Levothyroxine is used to treat an underactive thyroid gland (hypothyroidism).
You take it to replace the missing thyroid hormone.
Levothyroxine is only available on prescription. It comes as a tablet.
The brand available in Ireland is Eltroxin.
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.
They will adjust the dose to suit you.
At the start of treatment you will have regular blood tests.
Your levels will usually be checked once a year when your:
- hormone levels are stable
- symptoms are under control
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 show the dose is not right
Check if you can take levothyroxine
Levothyroxine can be taken by adults and children.
Check with your GP before starting to take levothyroxine if you:
- already have a medical condition
- 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
Pregnancy and levothyroxine
Levothyroxine is safe to take in pregnancy.
Tell your GP if you're planning to become pregnant or think you may be pregnant.
Keep taking levothyroxine throughout your pregnancy. If your thyroid hormone levels are too high or too low in pregnancy it can cause problems for you and your baby.
You'll need to have regular blood tests during pregnancy to make sure you're on the right dose of levothyroxine. Your doctor may need to increase the dose while you’re pregnant.
Breastfeeding and levothyroxine
It's OK to breastfeed while you're on levothyroxine. Thyroid hormones are a normal part of breast milk. When taken as a supplement they only pass into breast milk in tiny amounts. They are too small to affect your baby.
If you're breastfeeding, continue to take levothyroxine. It is replacing the hormone your body would normally be making. Your body needs good levels of thyroid hormones to make enough breast milk for your baby.
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 tells you to.
Eating and drinking
Some foods and drink can reduce the amount of levothyroxine your body takes in. This usually happens if you take your medicine at the same time as the food or drink.
These include:
- drinks containing caffeine, such as coffee, tea and some fizzy drinks
- dairy products such as milk, cheese and yoghurt
- broccoli
Kelp
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.
Soya
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.
Alcohol
You can drink alcohol while taking levothyroxine. Try to stay within the weekly low-risk alcohol guidelines.
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 mistake is unlikely to harm you.
Non-urgent advice: Contact your GP or pharmacist if you:
- accidentally take more than 1 extra dose
Side effects may not happen immediately. It can take up to 5 days before they happen.
Signs of an overdose of levothyroxine 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, you are less likely to get side effects.
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 reduces the dose.
Do not change your dose or stop taking your medicine before talking to your GP.
Common side effects are similar to symptoms of an overactive thyroid.
Talk to your GP, a pharmacist or nurse if side effects bother you or do not go away.
There are things you can do to manage common side eff
Feeling sick (nausea)
Stick to simple meals and do not eat rich or spicy food.
Getting sick (vomiting)
If you're getting sick, try small, frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.
If you are vomiting due to a stomach bug or illness, tell your doctor. Do not take any other medicines to treat vomiting without speaking to a pharmacist or doctor first.
If you take the combined contraceptive pill or the progestogen-only pill and you're getting sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Diarrhoea
Drink lots of fluids, such as water or squash, to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.
Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
If you take the combined contraceptive pill or the progestogen-only pill and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Headaches
Make sure you rest and drink plenty of fluids. Do not drink too much alcohol.
Ask your pharmacist to recommend a painkiller. Talk to your GP if the headaches last longer than a week or are severe.
Feeling restless or excitable, or having problems sleeping
These symptoms should improve as your body gets used to levothyroxine. If they do not go away, or they are causing you problems, contact your doctor.
Flushing and sweating
Try cutting down on coffee, tea and alcohol. It might help to keep the room cool and use a fan. You could also spray your face with cool water or sip cold or iced drinks. If it does not go away, contact your doctor as they may need to review your dose.
Muscle cramps
If you get unusual muscle aches, which is not from exercise or hard work, talk to your doctor. You may need a blood test to find the cause.
Shaking, usually of the hands
Talk to your doctor as you may need to have your dose reduced.
Weight loss
One of the symptoms of an underactive thyroid gland is weight gain. When you start taking levothyroxine, you may lose weight as your body's hormones rebalance.
Talk to your GP as you may need to have your dose reduced. Losing weight can also be a sign that your dose of levothyroxine is too high.
Serious side effects
Serious side effects are rare. But some may be signs of a thyroid crisis. A thyroid crisis is a severe reaction to high levels of thyroid hormone. You will need urgent medical help.
Emergency action required: Contact your GP immediately if you:
- have chest pain
- have fast or irregular heartbeats, or palpitations
- have a high temperature (38 degrees Celsius or above)
- feel dizzy or lightheaded - this may be a symptom of low blood pressure
- feel breathless, tired all the time or have swollen ankles or legs - these could be signs of heart failure
- have jaundice (yellowing of your skin and whites of your eyes)
- feel confused
- have fits
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 can be signs of a serious allergic reaction.
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 Health Products Regulatory Authority (HPRA): report an issue - hpra.ie
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.
Some medicines can affect thyroid hormones, so the dose of levothyroxine may need to be changed.
These include:
- epilepsy medicines such as carbamazepine and phenytoin
- rifampicin (antibiotic)
- amiodarone
- proton pump inhibitors such as omeprazole, esomeprazole and pantoprazole
- oestrogens - such as in combined contraceptive pills or hormone replacement therapy (HRT)
Levothyroxine can affect how other medicines work, so your GP may need to change their doses.
These medicines include:
- medicines for diabetes - either insulin or tablets
- warfarin, used to prevent blood blots
Some medicines should not be taken at the same time of day as levothyroxine as they can reduce the amount of levothyroxine your body takes in.
These include:
- antacids
- calcium salts
- iron salts
- orlistat, a medicine used for weight loss
- sucralfate, a medicine used to treat stomach ulcers
- some cholesterol-lowering medicines such as colestyramine, colestipol or colesevelem
Herbal remedies and supplements
Biotin supplements can affect how thyroid function tests work. Do not take biotin without talking to your GP.
Kelp (a type of seaweed) can contain high levels of iodine. It sometimes makes an underactive thyroid worse. Do not take supplements containing kelp if you're taking levothyroxine.
Do not take the herbal remedy St John's wort - it may reduce the level of levothyroxine in your blood.
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).
