Several complications can occur with an overactive thyroid (hyperthyroidism), particularly if the condition is not treated.

Graves' ophthalmopathy

If you have Graves' disease, you may have problems with your eyes. This is known as Graves' ophthalmopathy and is thought to be caused by the immune system mistakenly attacking the tissues of the eyes. It affects around 1 in 20 people with Graves’ disease.

Symptoms of Graves' ophthalmopathy include:

• eyes feeling dry and gritty
• sensitivity to light (photophobia)
• excessive tearing
• double vision 
• some loss of vision
• a feeling of pressure behind the eyes

In more severe cases, your eyes can bulge prominently from your eye sockets.

If you do develop Graves' ophthalmopathy, you will probably be referred to an eye specialist (ophthalmologist) for treatment.

Treatment options include:

• eyedrops to ease the symptoms
• sunglasses to protect the eyes against bright lights
• corticosteroids to reduce inflammation
• radiotherapy
• surgery

Pregnancy and overactive thyroid

Some women are pregnant when they are first diagnosed with an overactive thyroid gland. Becoming pregnant can lead to a relapse of symptoms, especially in someone with a history of Graves’ disease.

Pregnant women with an overactive thyroid are at an increased risk of developing complications during pregnancy and birth, such as miscarriage and eclampsia.

They are also more at risk of going into labour prematurely and having a baby with a low birthweight.

Pregnant women will need specialist treatment, so the condition should be managed using medications that don't affect the baby. This is likely to be a medication called propylthiouracil.

Underactive thyroid

In many cases, treatment causes the thyroid gland to release levels of hormones that are too low. This is known as having an underactive thyroid gland (hypothyroidism).

Sometimes this will only be a temporary side effect of treatment, but it can often be permanent.

Symptoms of an underactive thyroid gland include:

• being sensitive to cold
• weight gain
• constipation 
• depression 
• tiredness 

An underactive thyroid gland is treated using medications to help replicate the effects of the thyroid hormones. Read more about the treating an underactive thyroid gland.

Thyroid storm

An undiagnosed or poorly controlled overactive thyroid can lead to a rare but serious reaction called a thyroid storm. It affects around 1 in 100 people with an overactive thyroid gland.

A thyroid storm is a severe and sudden flare-up of symptoms caused by the metabolism going into overdrive, often due to triggers such as:

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

Symptoms of a thyroid storm include:

• a very rapid heartbeat (over 140 beats a minute)
• fever (a temperature higher than 38C/100.4F)
• dehydration, with diarrhoea and vomiting
• jaundice – a yellow tinge to your skin and eyes
• severe agitation and confusion
• hallucinations – seeing or hearing things that are not real
• psychosis – being unable to tell the difference between reality and your imagination
• excessive sweating 
• chest pain
• muscle weakness

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