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 only available 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.

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 (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:

• a heart problem including angina, heart disease or heart failure
• high blood pressure
• had a heart attack
• diabetes - the dose of your diabetes medicine may need to change because levothyroxine can raise blood sugar levels

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.

Food and caffeinated drinks can both stop your body taking in levothyroxine properly so it doesn't work as well.

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

How much will I take?

You may need to take several different tablets to make up your dose.

Levothyroxine comes in 25 microgram, 50 microgram and 100 microgram tablets. The word microgram is sometimes written with the Greek symbol μ followed by the letter g (μg). A microgram is 1,000 times smaller than a milligram (mg).

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 50 micrograms and 100 micrograms taken once a day. This may be increased gradually over a few weeks to between 100 micrograms and 200 micrograms 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 micrograms, 50 micrograms or 100 micrograms in it.

How to 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 a syringe or measuring spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it won't give the right amount.

Blood tests

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 the 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 often doses, 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 beat 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 to 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
• being sick (vomiting) or diarrhoea
• headaches
• 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.

Call a 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 (anayphylaxis).

These are not all the side effects of levothyroxine. For a full list see the leaflet inside your medicines packet.

What to do about:

• feeling sick - stick to simple meals and do not eat rich or spicy food.
• being sick (vomiting) or diarrhoea - drink plenty of water to prevent dehydration. If you're being sick try small, frequent sips. Speak to a pharmacist if you have signs of dehydration, such as peeing less than usual or having dark, strong-smelling pee. If you get severe diarrhoea or vomiting from a stomach bug or illness, tell your doctor.
• headaches - make sure you rest and drink plenty of fluids. Do not drink too much alcohol. Ask your pharmacist to recommend a painkiller. Headaches should usually go away after the first week of taking levothyroxine. Talk to your doctor if they last longer than a week or are severe.
• feeling restless or excitable, or problems sleeping - these symptoms should improve as your body gets used to levothyroxine. If it doesn't go away, or if it's causing you problems, contact your doctor.
• flushing or 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. The flushing should go away after a few days. If it doesn't go away, or if it's causing you problems, contact your doctor.
• muscle cramps - if you get unusual muscle ache, which isn't 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.

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.

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
• rifampicin
• amiodarone
• 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:

• 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

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

There's very little information about taking herbal remedies and supplements with levothyroxine.