic levothyroxine may have adverse side effects like: palpitations, nervousness, headache, difficulty sleeping, insomnia, swelling of the legs and ankles, weight loss and/or increased appetite. Allergies to the medicine are unlikely, but if the patient develops a severe reaction to this drug such as difficulty breathing, shortness of breath or swelling of the face and tongue it is imperative that the patient immediately seek medical attention.

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.

Studies have shown that using liothyronine in combination with levothyroxine does not produce synergistic effects; other studies discourage the use of these two medications concurrently.

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

Marketing and approvals in the United States

Levothyroxine 25 µg tablets

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 considered "generally regarded safe".⁴ 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 2002-07-24.⁵

Levothyroxine is on the negative formulary of several states, due the FDA's review of all l-thyroxine preparations in the 1990s. In the state of Florida for instance, Synthroid was only recently removed from the negative formulary, allowing pharmacies to dispense levothyroxine without prior physician approval.

References

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

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