Overactive thyroid (also known as hyperthyroidism) is a relatively common hormonal condition that occurs when there is too much thyroid hormone in the body.
Excess levels of thyroid hormones can then speed up the body’s metabolism, triggering a range of symptoms, such as:
nervousness and anxiety
hyperactivity – where a person can’t stay still and is full of nervous energy
unexplained or unplanned weight loss
swelling of the thyroid gland, which causes a noticeable lump, known as a goitre, to form in the throat
The severity, frequency and range of symptoms can vary from person to person.
What causes an overactive thyroid gland?
The thyroid gland is found in the neck. It produces hormones that are released into the bloodstream to control the body's growth and metabolism. These hormones are called thyroxine and triiodothyronine.
They affect processes such as heart rate and body temperature, and help convert food into energy to keep the body going.
In hyperthyroidism, the thyroid gland produces too much thyroxine or triiodothyronine, which speeds up the body's metabolism.
There are several possible underlying causes, the most common being Graves' disease, in which the body's immune system targets the thyroid gland and causes it to produce too much of the thyroid hormones.
An overactive thyroid usually responds well to treatment, and most people are able to control their symptoms.
The three 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.
Around 1 in 20 people with Graves' disease will also develop symptoms affecting their eyes, such as:
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.
Who is affected
Women are 10 times more likely to have an overactive thyroid gland than men.
It is estimated that around 1 in 50 women in England currently live with an overactive thyroid gland.
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.
Symptoms of overactive thyroid
An overactive thyroid (hyperthyroidism) has many signs and symptoms, although it is unlikely you would develop all of them.
Symptoms of hyperthyroidism
If you have hyperthyroidism, you may experience some of the following symptoms:
mood swings – such as anxiety, irritability and nervousness
difficulty sleeping (insomnia)
feeling tired all the time (fatigue)
needing to pass stools (faeces) or urine more frequently
excess fats in your stools – which can make them greasy and difficult to flush down the toilet (steatorrhoea)
sensitivity to heat and excess sweating
unexplained or unexpected weight loss – despite having an increased appetite (though in a small number of cases, the increase in appetite can lead to weight gain)
very infrequent or light periods, or periods stopping altogether
loss of interest in sex
If you have diabetes, your diabetic symptoms, such as extreme thirst and tiredness, may be made worse by hyperthyroidism.
Signs of hyperthyroidism
If you have hyperthyroidism, you may have some of the following physical signs:
a swelling in your neck caused by an enlarged thyroid gland (goitre)
irregular and/or unusually fast heart rate (palpitations)
trembling or shaking (tremor)
warm, moist skin
redness on the palms of your hands
loosening of your nails in their nail beds
patchy hair loss (alopecia)
twitching in your face and limbs
When to seek medical advice
See your GP if you are experiencing any of the above. They may not be the result of an overactive thyroid gland, but they will need further investigation.
It might be useful to make a list of your symptoms, as this can often be helpful in determining the correct diagnosis.
Causes of overactive thyroid
Overactive thyroid (hyperthyroidism) occurs when your thyroid gland produces too much of the thyroid hormones thyroxine or triiodothyronine.
Overproduction of thyroid hormones can be caused by a number of conditions, which are outlined below.
Graves' disease is the most common cause of overactive thyroid. It can run in families and can occur at any age, although it is most common in women aged 20-40 years old. You are more likely to develop Graves' disease if you smoke.
Graves' disease is an autoimmune condition. This means the immune system mistakes something in the body for a toxic substance and attacks it.
In Graves' disease, it attacks the thyroid gland, which leads to an overproduction of the thyroid hormones.
It is not known what triggers the immune system to do this. Like many autoimmune conditions, it is thought that a combination of both genetic and environmental factors could be involved.
If you have Graves' disease, your eyes may also be affected, causing discomfort and double vision. This is known as Graves' ophthalmopathy. You may find your eyes bulge out or appear more prominent.
For more information on Graves' ophthalmopathy, read our page on complications of an overactive thyroid gland.
It is possible for lumps to develop in your thyroid gland. These are known as nodules. It is not known why nodules develop, but they are usually non-cancerous (benign).
However, the nodules can contain abnormal thyroid tissue, which affects the normal production of thyroxine or triiodothyronine, causing an overactive thyroid. Nodules that contain abnormal thyroid tissue are described as toxic.
Toxic thyroid nodules account for about 1 in 20 cases of hyperthyroidism.
