Levothyroxine  

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.

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

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

Recent Levothyroxine Forums  View All  

Levothyroxine Side Effects I was recently diagnosed with an underactive thyroid. I have been taking levothyroxine for almost two weeks. For the last four or five days I have had a weird feeling. It is hard to explain. I don't really feel dizzy but I get a strange feeling in my head. I guess you would describe it as a head rush. Has anyone else had this feeling and will it go away as my body adjusts to the levothyroxine? Tuesday, 11/17/2009 4:02:14 AM  57 Replies

Levothyroxine Is Satan I don't feel so confused anymore. Now I know all of you are going through the same thing. My doctor put me on 50mcg of Levothyroxine about a month ago and I have gain 8 pounds! I get short-breathed and winded REALLY easily when working out. I used to go to the gym for 2 hours, now I can't make it longer than 30 minutes. I wake up ALL the time with tingly fingers because my hands have fallen asleep. I have headaches, I feel 100 times more lazy than before...and it's supposed to help your energy, not drain it. I e-mailed my doctor today, this is not making me happy. Not to mention I now have an acne type rash that burns like hell on my chin and I've always had clear skin. If this drug is supposed to make you feel better it sure as sh*t isn't working for me. I feel like a fat lazy a$$ with an itchhy burning rash on my face... stay away from this drug, that's my suggestion. :( Thursday, 11/5/2009 10:39:36 AM  8 Replies

Gaining Weight After Reducing Dose.. I was taking .100 of Levothyroxine and my dr. said that it was slightly over corrected...now I am taking .75 and have already gained almost 2 pounds in 5 days. I am really frustrated! My bloodwork says one thing, but my body is reacting differently...any advise? Is there a dose between .100 and .75? Wednesday, 11/4/2009 7:25:08 AM  Post a Reply

Does Levothyroxine Cause Acne I have been taking 25mcg for about 6 weeks and feel much better, no weight loss, but no gain. I have developed acne on my chin and around the right side of my mouth...is this a side effect? Anyone else had this happen? Tuesday, 11/3/2009 4:37:30 PM  Post a Reply

Levothyroxine Side Effects I had a total throdectomy 2 years ago due to thyroid cancer. I have been on 150 - 200mg Levothyroxine daily. Side effects are - now and again my heart will beat once really hard, when i wake up my hands feel numb and fingers tingle, get dizzy/drunk feelings, dry itchy skin, dry sore eyes, stomach pains and constipation. Sometimes i feel like i am turning into a hypercondreact. Does anyone else feel like this? I am due another blood test next month. Thursday, 10/29/2009 12:24:02 PM  9 Replies

Thyroid im takin 75mg of synthroid an im tryin to get pregnant my doctor is tellin me to wait i have this tingling feeling in my pinky finger an gaining weight like crazy someone help Tuesday, 10/27/2009 7:08:32 PM  2 Replies

Drug Strength I was on .175 of Levothyroxine. I had a test done and the Dr. said my thyroid was too low and said I had to go up to 0.25. Is this a lower dose or a higher dose? Would it be due to high T4 levels or low T3 levels? I am confused. I've been steadly going up to the next dose each time I test. I've been at .175 for about 5 to 6 months. Monday, 10/26/2009 6:57:38 PM  1 Reply

Gain Weigh With Thyroid Medication I have been on 50 mcg of levothyroxine for 5 months and I have gained 15 pounds. My doctor tells me that it should be the opposite and that I should loose weight. Is this possible? Have any of you taking this drug has experienced the same? Monday, 9/14/2009 9:40:21 AM  2 Replies

Viltilgo/high Level Thyroid I just took a blood test and my Throid is very high I also have viltilgo and the doctor Prescribe me 75MG. Just need to know what are the side affects of this medicine. Thursday, 9/3/2009 3:25:35 PM  Post a Reply

What's The Correct Time To Take Drug? I've been taking Levothyroxine for a bout a month and a half because I had a hyper thyroid and had to have radation to stop my thyroid. I feel much better, but I'm scared of becoming fat. My container says, Take medication on an empty stomach 1HR before or 2-3 hours after a meal. Does this mean it's okay if I take it at 5:30 am and then go back to sleep 'til 9am? Or do I HAVE to eat exactly an hour afterwards. It's very hard to gage when I'm going to be eating or not. Often after a meal, I might still have a snack within an hour. I've never been a meal kind of person. I like to graze throughout the day. It's not like I'm as big as a cow. I'm 5'1 and 3/4'' and 101 lbs. It's very strange to go from 84 lbs. to 101 in just 2 months. I've got to go to sleep now because I just took my Levothyroxine and I'm feeling so hungry for a snack.:( Tuesday, 8/18/2009 11:10:50 AM  7 Replies