Levothyroxine

Levothyroxine

Clinical data
Routes of administration	Oral, Intravenous
ATC code	H03AA01 (WHO)
Legal status
Legal status	US: WARNING In general: ℞ (Prescription only)
Pharmacokinetic data
Bioavailability	~100%
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	Through feces and urine
Identifiers
IUPAC name 3,5,3',5'-Tetraiodo-L-thyronine
CAS Number	51-48-9
PubChem CID	853
DrugBank	APRD00235
CompTox Dashboard (EPA)	DTXSID8023214
ECHA InfoCard	100.000.093
Chemical and physical data
Formula	C15H10I4NNaO4
Molar mass	798.86 (anhydrous) g·mol−1
3D model (JSmol)	Interactive image
SMILES OC(=O)C(N)Cc1cc(I)c(Oc2cc(I)c(O)c(I)c2)c(I)c1

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, and "Levoxyl" and "Synthroid" in the United States. There are also numerous generic versions approved by the Food and Drug Administration.

Uses

This medicine is a hormone replacement usually given to patients with thyroid problems, specifically, hypothyroidism. It is also given to people who have goitre or an enlarged thyroid gland.

Administration

It is recommended that levothyroxine be taken 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.

Precautions and side effects

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.

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.

Marketing and approvals in the United States

Levothyroxine 25 µg tablets

Synthroid is the most prescribed brand of T4 in United States. Synthroid was marketed in 1955, but was not FDA approved at that time as it was considered "generally regarded safe". 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. 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, it is illegal to dispense any other preparation than that which was written, without the approval of the prescriber (i.e. Synthroid to Levothyroxine or Levoxyl to Levothroid). Any third party claim for this must be submitted as a DAW (Dispense as Written) Code: "7-Brand Mandated by Law," or the pharmacy may face fines from the governing State Board of Pharmacy.

External links

• Detailed Euthyrox (Levothroid/Levothyroxine) Consumer Information: Uses, Precautions, Side Effects
• Leaflet
• [1] Some useful information on thyroid hormones, on a webpage dealing with chronic or acute overdoses of such hormones.