Several complications can occur with an overactive thyroid (hyperthyroidism), particularly if the condition isn't treated.
Eye problems, known as thyroid eye disease or Graves' ophthalmopathy, affect around one in three people with an overactive thyroid caused by Graves' disease.
These can include:
Many cases are mild and get better as your overactive thyroid is treated, but in around one in every 20 to 30 cases there's a risk of vision loss.
If you experience eye problems, you will probably be referred to an eye specialist (ophthalmologist) for treatment, such as eye drops, steroid medication or possibly surgery.
Read more about how thyroid eye disease is treated.
Treatment for an overactive thyroid often results in hormone levels that are too low – 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 requires long-term treatment with thyroid hormone medication.
Read more about the how an underactive thyroid is treated.
If you have an overactive thyroid during pregnancy and your condition isn't well controlled, it can increase the risk of:
Tell your doctor if you're planning a pregnancy or think you might be pregnant.
They will want to check whether your condition is under control and they may recommend switching to a treatment that won't affect the baby, such as the medication 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 reaction called a thyroid storm.
This is a sudden flare-up of symptoms that can be triggered by:
- an infection
- not taking your medication 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 (fever) over 38C (100.4F)
- diarrhoea and vomiting
- 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 for an ambulance immediately.
An overactive thyroid can also increase your chances of developing: