Levothyroxine
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| AHFS/Drugs.com | Monograph |
| MedlinePlus | a682461 |
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| Routes of administration |
by mouth, intravenous |
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| Pharmacokinetic data | |
| Bioavailability | ~100% |
| Metabolism | mainly in liver, kidneys, brain and muscles |
| Biological half-life | ca. 7 days (in hyperthyroidism 3–4 days, in hypothyroidism 9–10 days) |
| Excretion | feces and urine |
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| Synonyms | 3,5, 3', 5'-Tetraiodo-L-thyronine |
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| ECHA InfoCard | 100.000.093 |
| Chemical and physical data | |
| Formula | C15H11I4NO4 |
| Molar mass | 776.87 g·mol−1 |
| 3D model (Jmol) | |
| Melting point | 231 to 233 °C (448 to 451 °F) |
| Solubility in water | Slightly soluble (0.105 mg·mL−1 at 25 °C) mg/mL (20 °C) |
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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.
Side effects from excessive doses include weight loss, trouble tolerating heat, sweating, anxiety, trouble sleeping, tremor, and fast heart rate. Use is not recommended in people who have had a recent heart attack. Use during pregnancy has been found to be safe. It is recommended that dosing be based on regular measurements of TSH and T4 levels in the blood. Much of the effect of levothyroxine is following its conversion to triiodothyronine (T3).
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. The wholesale cost in the developing world is about 0.58 to 12.28 USD a month. In the United States a typical month of treatment is less than 25 USD.
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.
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