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Levothyroxine: uses, benefits and side effects

July 2nd, 2018 by Joe Lofts

Quick facts

What is levothyroxine?

Levothyroxine (leh-voh-thy-ROX-een) is a medicine used to control an underactive thyroid gland, which is also known as hypothyroidism.

Hypothyroidism affects 1 in 70 women and 1 in 1000 men in the UK. Symptoms include fatigue, weight gain, sensitivity to the cold and many more.


How does levothyroxine work?

Levothyroxine is given to replace the body’s own naturally-produced (endogenous) thyroxine (T4) thyroid hormone. Having enough thyroid hormone is important for maintaining normal mental and physical health.

The medication can also be used in induced hypothyroidism. When someone has an overactive thyroid, it can be removed or partially inactivated. This leaves the person with hypothyroidism and so the thyroid hormone needs to be replaced.

Levothyroxine is only available on prescription, and there are different types of levothyroxine. The medication comes as tablets, capsules or a liquid that you can drink.

It starts working immediately, although it may take a few weeks for your symptoms to noticeably improve.

Levothyroxine dose information

The most common levothyroxine dosage options are 25mcg, 50mcg and 100mcg tablets. Other less common dosage options include:

The prescribed dosage usually depends on the person, and your dose will dictate how many tablets you’ll need to take. Please note that doses are in micrograms (mcg) and not milligrams (mg).

Most adults will usually start between 50mcg and 100mcg tablets, which can then increase to between 100mcg and 200mcg. Over-50s and people with heart problems may start on a lower dose.

Before you are prescribed levothyroxine

Your GP will take a blood test to help decide the appropriate dose for you. You’ll be expected to have regular blood tests once you start taking the thyroid medication to see how well it’s working.

If there are no complications, you will initially have a review with your GP every 3 to 4 weeks. Once the condition is under control you will be required to see your doctor every 4 to 6 months and then annually for further checks.

If your GP has confirmed that you have hypothyroidism (which needs thyroid hormone replacement), you’ll be entitled to free NHS prescriptions via a medical exemption certificate.


You'll take a blood test with your GP to help them decide the appropriate dose for you.

How to take levothyroxine

Levothyroxine is taken orally as a tablet, capsule or liquid.

It’s best to take levothyroxine first thing in the morning, ideally 30 to 60 minutes before breakfast or a caffeinated drink.

Swallow the tablets whole with a glass of water. Do not spit, chew or crush.

Both breakfast and caffeine can prevent your body from taking in levothyroxine properly, so make sure you take it on an empty stomach.

By taking the medication at the same time each day and without the presence of food to alter absorption, you minimise variation in blood levels. This also means that monitoring blood tests are likely to show an accurate picture of how effective the dose is and therefore whether or not changes need to be made to the dose.

Avoid taking levothyroxine at the same time as calcium supplement, iron supplements or antacids as they can also reduce absorption. You can still take these, but give yourself four hours after taking the levothyroxine.

If you’ve missed a dose, take it as soon as possible unless it’s almost time for your next dose. Do not take two doses together. Using Echo can help you keep on top of your doses.


What are the side effects of levothyroxine?

Side effects on levothyroxine are uncommon.

The most common side effects of levothyroxine usually occur because the dose you are taking is higher than you need. These side effects usually go away once you are put on a lower dose or stop your treatment.

Too much levothyroxine can cause hyperthyroidism-like symptoms.

The common side effects are actually very similar to the effects of an overactive thyroid - the very problem the medication is tackling. These side effects include:

Serious side effects are even more uncommon, but you should see your GP straight away if you experience:

Serious allergic reactions to levothyroxine are very rare. Seek medical help straight away if you notice any symptoms of a serious allergic reaction including rash, itching or swelling, trouble breathing, or severe dizziness.

Levothyroxine interactions

Levothyroxine interacts with a number of medications, including calcium carbonate, magnesium supplements and iron supplements. These substances bind to levothyroxine and stop it from being properly absorbed. A comprehensive list of interactions can be found here.

Soya can stop levothyroxine from working so speak to your GP or pharmacist if you eat it regularly or take soya supplements.

Avoid eating kelp, too - it contains iodine, which can make an underactive thyroid worse.

If you are currently taking any medication or food that could affect how much levothyroxine you absorb, please don’t suddenly change anything if your thyroid hormone levels are stable. It’s important that you discuss it with your GP the next time you have an appointment. If the dose of levothyroxine needs to be adjusted, it may be necessary to monitor your levels closely while you change your dose timings or food habits.

If you’re buying a new medicine over the counter, always let your pharmacist know that you are taking levothyroxine so they can let you know if there are any interactions.


Levothyroxine contraindications

There are some instances when it’s not suitable to take levothyroxine. Levothyroxine contraindications include:

If you have concerns about your own suitability to levothyroxine, talk to your GP or pharmacist.

What’s the easiest way to get levothyroxine?

If your GP has authorised you to take levothyroxine on repeat prescription, you can order with Echo today.

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Clinically reviewed by Ana Ciubotaru MRPharmS: 30/6/18

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