Levothyroxine

trade names: Levo-T, Synthroid; drug class: thyroid hormone; action: increases metabolic rate, with increase in cardiac output, O₂ consumption, body temperature, blood volume, growth/development at cellular level; uses: hypothroidism, myxedema coma, thyroid hormone replacement, cretinism.

Brand names: Estre™, Levo-T®, Levothroid®, Levoxyl®, Synthroid®, Thyro-Tabs®, Unithroid®

Chemical formula:

Drug Forms:
• Levothyroxine Sodium Oral tablet (below)
• Levothyroxine Sodium Solution for injection
• Levothyroxine injection
• Levothyroxine capsules

Español:
• Levotiroxina Sódica, Tableta oral
• Levotiroxina Sódica, Solución para inyección
• Inyección de levotiroxina

Levothyroxine Sodium Oral tablet

What is this medicine?

LEVOTHYROXINE is a thyroid hormone. This medicine can improve symptoms of thyroid deficiency such as slow speech, lack of energy, weight gain, hair loss, dry skin, and feeling cold. It also helps to treat goiter (an enlarged thyroid gland).

This medicine may be used for other purposes; ask your health care provider or pharmacist if you have questions.

What should I tell my health care provider before I take this medicine?

They need to know if you have any of these conditions:
• angina
• diabetes
• dieting or on a weight loss program
• fertility problems
• heart disease
• high levels of thyroid hormone
• pituitary gland problem
• previous heart attack
• an unusual or allergic reaction to levothyroxine, thyroid hormones, other medicines, foods, dyes, or preservatives
• pregnant or trying to get pregnant
• breast-feeding

How should I use this medicine?

Take this medicine by mouth with plenty of water. It is best to take on an empty stomach, at least 30 minutes before or 2 hours after food. Follow the directions on the prescription label. Take at the same time each day. Do not take your medicine more often than directed.

Contact your pediatrician regarding the use of this medicine in children. While this drug may be prescribed for children and infants as young as a few days of age for selected conditions, precautions do apply. For infants, you may crush the tablet and place in a small amount of (5-10 ml or 1 to 2 teaspoonfuls) of water, breast milk, or non-soy based infant formula. Do not mix with soy-based infant formula. Give as directed.

Overdosage: If you think you have taken too much of this medicine contact a poison control center or emergency room at once.
NOTE: This medicine is only for you. Do not share this medicine with others.

What if I miss a dose?

What may interact with this medicine?

• amiodarone
• antacids
• anti-thyroid medicines
• calcium supplements
• carbamazepine
• cholestyramine
• colestipol
• digoxin
• female hormones, including contraceptive or birth control pills
• iron supplements
• ketamine
• liquid nutrition products like Ensure
• medicines for colds and breathing difficulties
• medicines for diabetes
• medicines for mental depression
• medicines or herbals used to decrease weight or appetite
• phenobarbital or other barbiturate medications
• phenytoin
• prednisone or other corticosteroids
• rifabutin
• rifampin
• soy isoflavones
• sucralfate
• theophylline
• warfarin

This list may not describe all possible interactions. Give your health care provider a list of all the medicines, herbs, non-prescription drugs, or dietary supplements you use. Also tell them if you smoke, drink alcohol, or use illegal drugs. Some items may interact with your medicine.

What should I watch for while using this medicine?

Be sure to take this medicine with plenty of fluids. Some tablets may cause choking, gagging, or difficulty swallowing from the tablet getting stuck in your throat. Most of these problems disappear if the medicine is taken with the right amount of water or other fluids.

Do not switch brands of this medicine unless your health care professional agrees with the change. Ask questions if you are uncertain.

You will need regular exams and occasional blood tests to check the response to treatment. If you are receiving this medicine for an underactive thyroid, it may be several weeks before you notice an improvement. Check with your doctor or health care professional if your symptoms do not improve.

It may be necessary for you to take this medicine for the rest of your life. Do not stop using this medicine unless your doctor or health care professional advises you to.

This medicine can affect blood sugar levels. If you have diabetes, check your blood sugar as directed.

You may lose some of your hair when you first start treatment. With time, this usually corrects itself.

If you are going to have surgery, tell your doctor or health care professional that you are taking this medicine.

What side effects may I notice from receiving this medicine?

Side effects that you should report to your doctor or health care professional as soon as possible:
• difficulty breathing, wheezing, or shortness of breath
• chest pain
• excessive sweating or intolerance to heat
• fast or irregular heartbeat
• nervousness
• skin rash or hives
• swelling of ankles, feet, or legs
• tremors

Side effects that usually do not require medical attention (report to your doctor or health care professional if they continue or are bothersome):
• changes in appetite
• changes in menstrual periods
• diarrhea
• hair loss
• headache
• trouble sleeping
• weight loss

This list may not describe all possible side effects. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Where should I keep my medicine?

Keep out of the reach of children.

Store at room temperature between 15 and 30 degrees C (59 and 86 degrees F). Protect from light and moisture. Keep container tightly closed. Throw away any unused medicine after the expiration date.

Last updated: 3/20/2006 11:23:00 AM

Important Disclaimer: The drug information provided here is for educational purposes only. It is intended to supplement, not substitute for, the diagnosis, treatment and advice of a medical professional. This drug information does not cover all possible uses, precautions, side effects and interactions. It should not be construed to indicate that this or any drug is safe for you. Consult your medical professional for guidance before using any prescription or over the counter drugs.

The pharmaceutical name for the thyroid hormone thyroxine (T₄); used for replacement therapy in hypothyroidism.

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

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]

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.

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.

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.

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 "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 2002-07-24.^[6]

References

1. ^ Vaidya B, Pearce SH (2008). "Management of hypothyroidism in adults". BMJ (Clinical research ed.) 337: a801. doi:10.1136/bmj.a801. PMID 18662921. http://bmj.com/cgi/pmidlookup?view=long&pmid=18662921. 
2. ^ Svensson J, Ericsson UB, Nilsson P, et al. (May 2006). "Levothyroxine treatment reduces thyroid size in children and adolescents with chronic autoimmune thyroiditis". The Journal of clinical endocrinology and metabolism 91 (5): 1729–34. doi:10.1210/jc.2005-2400. PMID 16507633. http://jcem.endojournals.org/cgi/pmidlookup?view=long&pmid=16507633. 
3. ^ Ruth H. Michel, Patricia J. Neafsey, Laura Cox Dzurec: Self Medication Practices among Patients taking Levothyroxine. The Internet Journal of Advanced Nursing Practice. 2004. Volume 6 Number 2
4. ^ Benvenga, Salvatore; Bartolone, L.; Pappalardo, M.A.; Russo, A.; Lapa, D.; Giorgianni, G.; Saraceno, G.; Trimarchi, F. (March 2008). "Altered Intestinal Absorption of L-Thyroxine Caused by Coffee" (in English). Thyroid (New York: Mary Ann Liebert, Inc.) 18 (3): 293–301. doi:10.1089/thy.2007.0222. PMID 18341376. http://www.ncbi.nlm.nih.gov/pubmed/18341376?dopt=Citation. Retrieved 2009-05-16. 
5. ^ "Abbott Laboratories Asks Federal Court to Help Protect Patients Against Misleading Information About Synthroid(R) (levothyroxine sodium, USP)". www.companyreports.com. http://www.companyreports.com/cgi-bin/stories.pl?ACCT=105&STORY=/www/story/06-12-2001/0001512746. Retrieved 2008-08-14. 
6. ^ "Drugs@FDA". www.accessdata.fda.gov. http://www.accessdata.fda.gov/scripts/cder/drugsatfda/index.cfm?fuseaction=Search.DrugDetails. Retrieved 2008-08-14. 

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

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