An underactive thyroid, also called hypothyroidism, means your thyroid gland does not produce enough chemicals called hormones.
Common signs of an underactive thyroid are tiredness, weight gain and feeling depressed. An underactive thyroid is not usually serious. It can often be treated successfully by taking daily hormone tablets to replace the hormones your thyroid isn't making.
An underactive thyroid cannot be prevented. Most cases are caused either by your immune system attacking your thyroid or by damage to your thyroid that can occur during some treatments for thyroid cancer and an overactive thyroid.
When to see your GP
Symptoms of an underactive thyroid are often confused for something else by patients and doctors and they develop slowly so you may not notice them for years.
You should see your GP and ask to be tested for an underactive thyroid if you have symptoms including:
being sensitive to the cold
dry skin and hair
The only accurate way to find out if you have a thyroid problem is to have a blood test to measure your hormone levels.
Who is affected
Both men and women can have an underactive thyroid, although it's more common in women. In the UK, it affects 15 in every 1,000 women and 1 in 1,000 men. The condition can also develop in children.
Around 1 in 3,500-4,000 babies is born with an underactive thyroid (called congenital hypothyroidism). All babies born in the UK are screened for congenital hypothyroidism using a heel-prick blood test when the baby is about five days old.
Treating an underactive thyroid
Treatment for an underactive thyroid involves taking daily hormone-replacement tablets, called levothyroxine, to raise your thyroxine levels. You will usually need treatment for the rest of your life. However, with proper treatment you should be able to lead a normal, healthy life.
If it's not treated, an underactive thyroid can lead to complications, including heart disease, goitre (a lump in the throat caused by a swollen thyroid), pregnancy problems and a life-threatening condition called myxoedema coma (although this is very rare).
The thyroid gland
The thyroid gland is a small butterfly-shaped gland in the neck, just in front of the windpipe (trachea).
One of its main functions is to produce hormones that help regulate the body's metabolism (the process that turns food into energy). These hormones are called triiodothyronine (T3) and thyroxine (T4).
When the thyroid does not produce enough thyroxine, many of the body's functions slow down.
Symptoms of underactive thyroid
Many symptoms of an underactive thyroid (hypothyroidism) are the same as those for other conditions, so they can easily be confused for something else.
Symptoms usually begin slowly and you may not realise you have a medical problem for several years.
Common symptoms include:
being sensitive to cold
slow movements and thoughts
muscle aches and weakness
dry and scaly skin
brittle hair and nails
loss of libido
pain, numbness and a tingling sensation in the hand and fingers (carpal tunnel syndrome)
irregular or heavy periods
Elderly people with an underactive thyroid may develop memory problems and depression. Children may experience slower growth and development. Teenagers may start puberty earlier than normal. If you have any of these symptoms, see your GP and ask to be tested for an underactive thyroid.
If underactive thyroid is not treated
It is unlikely that you would have many of the later symptoms of an underactive thyroid as the condition is often spotted before more serious symptoms appear.
Later symptoms of underactive thyroid include:
a low-pitched and hoarse voice
a puffy-looking face
thinned or partly missing eyebrows
a slow heart rate
Causes of underactive thyroid
An underactive thyroid (hypothyroidism) is when your thyroid gland doesn't produce enough of the hormone thyroxine, also called T4.
Most cases of underactive thyroid are due to either the immune system attacking the thyroid gland or a damaged thyroid.
Most cases of underactive thyroid happen when the immune system, which normally fights infection, attacks the thyroid gland. This damages the thyroid, which means it is not able to make enough of the hormone thyroxine, and leads to the symptoms of an underactive thyroid.
A condition called Hashimoto's disease is the most common type of autoimmune reaction that causes an underactive thyroid.
It is not clear what causes Hashimoto’s disease, but the condition runs in families. It is also common in people with another disorder related to the immune system, such as type 1 diabetes and vitiligo.
Previous thyroid treatment
An underactive thyroid can also occur as a side effect or complication of previous treatment to the thyroid gland, such as surgery or a treatment called radioactive iodine therapy.
These treatments are sometimes used for overactive thyroid (where the thyroid gland produces too much hormone) or thyroid cancer.
Less common causes
Worldwide, a lack of dietary iodine is a common cause of an underactive thyroid because your body needs iodine to make thyroxine. However, iodine deficiency is uncommon in the UK.
In some cases, babies are born with an underactive thyroid because the gland does not develop properly in the womb. This is called congenital hypothyroidism and it is usually picked up during routine screening soon after birth.
A problem with the pituitary gland could lead to an underactive thyroid. The pituitary gland is located at the base of the brain and regulates the thyroid. Therefore, damage to the pituitary may lead to an underactive thyroid.
An underactive thyroid has also been linked to some viral infections or some medications used to treat other conditions, such as:
lithium - a medication sometimes used to treat certain mental health conditions, including depression and bipolar disorder
amiodarone - a medication sometimes used to treat irregular heartbeats (arrhythmias)
interferons - a class of medication sometimes used to treat certain types of cancer and hepatitis C
Speak to your GP or specialist if you are concerned that a medication you are taking may be affecting your thyroid hormone levels.
Diagnosing underactive thyroid
If you have symptoms of an underactive thyroid (hypothyroidism), see your GP and ask for a blood test.
