An underactive thyroid gland (hypothyroidism) is where your thyroid gland doesn't produce enough hormones.
Common signs of an underactive thyroid are tiredness, weight gain and feeling depressed.
An underactive thyroid can often be successfully treated by taking daily hormone tablets to replace the hormones your thyroid isn't making.
There's no way of preventing an underactive thyroid. Most cases are caused either by the immune system attacking the thyroid gland and damaging it, or by damage to the thyroid that occurs during some treatments for an overactive thyroid or thyroid cancer.
Read more about the causes of an underactive thyroid.
When to see your doctor
Symptoms of an underactive thyroid are often similar to those of other conditions, and they usually develop slowly, so you may not notice them for years.
You should see your doctor and ask to be tested for an underactive thyroid if you have symptoms including:
- weight gain
- being sensitive to the cold
- dry skin and hair
- muscle aches
The only accurate way of finding out whether you have a thyroid problem is to have a thyroid function test , where a sample of blood is tested to measure your hormone levels.
Read more about testing for an underactive thyroid.
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. Children can also develop an underactive thyroid.
Around 1 in 3,500-4,000 babies are born with an underactive thyroid (congenital hypothyroidism). All babies born in the UK are screened for congenital hypothyroidism using a blood spot 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'll usually need treatment for the rest of your life. However, with proper treatment, you should be able to lead a normal, healthy life.
If an underactive thyroid isn't treated, it can lead to complications, including heart disease, goitre, pregnancy problems and a life-threatening condition called myxoedema coma (although this is very rare).
Symptoms of an underactive thyroid
Many symptoms of an underactive thyroid (hypothyroidism) are the same as those of other conditions, so it can easily be confused for something else.
Symptoms usually develop slowly and you may not realise you have a medical problem for several years.
Common symptoms include:
- being sensitive to cold
- weight gain
- slow movements and thoughts
- muscle aches and weakness
- muscle cramps
- dry and scaly skin
- brittle hair and nails
- loss of libido (sex drive)
- pain, numbness and a tingling sensation in the hand and fingers (carpal tunnel syndrome)
- irregular periods 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 doctor and ask to be tested for an underactive thyroid.
Read more about getting tested for an underactive thyroid.
If an underactive thyroid isn't treated
It's unlikely that you'd have many of the later symptoms of an underactive thyroid, because the condition is often identified before more serious symptoms appear.
Later symptoms of an underactive thyroid include:
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 an underactive thyroid are caused by the immune system attacking the thyroid gland and damaging it, or by damage that occurs as a result of treatments for thyroid cancer or an overactive thyroid.
An underactive thyroid often occurs when the immune system, which usually fights infection, attacks the thyroid gland. This damages the thyroid, which means it's not able to make enough of the hormone thyroxine, leading 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.
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 an 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 the body needs iodine to make thyroxine. However, iodine deficiency is uncommon in the UK.
Babies are sometimes born with an underactive thyroid because the thyroid gland doesn't develop properly in the womb. This is called congenital hypothyroidism and is uncommon, affecting around 1 in 3,000 babies. It's usually picked up during routine screening soon after birth.
The Great Ormond Street Hospital for Children website has more information about congenital hypothyroidism and how it's treated.
A problem with the pituitary gland could lead to an underactive thyroid. The pituitary gland sits at the base of the brain and regulates the thyroid. Therefore, damage to the pituitary gland 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 doctor or specialist if you're concerned that a medication you're taking may be affecting your thyroid hormone levels.
Diagnosing underactive thyroid
It's very important that an underactive thyroid (hypothyroidism) is diagnosed as soon as possible.
Low levels of thyroid-producing hormones, such as triiodothyronine (T3) and thyroxine (T4), can change the way the body processes fat.
Thyroid function test
A blood test measuring your hormone levels is the only accurate way to find out whether there's 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 results show raised TSH but normal T4, you may be at risk of developing an underactive thyroid in the future.
Your doctor may recommend that you have a repeat blood test every so often to see whether you eventually develop an underactive thyroid.
Blood tests are also sometimes used for other measurements, such as checking the level of a hormone called triiodothyronine (T3). However, this isn't routine, because T3 levels can often remain normal, even if you have a significantly underactive thyroid.
The Lab Tests Online UK website has more information about the different types of thyroid function tests.
Your doctor may refer you to an endocrinologist (a specialist in hormone disorders) 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 doesn't make enough of.
You'll 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're taking the correct dose, you'll 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 don't have any symptoms or they're very mild, you may not need any treatment. In these cases, your doctor will usually monitor your hormone levels every few months and prescribe levothyroxine if you develop symptoms.
If you're prescribed levothyroxine, you should take one tablet at the same time every day. It's usually recommended that you take the tablets in the morning, although some people prefer to take them at night.
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.
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 don't remember until later than this, skip the dose and take the next dose at the usual time, unless advised otherwise by your doctor.
An underactive thyroid is a lifelong condition, so you'll usually need to take levothyroxine for the rest of your life.
If you're prescribed levothyroxine because you have an underactive thyroid, you're entitled to a medical exemption certificate. This means you don't have to pay for your prescriptions. See getting help with prescription costs for more information on this.
Levothyroxine doesn't usually have any side effects, because the tablets simply replace a missing hormone.
Tell your doctor if you develop new symptoms while taking levothyroxine. You should also let them know if your symptoms get worse or don't improve.
In the UK, combination therapy – using levothyroxine and triiodothyronine (T3) together – isn't routinely used because there's insufficient evidence to show it’s better than using levothyroxine alone (monotherapy).
In most cases, suppressing thyroid-stimulating hormone (TSH) using high dose thyroid replacement therapy should be avoided because it carries a risk of causing adverse side effects, such as atrial fibrillation (an irregular and abnormally fast heart rate), strokes , osteoporosisand fracture.
However, this type of treatment may sometimes be recommended in cases where a person has a history of thyroid cancer and there’s a significant risk of it reoccurring.
Complications of underactive thyroid
Several complications can occur if you have an underactive thyroid that isn't treated.
If you have an untreated underactive thyroid, your risk of developing cardiovascular disease is increased.
This is because having low levels of the hormone thyroxine can lead to increased levels of cholesterol in your blood. High cholesterol can cause fatty deposits to build up in your arteries, restricting the flow of blood.
Contact your doctor if you're 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. Goitres can develop in people with an underactive thyroid when the body tries to stimulate the thyroid to produce more thyroid hormones.
If an underactive thyroid isn't treated during pregnancy, there's a risk of problems occurring. These include:
- 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
- birth defects
- 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 treatment under the guidance of a specialist in hormone disorders (an endocrinologist). Therefore, tell your doctor if you have an underactive thyroid and you're pregnant or trying to get pregnant.
In very rare cases, a severe underactive thyroid may lead to a life-threatening condition called 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's usually treated with thyroid hormone replacement medication given directly into a vein. In some cases, other treatments such as breathing support, antibiotics and steroid medication (corticosteroids) are also required.