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Several complications can develop if you have an overactive thyroid (hyperthyroidism), particularly if the condition is not treated.

Eye problems, known as thyroid eye disease or Graves' ophthalmopathy, affect around 1 in 4 people with an overactive thyroid caused by Graves' disease.

Problems can include:

• 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 people have mild eye symptoms, which get better as their overactive thyroid is treated. But some people may continue to have problems with their eyes.

If Graves’ ophthalmology is particularly severe it can cause vision loss.

If you have eye problems, you may be referred to an eye specialist (ophthalmologist) for treatment, such as eye drops, steroid medicine or possibly surgery.

Find out how bulging eyes caused by thyroid eye disease is treated

Radioactive iodine treatment for an overactive thyroid often causes hormone levels to become 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 long-term treatment with thyroid hormone medicine is needed.

Find out more about how an underactive thyroid is treated

It's very important that you use contraception if:

• you’re taking carbimazole
• you’ve had radioactive iodine treatment in the last 6 months

Both treatments can harm your unborn baby.

Tell your doctor immediately if you’re planning a pregnancy or become pregnant while taking carbimazole, or after having radioactive iodine treatment within the past 6 months.

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

• pre-eclampsia
• miscarriage
• premature labour and birth (before the 37th week of pregnancy)
• your baby having a low birthweight

Your doctor will want to check whether your condition is under control, and if you’re taking carbimazole they may recommend switching to a treatment that will not affect your baby, such as the medicine propylthiouracil.

Rarely, an undiagnosed or poorly controlled overactive thyroid can lead to a serious, life-threatening condition called a thyroid storm.

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

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

Symptoms of a thyroid storm include:

• a rapid heartbeat
• a high temperature
• high blood pressure (hypertension)
• yellowing of the skin and eyes (jaundice)
• severe agitation and confusion
• loss of consciousness

A thyroid storm is a medical emergency. If you think you or someone in your care is experiencing it, call 999 to ask for an ambulance immediately.

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) – a condition that makes your bones fragile and more likely to break
• heart failure – where the heart is unable to pump blood around your body properly