Skip to main content

Levothyroxine - Brand name: Eltroxin

On this page

  1. About levothyroxine
  2. Key facts
  3. Who can and cannot take levothyroxine
  4. How and when to take levothyroxine
  5. Side effects
  6. Pregnancy and breastfeeding
  7. Cautions with other medicines
  8. Common questions about levothyroxine

1. About levothyroxine

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

The thyroid gland makes thyroid hormones which help to control energy levels and growth. Levothyroxine is taken to replace the missing thyroid hormone thyroxine.

Levothyroxine is only available on prescription. It comes as tablets or as a liquid that you swallow.

2. Key facts

  • Levothyroxine is a synthetic version of a hormone called thyroxine. It replaces thyroxine if your thyroid gland cannot produce it and prevents the symptoms of hypothyroidism.
  • 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. 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, it's important to talk to your doctor to get the right care for you and your baby.

3. Who can and cannot take levothyroxine

Levothyroxine can be taken by most adults and children. However, it’s not suitable for some people.

Check with your doctor before taking levothyroxine if you:

  • have ever had an allergic reaction to levothyroxine or any other medicine
  • you have an overactive thyroid that produces too much thyroid hormone (thyrotoxicosis)
  • have a health problem that affects your adrenal glands (your doctor will be able to tell you if you're not sure)
  • have a heart problem including angina, heart disease or heart failure
  • have high blood pressure
  • have ever had a heart attack
  • have diabetes – the dose of your diabetes medicine may need to change because levothyroxine can raise blood sugar levels

4. How and when to take levothyroxine

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 does not work as well.

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

Dosage and strength

The dose of levothyroxine varies from person to person.

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.

Levothyroxine comes in 12.5 microgram, 25 microgram, 50 microgram, 75 microgram and 100 microgram tablets.

If you're taking levothyroxine as a liquid, 5ml can have 25 micrograms, 50 micrograms, 100 micrograms or 125micrograms in it.

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.

How to take levothyroxine

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. It’s available in different strengths.

If you or your child are taking levothyroxine as a liquid, it will usually come with a plastic syringe or spoon to help you measure out the right dose.

If you do not have a syringe or measuring spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it will not give the right amount.

If you 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 this case just skip the forgotten dose and take the next one at the usual time.

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

If you often forget 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.

If you take too much

Taking more than your prescribed dose of levothyroxine can give you symptoms such as a racing heartbeat (palpitations).

Urgent advice: Contact 111 for advice now if:

  • you take more than 1 extra dose of levothyroxine

Go to 111.nhs.uk or call 111

Immediate action required: Call 999 if:

  • you get chest pains – these may not happen immediately, it can be several days before they begin

Having regular 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 quite often but once your hormone levels are stable and your symptoms are under control, your levels will usually be checked once a year.

You may need blood tests more often if you:

  • are pregnant
  • start or stop a medicine that can affect the way levothyroxine works
  • have any symptoms that could mean your dose is not quite right

5. Side effects

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.

Talk to your doctor or pharmacist if you have switched to a different brand of levothyroxine and start to get:

  • symptoms of an underactive thyroid including feeling tired, weight gain or feeling depressed
  • symptoms of an overactive thyroid (similar to the common side effects below)

You may be sensitive to the new brand of levothyroxine you have been prescribed and may need to stay on the one you were taking before.

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. There are things you can do to help cope with them.

Feeling sick (nausea)

Stick to simple meals and do not eat rich or spicy food.

Being sick (vomiting)

If you're being sick try to take small, frequent sips of fluids, such as water or squash, to prevent dehydration. Speak to a pharmacist if you have signs of dehydration, such as 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 contraceptive pills and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.

Diarrhoea

Drink plenty of water to prevent dehydration. 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 from a stomach bug or illness, tell your doctor. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor first.

If you take contraceptive pills and you have severe diarrhoea for over 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 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 does not 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. 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.

Talk to your doctor or pharmacist if these side effects bother you or do not go away.

Serious side effects

It happens rarely, but some people may have serious side effects when taking levothyroxine.

Tell your doctor or contact 111 now if:

  • you get fast or irregular heartbeats (palpitations)

Go to 111.nhs.uk or call 111

Immediate action required: Call 999 now if:

  • you get chest pain

Find your nearest A&E

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to levothyroxine.

Immediate action required: Call 999 or go to A&E now if:
  • you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
  • you're wheezing
  • you get tightness in the chest or throat
  • you have trouble breathing or talking
  • your mouth, face, lips, tongue or throat start swelling

You could be having a serious allergic reaction and may need immediate treatment in hospital.

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

Information:

You can report any suspected side effect using the Yellow Card safety scheme.

Visit Yellow Card for further information.

6. Pregnancy and breastfeeding

Levothyroxine and pregnancy

Levothyroxine is safe to take in pregnancy.

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. Your doctor may need to increase your dose of levothyroxine while you’re pregnant.

Levothyroxine and breastfeeding

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 that are too small to affect your baby.

If you're breastfeeding, it's important that you continue to take levothyroxine, as this is replacing what your body would normally be making. Your body needs good levels of thyroid hormones to make enough breast milk for your baby.

