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 3 people with an overactive thyroid caused by Graves' disease.
Problems can include:
Many cases are mild and get better as your overactive thyroid is treated, but for around 1 in every 20 to 30 cases there's a risk of vision loss.
If you experience eye problems, you'll probably be referred to an eye specialist (ophthalmologist) for treatment, such as eye drops, steroid medicine or possibly surgery.
Find out more about how thyroid eye disease is treated.
Treatment for an overactive thyroid often results in hormone levels becoming 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.
If you have an overactive thyroid during pregnancy and your condition is not well controlled, it can increase the risk of:
Tell your doctor if you're planning a pregnancy or think you might be pregnant.
They'll want to check whether your condition is under control and may recommend switching to a treatment that will not affect your baby, such as the medicine propylthiouracil.
If you're not planning a pregnancy, it's important to use contraception because some treatments for an overactive thyroid can harm an unborn baby.
In rare cases, 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.
An overactive thyroid can also increase your chances of developing: