Levothyroxine is a medicine used to treat an underactive thyroid gland (hypothyroidism).
The thyroid gland makes thyroid hormone which helps to control energy levels and growth. Levothyroxine is taken to replace the missing thyroid hormone.
Levothyroxine is available only on prescription. It comes as tablets or as a liquid that you drink.
- Levothyroxine starts working straight away, 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 doctor can lower your dose to help reduce any side effects.
- Before you start taking levothyroxine, your doctor will do a blood test to see what dose you need. Once you start taking the medicine you'll have regular blood tests to see how well it’s working.
- Levothyroxine doses need to be carefully monitored during pregnancy. If you're planning to become pregnant or think you may be pregnant while taking levothyroxine, it's important to visit your doctor to make sure you get the correct care for you and your baby.
- Levothyroxine is also called by the brand name Eltroxin.
Who can and can't take levothyroxine
Levothyroxine can be taken by adults and children.
However, levothyroxine isn’t suitable for some people.
Do not take levothyroxine and go back to your doctor to discuss your treatment if you have:
- had an allergic reaction to levothyroxine or any other medicines in the past
- an overactive thyroid gland (hyperthyroidism)
- a health problem that affects your adrenal glands (your doctor will be able to tell you if you’re not sure)
Check with your doctor before taking levothyroxine if you have:
How and when to take it
Take levothyroxine once a day in the morning, ideally at least 30 minutes before having breakfast or a drink containing caffeine like tea or coffee.
Both breakfast and caffeinated drinks can stop your body taking in levothyroxine properly so it doesn't work as well.
How much will I take?
Levothyroxine comes in 25 microgram, 50 microgram and 100 microgram tablets. A microgram is a tiny amount - 1,000 times smaller than a milligram.
You may need to take several different tablets to make up your dose. Your doctor will tell you how many tablets to take each day.
The dose of levothyroxine varies from person to person.
Although starting doses are usually the same, the dose of levothyroxine you end up taking, or how quickly the dose is increased, depends on your symptoms, hormone levels, age and whether you have any other health problems.
Adults usually start with a dose between 50mcg and 100mcg taken once a day. This may be increased gradually over a few weeks to between 100mcg and 200mcg taken once a day.
Some people, such as over-50s or people with heart disease, may start on a lower dose.
If you’re taking levothyroxine as a liquid, 5mls has 25, 50, or 100mcg in it.
How will I take it?
Swallow the tablets whole with a drink of water.
Levothyroxine is available as a liquid for children and people who find it difficult to swallow tablets.
If you or your child are taking levothyroxine as a liquid, it will usually be made up for you by the pharmacist. It will come with a plastic syringe or spoon to help you measure out the right dose. If you don’t have one, ask your pharmacist for one. Don't use a kitchen teaspoon as it won't give the right amount.
Your doctor will do regular blood tests to check the levels of thyroid hormones in your body before and after starting levothyroxine. These will allow your doctor to adjust your dose to suit you.
At the start of treatment you can expect to have blood tests often. Once 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
What if I forget to take it?
If you forget to take a dose, take it as soon as you remember, unless it's almost time for your next dose. In which case just skip the forgotten dose. Do not take 2 doses together to make up for a missed dose.
If you forget doses often, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.
What if I take too much?
Taking an extra dose of levothyroxine by accident is unlikely to harm you.
Speak to your doctor if:
- you accidentally take more than 1 extra dose
- you get side effects such as a racing heart or chest pain. These may not happen straight away - it can be several days before they come on.
Like all medicines, levothyroxine can cause side effects although not everyone gets them.
Once you are on the right dose of levothyroxine, side effects should go away.
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 you go on a lower dose of levothyroxine or stop treatment.
Common side effects are the same as the symptoms of an overactive thyroid. Talk to your doctor or pharmacist if these side effects bother you or don’t go away:
- feeling sick, vomiting, or having diarrhoea
- feeling restless or excitable, or having problems sleeping
- flushing or sweating
- muscle cramps
- shaking, usually of the hands
Serious side effects
It happens rarely, but some people may have serious side effects when taking levothyroxine.
Tell your doctor straight away if you get:
- chest pain
- fast or irregular heartbeats, or palpitations
Serious allergic reaction
In rare cases, it’s possible to have a serious allergic reaction to levothyroxine.
A serious allergic reaction is an emergency. Contact a doctor straight away if you think you or someone around you is having a serious allergic reaction.
The warning signs of a serious allergic reaction are:
- getting a skin rash that may include itchy, red, swollen, blistered or peeling skin
- tightness in the chest or throat
- having trouble breathing or talking
- swelling of the mouth, face, lips, tongue, or throat
These are not all the side effects of levothyroxine. For a full list see the leaflet inside your medicines packet.
You can report any suspected side effect to the UK safety scheme.
Pregnancy and breastfeeding
Levothyroxine is generally safe to take in pregnancy and while breastfeeding.
In fact, it’s important to carry on taking levothyroxine throughout your pregnancy. Having too low or too high levels of thyroid hormone in pregnancy 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 for you and your baby. Most women need to take a higher dose of levothyroxine than usual while they’re pregnant.
Levothyroxine and breastfeeding
It’s generally safe to breastfeed while you’re on levothyroxine. Thyroid hormones pass into breast milk in very low levels that are too small to affect the baby.
If you're breastfeeding, it’s important that you continue to take levothyroxine. Your body needs good levels of thyroid hormones to make enough breast milk to feed your baby.
Tell your doctor if you're trying to get pregnant, are already pregnant or if you're breastfeeding.
Cautions with other medicines
Some medicines can interfere with thyroid hormones, so the dose of levothyroxine may need to be changed. They include:
- medicines for seizures, carbamazepine and phenytoin
- oestrogens - such as in combined contraceptive pills or hormone replacement therapy (HRT)
Levothyroxine can change how other medicines work, so their doses may need to be altered. These medicines include:
- medicines for diabetes, either insulin or tablets
- the blood thinning medicine, warfarin
Some medicines shouldn't be taken at the same time of day as levothyroxine as they can reduce the amount of levothyroxine your body takes in, including:
- 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.
Read the information leaflet supplied with these medicines or speak to your pharmacist for advice on how much time to leave between taking these medicines and taking levothyroxine.
Mixing levothyroxine with herbal remedies and supplements
Tell your doctor or pharmacist if you're taking any other medicines, including herbal remedies, vitamins or supplements.
How does levothyroxine work?
How long does levothyroxine take to work?
How long will I take levothyroxine for?
Is levothyroxine safe to take for a long time?
Can I drink alcohol with it?
Will it make me lose weight?
Can I drive or ride a bike with it?
Is there any food or drink I need to avoid?
Can I get thyroid medicines for free?
Will it affect my contraception?
Will I lose my hair?