Levothyroxine was first made in 1927.It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system.Levothyroxine is available as a generic medication.
Levothyroxine, also known as L-thyroxine, is a manufactured form of the thyroid hormone, thyroxine (T4).It is used to treat thyroid hormone deficiency including the severe form known as myxedema coma.It may also be used to treat and prevent certain types of thyroid tumors.It is not indicated for weight loss. Levothyroxine is taken by mouth or given by injection into a vein.Maximum effect from a specific dose can take up to six weeks to occur.
Common brand names include Eltroxin, Euthyrox, Letrox, Levaxin, Lévothyrox, L-thyroxine, Thyrax, and Thyrax Duotab in Europe; Thyrox, Thyronorm in South Asia; Unithroid, Eutirox, Levoxyl, Synthroid, and Tirosint in North and South America; and Thyrin and Thyrolar in Bangladesh. There are also numerous generic versions.
Levothyroxine for systemic administration is available as an oral tablet, an intramuscular injection, and as a solution for intravenous infusion. Furthermore, levothyroxine is available as both brand-name and generic products. While the United States Food and Drug Administration (FDA) approved the use of generic levothyroxine for brand-name levothyroxine in 2004, the decision was met with disagreement by several medical associations.
The American Association of Clinical Endocrinologists (AACE), the Endocrine Society, and the American Thyroid Association did not agree with the FDA that brand-name and generic formulations of levothyroxine were bioequivalent.As such, it was recommended that people be started and kept on either brand-name or generic levothyroxine formulations and not changed back and forth from one to the other.For people who do switch products, it is recommended that their TSH and free T4 levels be tested after six weeks to check that they are within normal range.
Absorption of orally administered levothyroxine from the gastrointestinal tract ranges from 40 to 80%, with the majority of the drug absorbed from the jejunum and upper ileum.Levothyroxine absorption is increased by fasting and decreased in certain malabsorption syndromes, by certain foods, and with age. The bioavailability of the drug is decreased by dietary fiber.
Greater than 99% of circulating thyroid hormones are bound to plasma proteins including thyroxine-binding globulin, thyroxine-binding prealbumin, and albumin.Only free hormone is metabolically active.
The primary pathway of thyroid hormone metabolism is through sequential deiodination.The liver is the main site of T4 deiodination, and along with the kidneys are responsible for about 80% of circulating T3. In addition to deiodination, thyroid hormones are also excreted through the kidneys and metabolized through conjugation and glucuronidation and excreted directly into the bile and the gut where they undergo enterohepatic recirculation.
Half-life elimination is 6–7 days for people with normal lab results; 9–10 days for people with hypothyroidism; 3–4 days for people with hyperthyroidism.Thyroid hormones are primarily eliminated by the kidneys (approximately 80%), with urinary excretion decreasing with age.The remaining 20% of T4 eliminated in the stool.
Levothyroxine is a synthetic form of thyroxine (T4), an endogenous hormone secreted by the thyroid gland, which is converted to its active metabolite, L-triiodothyronine (T3). T4 and T3 bind to thyroid receptor proteins in the cell nucleus and cause metabolic effects through the control of DNA transcription and protein synthesis.Like its naturally secreted counterpart, levothyroxine is a chiral compound in the L-form.
Thyroid hormone should not be used to treat obesity in patients with normal thyroid function. Levothyroxine is ineffective for weight reduction in normal thyroid patients and may cause serious or life-threatening toxicity, especially when taken with amphetamines. Talk to your doctor about the potential risks associated with this medication.
tell your doctor and pharmacist if you are allergic to levothyroxine, thyroid hormone, any other medications, or any of the ingredients in levothyroxine. Ask your pharmacist for a list of the ingredients.
