Overactive thyroid

What causes an overactive thyroid gland?

The thyroid gland sits in your neck. It makes hormones called thyroxine and triiodothyronine, which travel through the blood and control the body's growth and metabolism. They influence things like heart rate and body temperature, and help turn food into the energy your body runs on.

In hyperthyroidism, the thyroid gland makes too much thyroxine or triiodothyronine, which speeds up your metabolism.

There are several possible underlying causes. The most common is Graves' disease, in which the immune system attacks the thyroid gland and pushes it to produce too much of these hormones.

Causes of an overactive thyroid

Overactive thyroid (hyperthyroidism) happens when your thyroid gland produces too much thyroxine or triiodothyronine. A number of conditions can lead to this overproduction.

Graves' disease

Graves' disease is the most common cause of an overactive thyroid. It can run in families and appear at any age, though it is most common in women between 20 and 40 years old. Smoking makes you more likely to develop it.

Graves' disease is an autoimmune condition, meaning the immune system mistakes something in the body for a harmful substance and attacks it. In Graves' disease, it attacks the thyroid gland, leading to an overproduction of thyroid hormones.

It isn't known exactly what triggers the immune system to do this. As with many autoimmune conditions, a mix of genetic and environmental factors is thought to play a role.

If you have Graves' disease, your eyes may also be affected, causing discomfort and double vision. This is known as Graves' ophthalmopathy, and it can make your eyes appear to bulge or look more prominent.

For more on this, see the section on complications of an overactive thyroid gland.

Thyroid nodules

Lumps, known as nodules, can sometimes develop in the thyroid gland. It isn't clear why they form, but they are usually non-cancerous (benign).

Even so, a nodule can contain abnormal thyroid tissue that disrupts the normal production of thyroxine or triiodothyronine, leading to an overactive thyroid. Nodules with this abnormal tissue are called toxic nodules.

Toxic thyroid nodules are responsible for around 1 in 20 cases of hyperthyroidism.

Iodine supplements

Your thyroid gland uses the iodine in your food to make thyroxine and triiodothyronine. But taking extra iodine through supplements can push the gland to produce too much of these hormones.

This is known as iodine-induced hyperthyroidism, sometimes called the Jod-Basedow phenomenon. It usually only happens if you already have nodules in your thyroid gland.

Amiodarone

Amiodarone is a medicine known as an anti-arrhythmic, used to control an irregular heartbeat (atrial fibrillation). If you have non-toxic nodules in your thyroid gland, taking amiodarone can trigger hyperthyroidism because the drug contains iodine.

Amiodarone can also cause a form of hyperthyroidism that tends to be more severe and harder to treat, because it harms thyroid tissue directly. This is called amiodarone-induced hyperthyroidism.

Follicular thyroid cancer

In rare cases, an overactive thyroid can result from thyroid cancer that starts in the thyroid follicles. This happens when cancer cells in the thyroid gland begin producing thyroxine or triiodothyronine. It is also known as functioning thyroid cancer.

Finding the underlying cause

If tests confirm you have an overactive thyroid, you may be referred for further tests to find out what's causing it.

One such test is an isotope thyroid scan. This involves swallowing a small amount of a radioactive substance, usually technetium, in capsule or liquid form. A scan is then taken to see how much of it has been absorbed by your thyroid gland.

If your thyroid absorbs a large amount of the isotope, the underlying cause is likely to be Graves' disease or thyroid nodules.

If the amount absorbed is low, the cause could instead be:

• swelling (inflammation) of the thyroid gland, known as thyroiditis, often caused by the immune system mistakenly attacking thyroid tissue, or less commonly by infection
• having too much iodine in your diet
• in rare cases, thyroid cancer

If you think you might have an overactive thyroid gland (hyperthyroidism), it is best to see your doctor. The diagnosis is made by looking at your symptoms together with blood tests that check how well your thyroid is working, known as thyroid function tests.

