Complications

Several complications can occur with an overactive thyroid (hyperthyroidism), particularly if the condition isn't treated.

Eye problems

Eye problems,known as thyroid eye disease orGraves' ophthalmopathy, affect aroundone in threepeople with an overactive thyroid caused by Graves' disease.

These can include:

• the 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 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 youexperience eye problems, you willprobably be referred to an eye specialist (ophthalmologist) for treatment, such as eye drops, steroid medication or possibly surgery.

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 requires long-term treatment with thyroid hormone medication.

Pregnancy problems

If you have an overactive thyroid during pregnancyand your condition isn't well controlled, it can increasethe risk of:

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

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 thatwon'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 anunborn baby.

Thyroid storm

In rare cases, an undiagnosed or poorly controlled overactive thyroid can lead to a serious, life-threateningreaction called a thyroid storm.

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

• an infection
• pregnancy
• 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.

Other problems

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) this can make your bones fragile and more likely to break
• heart failure wherethe heart is unable to pump blood around the body properly