What is 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).

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)
Being sick (vomiting)
Feeling restless or excitable, or problems sleeping
Flushing or sweating
Muscle cramps
Shaking, usually of the hands

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

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.

7. Cautions with other medicines

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

  • epilepsy medicines like carbamazepine and phenytoin
  • rifampicin
  • amiodarone
  • oestrogens – such as in combined contraceptive pills or hormone replacement therapy (HRT)

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.

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