tell your doctor and pharmacist what prescription and nonprescription medications, vitamins, nutritional supplements, and herbal products you are taking or plan to take. Be sure to mention any of the following: amiodarone (Cordarone, Pacerone); amphetamines; anticoagulants ('blood thinners') such as warfarin (Coumadin); aspirin or aspirin-containing products and other nonsteroidal anti-inflammatory drugs; beta-blockers such as metoprolol (Lopressor),
propranolol (Inderal) or timolol (Timoptic); carbamazepine (Equetro, Tegretol); corticosteroids such as dexamethasone; digoxin (Lanoxin); estrogens; furosemide (Lasix); imatinib (Gleevac); insulin or other medications to treat diabetes; lithium (Lithobid); maprotiline; oral contraceptives; phenobarbital; phenytoin (Dilantin, Phenytek); rifampin (Rifater, Rifamate, Rifadin); selective serotonin reuptake inhibitors (SSRIs) such as sertraline (Zoloft); theophylline (TheoDur); and tricyclic antidepressants such as amitriptyline (Elavil). Many other medications may also interact with levothyroxine, so be sure to tell your doctor about all the medications you are taking, even those that do not appear on this list. Your doctor may need to change the doses of your medications or monitor you carefully for side effects.
if you take an antacid, calcium carbonate (Tums), cholestyramine (Questran), colesevelam (Welchol), colestipol (Colestid), iron, orlistat (Alli, Xenical), simethicone (Phazyme, Gas X), sevelamer (Renvela, Renagel), sodium polystyrene sulfonate (Kayexalate), or sucralfate (Carafate), take it at least 4 hours before or 4 hours after you take levothyroxine.
tell your doctor if you have or have ever had diabetes; hardening of the arteries (atherosclerosis); kidney disease; liver disease or hepatitis; bleeding problems; porphyria (condition in which abnormal substances build up in the blood and cause problems with the skin or nervous system); cardiovascular disease such as high blood pressure, chest pain (angina), arrhythmias, or heart attack; an underactive adrenal or pituitary gland, or any condition that makes it difficult for you to swallow.
tell your doctor if you are pregnant, plan to become pregnant or are breast-feeding. If you become pregnant while taking levothyroxine, call your doctor.
if you have surgery, including dental surgery, tell the doctor or dentist that you are taking levothyroxine.
Levothyroxine comes as a tablet and a capsule to take by mouth. It usually is taken once a day on an empty stomach, one half to one hour before breakfast. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take levothyroxine exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.
Swallow capsules whole; do not chew or crush them.
The tablets may get stuck in your throat or cause choking or gagging; therefore, the tablet should be taken with a full glass of water.
If you are giving levothyroxine to an infant, child, or adult who cannot swallow the tablet, crush the tablet and mix it in 1 to 2 teaspoons of plain water. Give this mixture by spoon or dropper right away. Do not store this mixture. Only mix the crushed tablets with water. Do not mix with food or soybean infant formula.
Your doctor will probably start you on a low dose of levothyroxine and gradually increase your dose.
Levothyroxine controls hypothyroidism, but does not cure it. It may take several weeks before you notice a change in your symptoms. Continue to take levothyroxine even if you feel well. Do not stop taking levothyroxine without talking to your doctor.
Levothyroxine is typically used to treat hypothyroidism, and is the treatment of choice for people with hypothyroidism, who often require lifelong thyroid hormone therapy. It may also be used to treat goiter via its ability to lower thyroid-stimulating hormone (TSH), a hormone that is considered goiter-inducing.Levothyroxine is also used as interventional therapy in people with nodular thyroid disease or thyroid cancer to suppress thyroid-stimulating hormone (TSH) secretion.A subset of people with hypothyroidism treated with an appropriate dose of levothyroxine will describe continuing symptoms despite TSH levels in the normal range.In these people, further laboratory and clinical evaluation is warranted as they may have another cause for their symptoms. Furthermore, it is important to review their medications and possible dietary supplements as several medications can affect thyroid hormone levels.
It is also used to treat myxedema coma, which is a severe form of hypothyroidism characterized by mental status changes and hypothermia. As it is a medical emergency with a high mortality rate, it should be treated in the intensive care unit with thyroid hormone replacement and aggressive management of individual organ system complications.
Dosages vary according to the age groups and the individual condition of the person, body weight and compliance to the medication and diet. Other predictors of the required dosage are sex, BMI, deiodinase activity (SPINA-GD) and etiology of hypothyroidism. Annual or semiannual clinical evaluations and TSH monitoring are appropriate after dosing has been established. Levothyroxine is taken on an empty stomach approximately half an hour to an hour before meals. As such, thyroid replacement therapy is usually taken 30 minutes prior to eating in the morning.For patients with trouble taking levothyroxine in the morning, bedtime dosing is effective as well.A recent study published in JAMA showed greater efficacy of levothyroxine when taken at bedtime. For normal people, it should be run at 100-150mcg/day, for short cycle, around 6weeks, for long cycle, 12-16weeks.
