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Complications - Overactive thyroid (hyperthyroidism)

Complications can happen with an overactive thyroid, particularly if the condition isn't treated.

Eye problems

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

Eye problems can include:

  • the eyes feeling dry and gritty
  • 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 as your overactive thyroid is treated. In around one in every 20 to 30 cases there's a risk of vision loss.

If you have eye problems, you will be referred to an eye specialist (ophthalmologist) for treatment. This could be eye drops, steroid medication or surgery.

How thyroid eye disease is treated

Underactive thyroid

Treatment for an overactive thyroid often results in hormone levels that are too low. This is known as an underactive thyroid (hypothyroidism).

Symptoms of an underactive thyroid can include:

  • sensitivity to cold
  • tiredness
  • weight gain
  • constipation
  • depression

An underactive thyroid is sometimes only temporary. But often it's permanent and needs long-term treatment with thyroid hormone medication.

Treating an underactive thyroid

Pregnancy problems

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

  • pre-eclampsia
  • miscarriage
  • giving birth prematurely
  • your baby having a low birthweight

Tell your GP if you're planning a pregnancy or you think you might be pregnant.

They will want to check that your condition is under control. They may recommend switching to a treatment that won't affect the baby.

If you're not planning a pregnancy, it's important to use contraception. This is because some treatments for an overactive thyroid can harm an unborn baby.

Thyroid storm

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

This is a sudden flare-up of symptoms that can be triggered by:

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

Emergency action required: Call 112 or 999 if

you think you or someone in your care is experiencing a thyroid storm.

Symptoms include:

  • a rapid heartbeat
  • a high temperature over 38 degrees Celsius 
  • diarrhoea and vomiting
  • yellowing of the skin and eyes (jaundice)
  • severe agitation and confusion
  • loss of consciousness

A thyroid storm is a medical emergency.

Other problems

An overactive thyroid can also increase your chances of developing:

  • atrial fibrillation - a heart condition that causes an irregular and often abnormally fast heart rate
  • 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

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: 14 March 2021
Next review due: 14 March 2024