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Key points about levothyroxine

  • Levothyroxine is a thyroid hormone, used to treat hypothyroidism.
  • Levothyroxine is also called Eltroxin or Synthroid.
  • Find out how to take it safely and possible side effects. 
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Levothyroxine replaces or provides thyroid hormone when your body cannot make enough. This is a condition called hypothyroidism. Levothyroxine is also used to treat other types of thyroid disorders such as certain types of goiters and thyroid cancer. Read more about hypothyroidism.

In New Zealand levothyroxine is available as tablets in different strengths (25 micrograms, 50 micrograms and 100 micrograms). 

  • The dose of levothyroxine will be different for different people. It depends on your medical condition, laboratory test results and your response to treatment.
  • Your doctor will usually start you on a low dose and increase the dose depending on how you respond.
  • The usual starting dose is 50–100 micrograms daily, which may be increased slowly to 100–200 micrograms daily. 
  • Levothyroxine is usually taken once a day.
  • Always take your levothyroxine exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much levothyroxine to take, how often to take it and any special instructions.

  • Timing: Levothyroxine is best taken on an empty stomach, 1 hour before food or 2 hours after food. Try to take it at the same time each day. If you keep forgetting to have it on an empty stomach, and you end up missing your medicine, it is better to just take it at the time that suits you best, and your doctor can increase the dose if needed. Speak to your doctor about this.
  • Missed dose: If you forget to take your dose, take it as soon as you remember. But if it is nearly time for your next dose, just take the next dose as usual. Do not take 2 doses to make up for a missed dose.
  • Keep taking levothyroxine regularly. It takes a few weeks before you feel the full effects of levothyroxine. Do not stop taking levothyroxine suddenly; speak to your doctor or nurse before stopping. Levothyroxine treatment is usually lifelong.

  • Different brands: There are different brands of levothyroxine tablets. It is best to continue taking the same brand of tablets. If you have any questions about the brand you are taking, ask your doctor or pharmacist. If your tablets look different to your last supply, ask your doctor or pharmacist for advice.
  • Different strengths: Levothyroxine tablets are available in different strengths. If your tablets look different to your last supply, ask your doctor or pharmacist for advice.
  • Blood tests: You will need blood tests while taking levothyroxine to check how well the medicine is working for you.

  • Are you pregnant, planning a pregnancy or breastfeeding?
  • Do you have problems with your heart such as angina?
  • Do you have high blood pressure (hypertension)?
  • Do you have diabetes?
  • Are you taking any other medicines? This includes any medicines you buy without a prescription, such as herbal and complementary medicines.

If so, it’s important that you tell your doctor or pharmacist before you start levothyroxine. Sometimes a medicine isn’t suitable for a person with certain conditions or it can only be used with extra care.

Like all medicines, levothyroxine can cause side effects, although not everyone gets them. Often side effects improve as your body gets used to the new medicine.

Side effects What should I do?
  • Hair loss
  • This usually happens during the first few months of treatment and should stop once your body gets used to the medicine.
  • Tell your doctor if this continues or gets worse. 
  • Feeling sick or nausea
  • Headache
  • Tell your doctor if troublesome.
  • Feeling restless
  • Increased sweating
  • Feeling flushed or hot 
  • Feeling nervous or mood swings 
  • Diarrhoea (runny poos)
  • Increased appetite
  • Having trouble sleeping
  • Tell your doctor.
  • Signs of an allergic reaction such as skin rash, itching, blisters, peeling skin, swelling of your face, lips or mouth or having problems breathing
  • Tell your doctor immediately or phone HealthLine 0800 611 116.
  • Fast or irregular heart beat
  • Chest pain
  • Shortness of breath
  • Difficulty breathing
  • Tell your doctor immediately or phone HealthLine 0800 611 116.
Did you know that you can report a side effect to a medicine to CARM (Centre for Adverse Reactions Monitoring)? Report a side effect to a product(external link)

Levothyroxine can interact with a number of medicines and herbal supplements so check with your doctor or pharmacist before starting levothyroxine and before starting any new medicines. Levothyroxine can interact with antacids (medicines used for indigestion), iron supplements and calcium supplements. If you are taking any of these medicines, separate them from levothyroxine by at least 2 hours.

Medsafe Consumer Information Sheets:

Eltroxin(external link)
Synthroid(external link)
Levothyroxine(external link)

New Zealand Formulary Patient Information: Levothyroxine(external link)


  1. Levothroxine sodium(external link) New Zealand Formulary (August 2016)
  2. Levothyroxine (Eltroxin)(external link) Patient Info, UK (August 2016)
  3. Management of thyroid dysfunction in adults(external link) BPAC, NZ, 2010

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Credits: Sandra Ponen, Pharmacist, Healthify He Puna Waiora. Healthify is brought to you by Health Navigator Charitable Trust.

Reviewed by: Angela Lambie, Pharmacist, Auckland

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