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Complications - Overactive thyroid (hyperthyroidism)

You can have complications with an overactive thyroid, especially if it has not been treated.

Eye problems

Eye problems affect about 1 in 3 people with an overactive thyroid caused by Graves' disease.

Eye problems can include:

  • dry and gritty eyes
  • sensitivity to light
  • watering eyes
  • blurred or double vision
  • red eyes
  • red, swollen or pulled-back eyelids
  • bulging eyes

Many cases are mild and get better with treatment. There is a risk of loss of vision in about 1 in every 20 to 30 cases.

If you have eye problems, your doctor will refer you to an eye specialist (ophthalmologist) for treatment. Treatment can include eye drops, steroid medicines or surgery.

Pregnancy problems

If you have an overactive thyroid during pregnancy and your condition is not well controlled, it can increase the risk of:

Tell your GP if you're planning to get pregnant or you think you might be pregnant.

They will check that your condition is under control. They may need to change your treatment.

Use contraception if you do not plan getting pregnant. Some thyroid treatments can harm a developing baby.

If you are taking carbimazole, your doctor will usually change this to propylthiouracil.

Thyroid storm

An undiagnosed or poorly controlled overactive thyroid can lead to a serious, life-threatening reaction. This is rare. It is called a thyroid storm.

A thyroid storm is a sudden flare-up of symptoms.

It can be triggered by:

  • an infection
  • pregnancy
  • not taking your medicine correctly
  • damage to the thyroid gland, such as a punch to the throat

Emergency action required: Phone 112 or 999 immediately if:

  • you think you or someone else is having a thyroid storm

Symptoms of a thyroid storm include:

Other problems

An overactive thyroid can also increase your chances of developing:

  • a heart condition that causes an irregular and often abnormally fast heart rate - atrial fibrillation
  • weakened bones (osteoporosis) - this can make your bones fragile and more likely to break
  • heart failure - where the heart is unable to pump blood around the body properly

Underactive thyroid

Treatment for an overactive thyroid can result in hormone levels becoming too low. This is called an underactive thyroid (hypothyroidism).

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: 20 December 2023
Next review due: 20 December 2026