Thyroid function tests

Your doctor will take a sample of your blood and check the levels of:

• thyroid stimulating hormone (TSH)
• thyroxine and triiodothyronine (the thyroid hormones)

TSH is made in the pituitary gland in your brain, and it controls how much thyroxine and triiodothyronine your thyroid produces. When your thyroxine and triiodothyronine levels are normal, your pituitary gland releases a normal amount of TSH. When your thyroid starts producing too much hormone, the pituitary gland stops releasing TSH. And when thyroxine or triiodothyronine levels drop, the pituitary gland makes more TSH to bring them back up.

If you have an overactive thyroid, your test will show TSH levels that are consistently lower than normal. A low TSH level tells your doctor that your thyroid gland is overactive and is probably producing too much thyroid hormone. This is the first part of the thyroid function test.

Your doctor will then check your blood levels of thyroxine and triiodothyronine. If your thyroid is overactive, both of these hormones will be higher than normal.

Subclinical overactive thyroid gland

Sometimes the tests show that your thyroid hormone levels are normal, but your TSH level is low or suppressed. This is called a subclinical overactive thyroid gland. If you are diagnosed with this, you may not need any treatment.

In most cases, the lowered TSH level returns to normal within a couple of months, and the condition resolves on its own. Even so, you will need a follow up thyroid function test so your doctor can keep track of how you are doing.

Treatment

An overactive thyroid usually responds well to treatment, and most people are able to control their symptoms.

The 3 most widely used treatments for an overactive thyroid gland are:

• thionamides – a group of medications, including carbimazole and methimazole, that stop the thyroid gland producing too much thyroid hormone
• radioiodine treatment – a radioactive substance called iodine that helps shrink the thyroid gland, reducing its activity (the radiation contained in iodine is a very low dose and does not pose a threat to health)
• surgery – in a small number of cases surgery may be required to remove some or all of the thyroid gland, particularly if there is a large goitre

Beta-blockers may also sometimes be used to temporarily relieve many symptoms of an overactive thyroid gland, although it doesn’t target the thyroid gland itself.

It’s common for treatment to lead to the thyroid not producing enough hormones. This is known as having an underactive thyroid gland (hypothyroidism). However, an underactive thyroid is not usually serious and is easily treated.

Read more about the treatment of an overactive thyroid gland

Treating an overactive thyroid

If you are diagnosed with an overactive thyroid gland (hyperthyroidism), your GP will refer you to a specialist in hormonal conditions (endocrinologist) to plan your treatment.

Thionamides

Thionamides, such as carbimazole and propylthiouracil, are a common treatment. They are a type of medication that stops your thyroid gland producing excess amounts of thyroxine or triiodothyronine.

As thionamides affect the production of thyroid hormone rather than their current levels, you will need to take them for several weeks before you notice an improvement (usually between 4 to 8 weeks).

Once the production of thyroid hormones is under control, your specialist may gradually reduce your medication.

You may need to continue taking thionamides for a long time, until the condition is under control.

Around 1 in 20 people will experience side effects when they first start taking thionamides, such as:

• itchy skin rash
• joint pain

These side effects should pass once your body is used to the effects of the medication.

In rare cases (around 1 in 500), thionamides cause a sudden drop in white blood cells (agranulocytosis), which can make you extremely vulnerable to infection.

Symptoms of agranulocytosis include:

• fever
• gum pain, swelling and bleeding
• sore throat
• mouth ulcers
• persistent cough
• shortness of breath

If you are taking thionamides and you experience any of the symptoms above, phone your GP immediately for advice and an urgent blood test. If this is not possible, phone 111 or your local out-of-hours service.

Beta-blockers

Beta-blockers, such as propranolol or atenolol, can relieve some of the symptoms of an overactive thyroid, including tremor (shaking and trembling), rapid heartbeat and hyperactivity.

Your specialist may prescribe you a beta-blocker while the condition is being diagnosed, or until thionamide brings your thyroid gland under control. However, beta-blockers are not suitable if you have asthma.