A blood test measuring your hormone levels is the only accurate way to find out if there is a problem.
The test, called a thyroid function test, looks at levels of thyroid-stimulating hormone (TSH) and thyroxine (T4) in the blood.
A high level of TSH and a low level of T4 in the blood could mean you have an underactive thyroid.
If your test shows raised TSH but normal T4, it means you may be at risk of developing an underactive thyroid in the future.
Your GP may advise a repeat blood test every so often to see if you eventually develop an underactive thyroid.
Blood tests may sometimes be used for other measurements, such as checking the level of a hormone called triiodothyronine (T3). However, this is not routine because levels of T3 can remain normal even if you have a significantly underactive thyroid.
Your GP may refer you to a specialist in hormone disorders, known as an endocrinologist, if you:
are younger than 16
are pregnant or trying to get pregnant
have just given birth
have another health condition, such as heart disease, which may complicate your medication
are taking a medication known to cause a reduction in thyroid hormones, such as amiodarone or lithium
Treating underactive thyroid
An underactive thyroid (hypothyroidism) is usually treated by taking daily hormone-replacement tablets called levothyroxine.
Levothyroxine replaces the thyroxine hormone which your thyroid does not make enough of.
You will initially have regular blood tests until the correct dose of levothyroxine is reached. This can take a little while to get right.
You may start on a low dose of levothyroxine, which may be increased gradually depending on how your body responds. Some people start to feel better soon after beginning treatment, while others don't notice an improvement in their symptoms for several months
Once you are taking the correct dose, you will usually have a blood test once a year to monitor your hormone levels.
If blood tests suggest you may have an underactive thyroid, but you do not have any symptoms or they are very mild, you may not need any treatment. In these cases, your GP will usually monitor your hormone levels every few months and prescribe levothyroxine if you develop symptoms.
If you are prescribed levothyroxine, you should take one tablet at the same time every day.
The effectiveness of the tablets can be altered by other medications, supplements or foods, so they should be swallowed with water on an empty stomach and you should avoid eating for 30 minutes afterwards. Taking the tablets in the morning is usually recommended, although some people prefer to take them at night.
If you forget to take a dose, take it as soon as you remember if this is within a few hours of your usual time. If you do not remember until later than this, skip the dose and take the next dose at the usual time unless advised otherwise by your doctor. Do not take two doses together to make up for a missed dose.
An underactive thyroid is a lifelong condition, so you will usually need to take levothyroxine for the rest of your life.
If you are prescribed levothyroxine because you have an underactive thyroid, you are entitled to a medical exemption certificate. This means you do not have to pay for any of your prescriptions. See help with prescription costs for more information about this.
Levothyroxine does not usually have any side effects as the tablets simply replace a missing hormone.
Side effects usually only occur if you are taking too much levothyroxine. This can cause problems including chest pain, sweating, headaches, diarrhoea and vomiting.
You should tell your doctor if you develop new symptoms while taking levothyroxine. You should also let them know if your symptoms get worse or do not improve.
Triiodothyronine (T3) treatment
In the UK, treatment for underactive thyroid involving a combination of levothyroxine and a hormone called triiodothyronine (T3) is not usually recommended because studies have found no evidence that this is more effective.
There are also concerns that taking both these hormone-replacement medications may increase your risk of serious side effects such as an irregular heartbeat (arrhythmia) and weak bones (osteoporosis).
Levothyroxine should be taken with water on an empty stomach
Underactive thyroid and pregnancy
Tell your GP if you're pregnant or are planning a pregnancy and you have an underactive thyroid. They will refer you to a specialist because there is a risk to your baby if your underactive thyroid is not treated properly during pregnancy.
See complications of an underactive thyroid for more information.
Complications of underactive thyroid
Several complications can occur if you have an underactive thyroid that is not treated.
Having an untreated underactive thyroid can increase your risk of developing cardiovascular disease (disease of the heart or blood vessels).
This is because having low levels of the hormone thyroxine can lead to increased levels of cholesterol in your blood. Having high levels of cholesterol can cause fatty deposits to build up in your arteries, restricting the flow of blood.
You should call your doctor if you are being treated for an underactive thyroid and you develop chest pain, so that any problems can be detected and treated if necessary.
A goitre is an abnormal swelling of the thyroid gland that causes a lump to form in the throat. They can develop in people with an underactive thyroid when the body tries to stimulate the thyroid to produce more thyroid hormones.
If underactive thyroid is not treated during pregnancy, there is a risk of problems including:
pre-eclampsia - which can cause high blood pressure and fluid retention in the mother, and growth problems in the baby
anaemia in the mother
an underactive thyroid in the baby
bleeding after birth
problems with the baby's physical and mental development
premature birth or a low birthweight
stillbirth or miscarriage
These problems can usually be avoided with proper treatment by a specialist, so you should tell your GP if you have an underactive thyroid and you are pregnant or planning a pregnancy.
In very rare cases, a severe underactive thyroid may lead to a life-threatening condition known as myxoedema coma. This is where the thyroid hormone levels become very low, causing symptoms such as confusion, hypothermia, and drowsiness.
Myxoedema coma requires emergency treatment in hospital. It is usually treated with thyroid hormone replacement medication given directly into a vein. In some cases, other treatments such as mechanical breathing support, antibiotics and steroid medication (corticosteroids) are also required.