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Treatment - Underactive thyroid (hypothyroidism)

An underactive thyroid can often be treated by taking daily hormone tablets. These replace the hormones your thyroid is not making.

You'll usually need treatment for the rest of your life. But with proper treatment, you can lead a normal healthy life.

Hormone replacement tablets - levothyroxine

You'll usually take levothyroxine. This is a tablet to replace the thyroxine hormone.

You'll have regular blood tests until the correct dose of levothyroxine is reached. This can take a little while to get right.

You may start on a low dose first. The dose may be increased gradually. But it depends on how your body responds. Some people start to feel better soon after beginning treatment. Others do not notice an improvement for several months.

When you're taking the correct dose, you'll usually have a blood test once a year. This is to check your hormone levels.

Taking levothyroxine

If you're prescribed levothyroxine, take 1 tablet at the same time every day.

You're usually told to take the tablets in the morning. But some people prefer to take them at night.

Other medicines, supplements or foods can affect the tablets. So you should only swallow them with water on an empty stomach. Avoid eating for 30 minutes afterwards.

If you forget to take a dose, take it as soon as you remember. If you do not remember until a long time later, skip the dose and take the next dose at the usual time. Your GP may tell you otherwise.

Side effects of levothyroxine

You'll usually only have side effects if the dose you have taken is too high. These will usually disappear when the dose is reduced or treatment is stopped for a few days.

Side effects you may notice include:

  • sweating
  • chest pain
  • muscle pain
  • headaches
  • diarrhoea
  • vomiting
  • restlessness or difficulty sleeping

Some patients may experience a severe reaction to high levels of thyroid hormone. This is called a thyroid crisis.

A thyroid crisis can cause the following symptoms:

  • very high fever
  • fast heart rate
  • low blood pressure
  • heart failure
  • jaundice
  • confusion
  • seizure
  • coma

Speak to your GP if you have any concerns.

When you might not need medicine

Blood tests might suggest you have an underactive thyroid. But you might not have any symptoms, or very mild symptoms. If this is the case, you may not need any treatment.

In these cases, your GP will usually check your hormone levels every few months. They'll prescribe levothyroxine if you develop symptoms.

If you do not have treatment


If an underactive thyroid is not treated, this can lead to problems.

These include:

  • heart disease
  • goitre
  • pregnancy problems
  • myxoedema coma - a rare, but life-threatening condition

Heart problems

If you do not have treatment you are at greater risk of cardiovascular disease.

Low levels of the hormone thyroxine can lead to high cholesterol. This can lead to serious heart-related problems, such as angina and a heart attack.

Non-urgent advice: Contact your GP if:

  • you're being treated for an underactive thyroid and you have chest pain


This is an abnormal swelling of the thyroid gland. It causes a lump to form in the throat. Goitre can develop in people with thyroid problems.

Underactive thyroid and pregnancy

An underactive thyroid should be treated properly before you become pregnant. This is important for the health of you and your baby.

Non-urgent advice: Contact your GP if:

  • you're pregnant or trying to become pregnant and you have an underactive thyroid

They may refer you to a specialist.

Pregnancy complications

If an underactive thyroid is not treated, there's a risk of problems.

These include:

  • pre-eclampsia – this can cause high blood pressure and fluid retention, leading to growth problems in your baby
  • an underactive thyroid in your baby
  • anaemia in the mother
  • birth defects
  • bleeding after birth
  • problems with your baby's physical and mental development
  • premature birth or a low birthweight
  • stillbirth or miscarriage

These problems can usually be avoided with treatment.

Myxoedema coma

This is a rare life-threatening condition when thyroid hormone levels become very low.

It can cause symptoms such as:

  • confusion
  • hypothermia
  • drowsiness

Myxoedema coma requires emergency treatment in hospital.

Content supplied by the NHS and adapted for Ireland by the HSE

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This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.

Page last reviewed: 25 March 2021
Next review due: 25 March 2024