triiodothyronine (T3) and thyroxine (T4)
Medical use
Both T3 and T4 are used to treat thyroid hormone deficiency (hypothyroidism). They are both absorbed well by the gut, so can be given orally. Levothyroxine is the pharmaceutical name (INN) of levothyroxine sodium (T4), which is metabolised more slowly than T3 and hence usually only needs once-daily administration. Natural desiccated thyroid hormonesare derived from pig thyroid glands, and are a "natural" hypothyroid treatment containing 20% T3 and traces of T2, T1 and calcitonin. Also available are synthetic combinations of T3/T4 in different ratios (such as liotrix) and pure-T3 medications (INN: liothyronine). Levothyroxine Sodium is usually the first course of treatment tried. Some patients feel they do better on desiccated thyroid hormones; however, this is based on anecdotal evidence and clinical trials have not shown any benefit over the biosynthetic forms.
Thyronamines have no medical usages yet, though their use has been proposed for controlled induction of hypothermia, which causes the brain to enter a protective cycle, useful in preventing damage during ischemic shock.
Synthetic thyroxine was first successfully produced by Charles Robert Harington and George Barger in 1926.
Formulations
Today most patients are treated with levothyroxine, or a similar synthetic thyroid hormone.However, natural thyroid hormone supplements from the dried thyroids of animals are still available.Natural thyroid hormones have become less popular, due to evidence that varying hormone concentrations in the thyroids of animals before they are slaughtered leads to inconsistent potency and stability. Levothyroxine contains T4 only and is therefore largely ineffective for patients unable to convert T4 to T3. These patients may choose to take natural thyroid hormone as it contains a mixture of T4 and T3, or alternatively supplement with a synthetic T3 treatment. In these cases, synthetic liothyronine is preferred due to the potential differences between drug lots of natural thyroid products. It would be counterintuitive to supplement with T4-only if the patient cannot convert T4 to T3. Some natural thyroid hormone brands are F.D.A. approved, but some are not. Thyroid hormones are generally well tolerated. Thyroid hormones are usually not dangerous for pregnant women or nursing mothers, but should be given under a doctor's supervision. In fact, if a woman who is hypothyroid is left untreated, her baby is at a higher risk for birth defects. When pregnant, a woman with a low functioning thyroid will also need to increase her dosage of thyroid hormone. One exception is that thyroid hormones may aggravate heart conditions, especially in older patients; therefore, doctors may start these patients on a lower dose & work up to avoid risk of heart attack.
Levothroxine Education:
- Use Synthroid as directed by your doctor.
- Take Synthroid by mouth on an empty stomach at least one-half to one hour before breakfast.
- Do not take an antacid or a product that has iron or calcium in it within 4 hours of taking Synthroid.
- If the patient is a child or if you cannot swallow the tablet whole, you may crush the correct dose of the medicine. Add the crushed medicine to 1 to 2 teaspoons (5 to 10 mL) of water. Mix well. Use a spoon or dropper to give the medicine as soon as possible. Do not store the mixture for later use. Do not mix crushed tablets in soybean infant formula. Ask your pharmacist if you have any questions.
- Synthroid works best if it is taken at the same time each day.
- Continue to take Synthroid even if you feel well. Do not miss any dose.
- It may take several weeks before you notice an improvement in your symptoms. Do not stop or change your dose of Synthroid without first checking with your doctor.
- If you miss a dose of Synthroid, take it as soon as possible. If it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not take 2 doses at once.
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