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Levothyroxine is a medicine used to treat an underactive thyroid gland (hypothyroidism).

It’s taken to replace the missing thyroid hormone.

Levothyroxine is only available on prescription. It comes as a tablet.

The brand available in Ireland is Eltroxin.

Get emergency help

You might need emergency help if you have a serious allergic reaction (anaphylaxis) or serious side effects.

Serious allergic reaction

A serious allergic reaction to levothyroxine is rare. But you will need to go to an emergency department (ED) if you get symptoms.

Emergency action required: Call 112 or 999 or go to an ED immediately if:

  • you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
  • you're wheezing
  • you feel generally unwell
  • your skin feels sensitive to the sun
  • you get tightness in the chest or throat
  • you have trouble breathing or talking
  • your mouth, face, lips, tongue or throat start swelling

Stop taking levothyroxine immediately. These are warning signs of a serious allergic reaction.

Serious side effects

Serious side effects are rare. But some may be signs of a thyroid crisis. You will need urgent medical help.

Emergency action required: Phone your GP immediately if you:

  • have chest pain
  • have fast or irregular heartbeats, or palpitations
  • have a high temperature (38 degrees Celsius or above)
  • have low blood pressure
  • feel breathless, tired all the time or have swollen ankles or legs - these could be signs of heart failure
  • look jaundice
  • feel confused
  • have fits

When you start taking levothyroxine

Levothyroxine starts working immediately, but it may be several weeks before your symptoms start to improve.

The most common side effects of levothyroxine are caused by taking a bigger dose than you need. Your GP can lower your dose to help reduce any side effects.

Blood tests

Your GP will do regular blood tests to check the levels of thyroid hormones in your body before and after you start levothyroxine.

Your GP will use the results to adjust the dose to suit you.

At the start of treatment you can expect to have blood tests often. When your hormone levels are stable, you'll usually have a blood test after 4 to 6 months, and after that once a year.

You may need blood tests more often if you:

  • are pregnant
  • start or stop a medicine that can interfere with levothyroxine
  • have any symptoms that could mean your dose is not quite right 

Check if you can take levothyroxine

Levothyroxine can be taken by adults and children. 

Non-urgent advice: Check with your GP before starting to take levothyroxine if you:

  • already have a medical condition
  • have had an allergic reaction to medicine in the past
  • are trying to get pregnant, already pregnant or breastfeeding
  • have an intolerance to some sugars, including lactose intolerance


Talk to your GP or pharmacist before taking levothyroxine if you are pregnant, trying to get pregnant or think you might get pregnant.

They will let you know if levothyroxine is safe to take in pregnancy.


Talk to your GP or pharmacist before taking levothyroxine if you are breastfeeding. They will let you know if levothyroxine is safe to take while breastfeeding.

How and when to take levothyroxine

Take levothyroxine once a day on an empty stomach. Try to take it an hour before having breakfast or drinking caffeine, such as tea or coffee.

You’ll usually need to take this medicine for the rest of your life.

If you stop taking levothyroxine, your symptoms are likely to come back.

Do not stop taking levothyroxine unless your GP has told you to.

Eating and drinking 

Avoid eating or drinking the following at the same time of day as levothyroxine:

  • drinks containing caffeine, such as coffee, tea and some fizzy drinks
  • dairy products such as milk, cheese and yogurt
  • broccoli

They can reduce the amount of levothyroxine your body takes in. 


Do not take supplements containing kelp if you're taking levothyroxine. Kelp (a type of seaweed) can contain high levels of iodine that sometimes makes an underactive thyroid worse. 


Your GP might need to do extra blood tests if you regularly eat soya or take soya supplements. Soya in food and supplements may stop levothyroxine working properly.


You can drink alcohol while taking levothyroxine. 

If you forget to take it

Do not take 2 doses together to make up for a missed dose.

If you forget to take a dose, take it as soon as you remember, unless it's almost time for your next dose. If it's almost time for your next dose, skip the forgotten dose. 

If you forget to give your child their dose, contact your GP or pharmacist for advice on what you should do.

If you take too much

Taking an extra dose of levothyroxine by accident is unlikely to harm you.

Non-urgent advice: Speak to your GP or pharmacist if you:

  • accidentally take more than 1 extra dose
  • get side effects such as a racing heart beat or chest pain.

Side effects may not happen immediately. It can take up to 5 days before they happen.

Signs of an overdose include:

  • a high temperature of 38 degrees Celsius or higher
  • an irregular heartbeat
  • muscle cramps
  • headaches
  • restlessness
  • feeling flushed
  • sweating
  • diarrhoea 

Side effects

When you are on the right dose of levothyroxine, the likelihood of developing side effects will be reduced.

Common side effects

The common side effects of levothyroxine usually happen because the dose you're taking is more than you need.

These side effects usually go away after your GP lowers your levothyroxine dose.

Do not change your dose or stop taking your medicine before talking to your GP.

Common side effects are the same as the symptoms of an overactive thyroid. 

Talk to your GP, a pharmacist or a nurse if the side effects bother you or do not go away.

Side effects include:

  • generally feeling unwell
  • feeling sick
  • being sick (vomiting) or diarrhoea
  • headaches
  • feeling restless or excitable, or having problems sleeping
  • flushing or sweating
  • a high temperature of 38 degrees Celsius or higher
  • muscle cramps or weakness
  • shaking, usually of the hands
  • weight loss
  • chest pain, pounding, irregular or fast heartbeat
  • irregular periods

Read the patient information leaflet that comes with your medicine for a full list of side effects.

Non-urgent advice: Find your patient information leaflet

Your patient information leaflet is the leaflet that comes with your medicine. You can find a digital version of the leaflet online.

Report side effects

You can report any suspected side effects to the the Health Products Regulatory Authority (HPRA): report an issue -

Taking levothyroxine with other medicines

If you’re on any other medicines or supplements, check with your GP, a pharmacist or nurse before you start taking levothyroxine.

Fact check

This content was fact checked by a pharmacist, a GP, the National Medication Safety Programme (Safermeds) and the Health Products Regulatory Authority (HPRA).

Page last reviewed: 24 September 2021
Next review due: 24 September 2024

This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.