If you notice that your baby is not feeding as well as usual, or if you have any other concerns about your baby, talk to your midwife, health visitor, pharmacist or doctor as soon as possible.

Non-urgent advice: Tell your doctor if you're:

  • trying to get pregnant
  • pregnant
  • breastfeeding

7. Cautions with other medicines

Some medicines can affect thyroid hormones, so the dose of levothyroxine may need to be changed. They include:

Levothyroxine can affect how other medicines work, so their doses may need to be changed. 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, 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

Find out more from the information leaflets that come with the medicines. Or get your pharmacist's 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.

However, biotin supplements can affect the accuracy of thyroid function tests. Do not take biotin without talking to your doctor.

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

Medicine safety

Tell your doctor or pharmacist if you're taking any other medicines, including herbal medicines, vitamins or supplements.

8. Common questions about levothyroxine

How does levothyroxine work?

The thyroid gland in your neck makes a hormone called thyroxine. Thyroxine controls how much energy your body uses (the metabolic rate). It's also involved in digestion, how your heart and muscles work, brain development and bone health.

When the thyroid gland does not make enough thyroxine (called hypothyroidism), many of the body's functions slow down. Some of the most common symptoms of an underactive thyroid gland are:

  • tiredness
  • feeling cold
  • difficulty concentrating
  • weight gain
  • feeling depressed

Levothyroxine is a synthetic version of thyroxine. You take levothyroxine tablets to replace the thyroxine that your thyroid gland cannot produce and prevent the symptoms of hypothyroidism.

How long does levothyroxine take to work?

Levothyroxine starts working straight away, but it may be several weeks before your symptoms start to improve and you feel any different.

How long will I take levothyroxine for?

Treatment with levothyroxine is usually lifelong. If you stop taking levothyroxine your symptoms are likely to come back.

Are there any long term side effects?

Yes, it's safe to take levothyroxine for a long time, even many years. However, high doses of levothyroxine over a long time can sometimes cause weakening of the bones (osteoporosis).

This should not happen if you are on the right dose. It's important to have regular blood tests to make sure your dose is not too high.

Will it affect my fertility?

There's no evidence to suggest that taking levothyroxine reduces fertility in either men or women.

However, speak to a pharmacist or your doctor if you're trying to get pregnant as they may want to review your treatment.

Can I drink alcohol with it?

Yes, you can drink alcohol while taking levothyroxine. Alcohol does not affect how this medicine works.

However, if you have side effects such as headaches, flushing or sweating with levothyroxine, alcohol may make these worse.

Will it make me lose weight?

One of the symptoms of an underactive thyroid gland is weight gain. So, once you start taking levothyroxine, you may lose weight as your body's hormones rebalance. Losing weight can also be a sign that your dose of levothyroxine is too high.

Once your thyroid hormone levels have returned to normal, your tendency to gain or lose weight is the same as for people who do not have thyroid problems.

Can I drive or ride a bike with it?

Yes, levothyroxine does not affect your ability to drive or ride a bike.

Is there any food or drink I need to avoid?

There are some foods and drinks that do not mix well with levothyroxine:

  • drinks containing caffeine, like coffee, tea and some fizzy drinks, can reduce the amount of levothyroxine your body takes in. Leave at least 30 minutes after taking levothyroxine before you drink them.
  • calcium-rich foods, such as milk, cheese, yoghurt and broccoli, can reduce the amount of levothyroxine your body takes in. Leave at least 4 hours between taking levothyroxine and eating calcium-rich foods.
  • soya in food and supplements may stop levothyroxine working properly. If you regularly eat soya or take soya supplements your doctor might need to do extra blood tests to make sure you're getting enough levothyroxine.
  • kelp (a type of seaweed) can contain high levels of iodine, which sometimes makes an underactive thyroid worse. Do not take supplements containing kelp if you're taking levothyroxine.
Can I get thyroid medicines for free?

If you have an underactive thyroid, you're entitled to free prescriptions for all of your medicines (not just your levothyroxine). To claim your free prescriptions you'll need to have a medical exemption certificate.

The application form for the medical exemption certificate is called FP92A. You can get this from your doctor's surgery. You will need to fill in the form, then your doctor will sign it and send it off.

Will it affect my contraception?

Levothyroxine does not affect any types of contraception, including the combined pill or emergency contraception.

However, the combined pill contains oestrogen which can reduce the amount of levothyroxine in your body.

Speak to your doctor if you start or stop taking the Pill as your dose of levothyroxine may need to change.

If levothyroxine makes you sick (vomit) or you have severe diarrhoea for over 24 hours while you’re taking levothyroxine, your contraceptive pills may not protect you from pregnancy.

If this happens, follow the instructions in the leaflet that comes with your contraceptive pills.

Find out more about what to do if you're on the pill and you're being sick or you have diarrhoea.

Will I lose my hair?

It's having an underactive thyroid that causes hair loss – not treatment with levothyroxine.

Because the natural hair growth cycle takes several months, hair loss related to thyroid disease might only be seen months after the condition has begun. If treatment with levothyroxine has already started, it may seem like the medicine – rather than the underlying illness – is causing the hair loss.

Hair usually regrows after treatment with levothyroxine, but it may take many months.

Page last reviewed: 17 December 2021
Next review due: 17 December 2024