Iodine contained in the food you eat is used by your thyroid gland to produce the thyroid hormones thyroxine and triiodothyronine. However, taking additional iodine in supplements can cause your thyroid gland to produce too much thyroxine or triiodothyronine.
This is known as iodine-induced hyperthyroidism – sometimes referred to as Jod-Basedow phenomenon. It usually only occurs if you already have nodules in your thyroid gland.
Amiodarone is a type of medication known as an anti-arrhythmic, which helps to control an irregular heartbeat (atrial fibrillation). If you have non-toxic nodules in your thyroid gland, taking amiodarone can cause hyperthyroidism because it contains iodine.
Amiodarone can cause a type of hyperthyroidism usually more severe and difficult to treat through a harmful effect on thyroid tissue. This type of hyperthyroidism is called amiodarone-induced hyperthyroidism.
Follicular thyroid cancer
In rare cases, you may develop an overactive thyroid as a result of thyroid cancer that starts in your thyroid follicles. This can occur if cancer cells in your thyroid gland begin to produce thyroxine or triiodothyronine. This is also known as functioning thyroid cancer.
Diagnosing overactive thyroid
See your GP if you think you may have an overactive thyroid gland (hyperthyroidism).
A diagnosis will be based on your symptoms and the results of blood tests that assess how well your thyroid gland is working. These are known as thyroid function tests.
Thyroid function tests
Your GP will take a sample of your blood and test it for levels of:
thyroid-stimulating hormone (TSH)
thyroxine and triiodothyronine (the thyroid hormones)
TSH is made in the pituitary gland in your brain and controls the production of thyroxine and triiodothyronine.
When the level of thyroxine and triiodothyronine in your blood are normal, your pituitary gland releases a normal level of TSH. When thyroid hormone production becomes excessive, the pituitary gland stops releasing TSH.
When the level of thyroxine or triiodothyronine drops, the pituitary gland produces more TSH to boost it.
If you have an overactive thyroid, the thyroid function test will show that levels of TSH in your blood are consistently lower than normal. Low levels of TSH mean your thyroid gland is overactive and likely to be making excessive thyroid hormones. This is the first part of the thyroid function test.
Your GP will then test your blood for levels of thyroxine and triiodothyronine. If you have an overactive thyroid, you will have higher than normal levels of both these hormones.
Subclinical overactive thyroid gland
In some cases, tests may show you have normal thyroid hormone levels, but low or suppressed levels of TSH.
This is known a subclinical overactive thyroid gland. If you are diagnosed with subclinical overactive thyroid, you may not need treatment.
In most cases, the reduced level of TSH in your blood returns to normal within a couple of months and your subclinical hyperthyroidism will resolve by itself.
However, you will need a further thyroid function test so your condition can be monitored.
Determining the underlying cause
If tests confirm an overactive thyroid gland, you may be referred for further tests to determine the underlying cause.
An additional test that may be used is an isotope thyroid scan. This involves swallowing small amounts of a radioactive substance (an isotope), usually technetium, in capsule or liquid form.
A scan is then used to measure how much of the isotope has been absorbed by your thyroid gland.
If your thyroid gland absorbs a high amount of the isotope, it is likely the underlying cause is either Graves' disease or thyroid nodules.
If the amount is low, the underlying cause could be due to:
swelling (inflammation) of the thyroid gland (thyroiditis), often caused by your immune system mistakenly attacking thyroid tissue or, less commonly, by infection
having too much iodine in your diet
in rare cases, thyroid cancer
Treating 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.
The most widely used treatments for an overactive thyroid are outlined below.
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 four to eight 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
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:
gum pain, swelling and bleeding
shortness of breath
If you are taking thionamides and you experience any of the symptoms above, call your GP immediately for advice and an urgent blood test.
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 tired all the time (fatigue)
cold hands and feet
trouble sleeping, sometimes with nightmares
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 six months after having radioiodine treatment. Men should not father a child for at least four 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 six months, and men should not father a child for at least four 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 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 of overactive thyroid
Several complications can occur with an overactive thyroid (hyperthyroidism), particularly if the condition is not treated.
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)
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
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.
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
An underactive thyroid gland is treated using medications to help replicate the effects of the thyroid hormones.
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:
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
A thyroid storm is a medical emergency. If you think you or someone in your care is experiencing this complication, you need to call 999 for an ambulance.