Complications

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

Eye problems

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:

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

Underactive thyroid

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:

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

Pregnancy problems

It's very important that you use contraception if:

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:

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.

Thyroid storm

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:

Symptoms of a thyroid storm include:

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.

Other problems

An overactive thyroid can also increase your chances of developing:

Page last reviewed: 24 September 2019
Next review due: 24 September 2022