Getting the right dosage is important, but don't expect the dosage you start out with to be the dosage that you eventually keep. Doctors often use weight as a guideline for determining dosage. Some use the formula of 1.6 micrograms of T4 for every 1 kilogram (or 2.2 pounds) of weight for a starting dosage. Others prefer a more conservative approach, starting patients at a very low dose (perhaps as low as 25 micrograms). Note the 1.6 micrograms is a full replacement dose. This means that if part of your thyroid still functions properly, you won't need this full dose because you will continue to make some of their own T4, in addition to the dose in the pill.
Because it's common for dosages to change at the start of treatment, your doctor will likely monitor your thyroid stimulating hormone (TSH) levels after two or three months (though some doctors check as soon as four weeks) from your first day of treatment. And since hormone replacement therapy is usually a lifelong treatment, you should get checked every year to make sure you're taking the right dose if you're on a stable dose. You should communicate with your doctor more frequently if your dose is being adjusted.
Since thyroid hormone occurs naturally in the body, almost anyone can take levothyroxine. However, you may not be able to take this medicine if you have certain medical conditions.
To make sure this medicine is safe for you, tell your doctor if you have:
a thyroid disorder called thyrotoxicosis;
heart disease, coronary artery disease, or a history of blood clots;
diabetes (insulin or oral diabetes medication doses may need to be changed when you start taking levothyroxine);
anemia (lack of red blood cells);
a blood-clotting disorder;
osteoporosis, or low bone mineral density;
problems with your pituitary gland;
any food or drug allergies;
an untreated or uncontrolled adrenal gland disorder;
if you also take a blood thinner (warfarin, Coumadin, Jantoven); or
if you have recently had a heart attack, or are having any symptoms of a heart attack (chest pain or heavy feeling, pain spreading to the jaw or shoulder, nausea, sweating, general ill feeling).
Tell your doctor if you have recently received radiation therapy with iodine (such as I-131).
Levothyroxine is not expected to harm an unborn baby. If you become pregnant while taking this medicine, do not stop taking the medicine without your doctor's advice. Having low thyroid hormone levels during pregnancy could harm both mother and baby. Your dose needs may be different during pregnancy.
Levothyroxine can pass into breast milk, but it is not expected to be harmful to a nursing baby. Tell your doctor if you are breast-feeding. Your dose needs may be different while you are nursing.
Do not give this medicine to a child without medical advice. Tirosint is not approved for use by anyone younger than 6 years old.
Get emergency medical help if you have signs of an allergic reaction to levothyroxine: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Call your doctor at once if you have a serious side effect such as:
fast or irregular heart rate;
chest pain, feeling short breath;
fever, hot flashes, sweating;
feeling unusually cold;
weakness, tiredness, sleep problems (insomnia);
memory problems, feeling depressed or irritable;
dryness of your skin or hair, hair loss;
changes in your menstrual periods; or
vomiting, diarrhea, appetite changes, weight changes.
Certain side effects may be more likely in older adults.
Common levothyroxine side effects may include:
leg cramps, muscle weakness;
feeling nervous or irritable;
skin rash, mild hair loss.
Many other medicines can be affected by your thyroid hormone levels. Other medicine may also increase or decrease the effects of levothyroxine.
Many drugs can interact with levothyroxine and not all possible interactions are listed in this medication guide.
Tell your doctor about all medications you use, start using, or stop using during your treatment with this medicine. This includes prescription, over-the-counter, vitamin, and herbal products. Give a list of all your medicines to any healthcare provider who treats you.
You shouldn't take more levothyroxine than your doctor prescribes.
If you take too much of the drug, you may develop symptoms of overactive thyroid, including fatigue, sensitivity to heat, increased appetite, and sweating.
Symptoms of a levothyroxine overdose may also include:
Confusion and disorientation
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