Must Know Side Effects of Levothyroxine

Levothyroxine is a prescription drug. It comes as a tablet or capsule to be taken orally. It is also available as an injectable solution for administration by healthcare providers only.

Oral levothyroxine tablets are available as the branded drugs Leboxil, Synthroid, and Unitroid. It is also sold as a generic. Generic medicines are usually cheaper than branded medicines. In some cases, branded drugs may not be available in all strengths or forms.


Oral levothyroxine tablets are used to treat hypothyroidism. This is a condition in which the thyroid gland produces very little thyroid hormone.

Levothyroxine can also be used to treat goitre, which is an enlarged thyroid gland. It also treats certain types of thyroid cancer.

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How does it work?

Levothyroxine belongs to a class of drugs called hormones. A drug class is a group of drugs that work similarly. These drugs are commonly used to treat similar symptoms.

Levothyroxine works by providing the thyroid hormones that the thyroid gland produces when it is functioning normally.

Side effects

Oral levothyroxine tablets may cause side effects.

  • Increased appetite
  • Weight loss
  • Heat sensitivity
  • Excessive sweating
  • Headache
  • Hyperactivity
  • Nervousness
  • Anxiety
  • Irritability
  • Mood swings
  • Trouble sleeping
  • Tiredness
  • Tremors
  • Muscle weakness
  • Changes in menstrual periods
  • Hair loss (usually temporary)
  • Diarrhoea
  • Vomiting
  • Stomach cramp

If these effects are mild, they may disappear within days or weeks. If symptoms worsen or do not improve, consult your doctor or pharmacist.

Serious side effects 

Call your doctor right away if you have any serious side effects. Call 911 if your symptoms feel life-threatening or if you think you have a medical emergency. Serious side effects and their symptoms include:

A. Heart attack

Symptoms include:

  • Chest pain
  • Difficulty breathing
  • Upper body discomfort

B. Heart failure: 

Symptoms include:

  • Difficulty breathing
  • Exhaustion
  • Swelling of feet, ankles and feet
  • Unexpected weight gain
  • Very fast heart rate
  • Arrhythmia



The aim is to provide the most relevant and up-to-date information. However, because drugs affect everyone differently, we cannot guarantee that this information includes all possible side effects. This information is not a substitute for medical advice. Always discuss possible side effects with a doctor who knows your medical history.

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Levothyroxine may interact with other medications

Levothyroxine oral tablets may interact with other medications, vitamins, or herbs you may be taking. An interaction is when a substance changes how a drug works. This can be harmful or cause the medicine to work less well.

All medicines should be carefully controlled by your doctor to avoid interactions. Be sure to tell your doctor about any medicines, vitamins, or herbs you take. Talk to your doctor or pharmacist to find out how this medicine may interact with other medicines you take.

Examples of drugs that may interact with levothyroxine are listed below.

Interactions that increase the risk of side effects

Taking levothyroxine with certain medications may increase side effects. 

Examples of these drugs are:

  • Antidepressants such as amitriptyline and maprotiline. The side effects of these two antidepressants and levothyroxine may increase when these drugs are taken together. This can put you at risk of abnormal heart rhythms (arrhythmias).
  • Sympathomimetics, such as pseudoephedrine and albuterol. Taking these drugs together may increase the effects of both sympathomimetics and levothyroxine. This can put you at risk for serious heart problems.
  • Blood thinners such as warfarin. Using these drugs with levothyroxine may increase the risk of bleeding. Your doctor may need to reduce your blood thinner dosage if you are also taking levothyroxine. 
  • Ketamine. Taking this drug with levothyroxine may increase the risk of high blood pressure and increased heart rate.

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Interactions that may make drugs less effective

  • If levothyroxine is ineffective: Taking levothyroxine with certain medications may not work well in treating symptoms because it may reduce the amount of levothyroxine in the body. Examples of these drugs are:
    • The antidepressant sertraline. If you are taking sertraline with levothyroxine, your doctor may need to increase your dose of levothyroxine.
    • Rifampin and antiepileptic drugs such as carbamazepine and phenobarbital.
    • calcium carbonate or iron sulfate. Take levothyroxine at least 4 hours before or after taking these medications to ensure levothyroxine is working properly.
    • Colesevelam, cholestyramine, colestipol, chiexalate, or sevelamer. Take levothyroxine at least 4 hours before taking these medicines to make sure levothyroxine is working properly.
    • Orlistat.
    • Simethicone and an antacid such as aluminium or magnesium.
    • Anticancer agents belonging to the class of tyrosine kinase inhibitors such as imatinib.


  • If other drugs are ineffective:

Certain medications may not work well together with levothyroxine. This is because the amount of these drugs in your body may decrease. Examples of these drugs are:

    • Diabetic drugs such as insulin, metformin, nateglinide, glipizide, pioglitazone. If you are taking these diabetes medications along with levothyroxine, your doctor may need to increase your dose of these medications.
    • Digoxin. If you are taking this drug along with levothyroxine, your doctor may need to increase your dose of digoxin. 
    • Theophylline. When taken with levothyroxine, your doctor can monitor the levels of theophylline in your body.



The goal is to provide the most relevant and up-to-date information. However, because drugs affect everyone differently, we cannot guarantee that this information includes all possible interactions. This information is not intended to replace medical advice. Always consult your doctor about possible interactions with all prescription medications, vitamins, herbs, dietary supplements, and over-the-counter medications you take.

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Too much levothyroxine Overdose:

You may have dangerous levels of the drug in your body. Symptoms of overdose include:

  • Confusion
  • Disorientation
  • Stroke
  • Shock
  • Coma

If you think you've taken too much of this medicine, call your doctor or local poison control centre. If your symptoms are severe, go to the nearest emergency room.

Important considerations

A. General 

  • Take levothyroxine on an empty stomach without food.
  • Take levothyroxine in the morning. Take 30 minutes to 1 hour before your first meal of the day.
  • You can cut or crush your tablet.


B. Storage 

  • Store Levothyroxine at room temperature between 59°F and 86°F (15°C and 30°C). Store away from light.
  • Do not store this medicine in damp or damp areas such as bathrooms.


C. Refill

  • Prescriptions for this drug are refillable. No new prescription is required to refill this medicine. Your doctor will record the number of refills allowed on your prescription.


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D. Travel

If travelling with medication:

  • Always carry your medicine with you. When boarding an aeroplane, do not put it in your checked baggage. Keep it in your hand luggage.
  • Don't worry about his X-ray machine at the airport. They cannot harm your medicine. You may need to show the pharmacy label of your medicine to the airport staff. Always carry the original container marked with your prescription.
  • Do not put this medicine in your car's glove box or leave it in your car. Avoid this at all costs when the weather is very hot or very cold.

E. Clinical monitoring

Your doctor will monitor your thyroid hormone levels during treatment with this drug. Your doctor will do blood tests to make sure your thyroid hormone levels are in the range that seems optimal for you. A test will show if your medication is working.


F. Diet 

Certain foods (such as soybean meal, cottonseed meal, walnuts, and other fibres) can affect how levothyroxine is absorbed by the body. Talk to your doctor about changing your diet.


G. Prior authorization 

Many insurance companies require prior approval for certain branded forms of this drug. This means that your doctor must get approval from your health insurance company before they can pay for your prescription.


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