Beta-blockers can sometimes cause side effects, including:

• feeling sick
• feeling tired all the time (fatigue)
• cold hands and feet
• trouble sleeping, sometimes with nightmares

Radioiodine treatment

Radioiodine treatment is a form of radiotherapy used to treat most types of overactive thyroid. Radioactive iodine shrinks your thyroid gland, reducing the amount of thyroid hormone it can produce.

Radioiodine treatment is given either as a drink or a capsule to swallow. The dose of radioactivity in the radioiodine is very low and is not harmful.

Radioiodine treatment is not suitable if you are pregnant or breastfeeding, and may not be suitable if you have eye problems, such as double vision or prominent (bulging) eyes.

Women should avoid getting pregnant for at least 6 months after having radioiodine treatment. Men should not father a child for at least 4 months after having radioiodine treatment.

Most people only require a single dose of radioiodine treatment. If a further follow-up dose is required, it is usually given 6 to 12 months after the first dosage.

A short course of thionamides treatment may be given a few weeks before radioiodine treatment, as this can lead to a more rapid relief of symptoms.

Thionamides or radioiodine?

In some cases, a particular treatment may be recommended based on factors such as your age, symptoms and the amount of extra thyroid hormone in your blood. However, there may be circumstances when you are offered a choice between a long-term course of thionamides or radioiodine treatment.

Both treatments have advantages and disadvantages.

Advantages of thionamides include:

• they are straightforward to take and you do not have to go to hospital to take them
• there is less risk of getting an underactive thyroid gland (hypothyroidism) as a result of treatment

Disadvantages of thionamides include:

• treatment may not be as successful as radioiodine treatment
• there is a higher risk of side effects

Advantages of radioiodine treatment include:

• treatment is usually very successful

Disadvantages of radioiodine treatment include:

• there is a higher chance of your thyroid gland becoming underactive as a result of treatment
• radioiodine treatment is usually not suitable for people with additional symptoms affecting their eyes (Graves’ ophthalmopathy)
• women have to avoid getting pregnant for at least 6 months, and men should not father a child for at least 4 months after treatment

You should discuss the potential risks and benefits of both types of treatment with the specialist in charge of your care.

Surgery

Surgery to remove all or part of the thyroid gland is known as a total or partial thyroidectomy. It is a permanent cure for recurrent overactive thyroid.

Your specialist may recommend surgery if your thyroid gland is severely swollen (a large goitre) and is causing problems in your neck.

Other reasons for surgery include:

• a person cannot be treated with radioiodine treatment as they are pregnant and are unable or unwilling to take thionamides
• a person has a severe form of Graves’ ophthalmopathy
• the symptoms return (relapse) after a previous successful course of treatment with thionamides

It is normally recommended that the entire thyroid gland is removed, as this means there will be no chance of a relapse.

However, you will need to take medication for the rest of your life to compensate for the lack of a functioning thyroid gland – these will be the same medications used to treat an underactive thyroid gland.

Complications

Around 1 in 20 people with Graves’ disease will also develop symptoms affecting their eyes, such as:

• double vision
• sensitivity to light (photophobia)
• tearing (excess production of tears)

This is known as Graves’ ophthalmopathy and should be seen by a doctor who specialises in treating eye conditions (an ophthalmologist).

A rarer and more serious complication is a sudden and severe flare-up of symptoms, known as a thyroid storm. A thyroid storm can be life-threatening, as it causes severe dehydration and heart problems.

Read more about the complications of an overactive thyroid gland

Who is affected

Women are 10 times more likely to have an overactive thyroid gland than men.

In most cases, symptoms will begin somewhere between the ages of 20 and 40, though they can start at any age, including in childhood.

An overactive thyroid gland occurs most frequently in white and Asian people, and less frequently in African-Caribbean people.

Complications of an overactive thyroid

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 phone 999 for an ambulance.

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