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Levothyroxine: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Feb 25, 2022.

1. How it works

  • Levothyroxine is a man-made form of thyroxine, a hormone that is produced naturally in the body by the thyroid gland.
  • Levothyroxine replaces missing thyroxine in people whose thyroid glands do not produce enough thyroxine naturally.
  • Thyroid hormones (such as thyroxine) play a vital role in our normal growth and development, and the maturation of our brain, spinal cord, nerves, and bone. Thyroid hormones help cells function and are also involved in their breakdown. They also help regulate our moods, reproductive function, metabolism, and gastrointestinal function.
  • Levothyroxine is generally taken life-long when used to treat hypothyroidism (low thyroid levels) unless the cause is transient.

2. Upsides

  • Used for the treatment of hypothyroidism (low thyroxine levels in the body). Replaces or supplements low or missing thyroxine and maintains normal intellectual and physical growth and development.
  • In children born with hypothyroidism, levothyroxine rapidly restores thyroid levels to normal, preventing detrimental effects on intellectual and physical development.
  • Levothyroxine is recommended by American guidelines as the preferred hormone for hypothyroidism. It is effective for all causes of hypothyroidism, except transient hypothyroidism caused by subacute thyroiditis (an inflammatory disease of the thyroid most likely caused by a virus).
  • May also be used in the management of goiter and some thyroid cancers.
  • Generic levothyroxine is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • May increase heart rate, cause arrhythmias (palpitations), precipitate angina, and have other adverse effects on the heart.
  • A headache, hair loss, flushing, diarrhea, and menstrual irregularities in women are also reasonably common.
  • May cause weight loss; however, thyroxine should not be used for the sole purpose of treating obesity as its risks far outweigh any benefits when used in people with normal thyroid function.
  • There is a fine line between having too much thyroxine resulting in toxicity, and having too little, meaning that it is ineffective.
  • Levothyroxine has been associated with a decrease in bone mineral density when used long-term, resulting in an increased risk of fractures.
  • Should not be used in people whose adrenal glands are not functioning properly or with thyrotoxicosis (high levels of thyroid hormone) or when TSH levels are suppressed and T3 and T4 levels are normal.
  • May not be suitable for people with cardiovascular disease or clotting disorders. Contraindicated following a heart attack (myocardial infarction).
  • May not be suitable for women who are intending to become pregnant or during pregnancy or breastfeeding. It should not be used for the treatment of male or female infertility unless it is associated with hypothyroidism.
  • Interacts with several other medications including amiodarone, anticoagulants, antidepressants, and digoxin. Foods such as soybean flour (found in infant formula), cottonseed meal, walnuts, and dietary fiber can decrease the absorption of levothyroxine.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Levothyroxine replaces missing thyroxine and is the preferred medicine to treat hypothyroidism. The dosage of levothyroxine needs to be tailored for each individual and there is a fine line between taking too much thyroxine and toxic side effects, and having too little, resulting in compromised functioning.

5. Tips

  • Take on an empty stomach, at least 30 to 60 minutes before food. Several foods, fiber, soy, coffee, and grapefruit juice can interfere with absorption. Supplements that contain iron and calcium may affect the absorption of levothyroxine (separate administration by 4 hours).
  • Dosages should be titrated every 6 to 8 weeks under a doctor's advice.
  • Take thyroxine EXACTLY as directed by your doctor. Taking too much can result in toxicity and potentially fatal side effects; taking too little may mean that you are not taking enough to supplement your low thyroid levels. Do not discontinue or take any other dose without your doctor's advice.
  • Swallow capsules whole; do not cut, crush, or attempt to dissolve in water.
  • The Levoxyl-branded tablet may rapidly swell and disintegrate, and cause choking or gagging if it becomes stuck in your throat. Take with a full glass of water, but talk with your doctor should you have difficulty swallowing it.
  • Tell your doctor if you experience a rapid or irregular heart rate, chest pain, or shortness of breath. Also discuss any other unusual symptoms such as excessive sweating, leg cramps, irritability, sleeplessness, headache, tremors, changes in appetite or weight, menstrual irregularities, or skin rashes.
  • Rarely, partial hair loss may occur during the first few months of treatment, but this is usually temporary.
  • Regular blood tests are usually needed.
  • Children may need intensive monitoring to prevent over or under dosage as both can have detrimental effects on development. The dosage of levothyroxine may need adjusting during pregnancy.

6. Response and effectiveness

  • 40-60% of orally administered levothyroxine is absorbed; the majority from the jejunum and upper ileum of the gastrointestinal tract. Absorption is increased when levothyroxine is taken on an empty stomach.
  • May take several weeks for an improvement in symptoms to be seen and up to 4 to 6 weeks for peak blood levels to be reached.
  • Absorption may be decreased with malabsorption syndromes and with certain foods such as soy-based infant formula.

7. Interactions

Medicines that interact with levothyroxine may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with levothyroxine. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with levothyroxine include:

  • amiodarone or other medications that affect iodine, such as radioactive iodine
  • amphetamines, such as dexamphetamine or phentermine
  • anticoagulants, such as warfarin
  • anticonvulsants such as carbamazepine, phenobarbital, or phenytoin
  • antidepressants, such as sertraline or anti-anxiety medications
  • aspirin
  • ciprofloxacin
  • estrogens and oral contraceptives
  • heart medications, such as digoxin, metoprolol, or propranolol
  • HIV medications (eg, atazanavir, indinavir, ritonavir, or saquinavir)
  • medications for diabetes, including insulin
  • medications that can affect the absorption of levothyroxine, such as antacids, calcium carbonate, cholestyramine, iron, orlistat sucralfate, sevelamer, or proton pump inhibitors
  • rifampin.

Note that this list is not all-inclusive and includes only common medications that may interact with levothyroxine. You should refer to the prescribing information for levothyroxine for a complete list of interactions.  


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use levothyroxine only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: February 25, 2022.