Page last reviewed: 13/07/2011

An underactive thyroid means your thyroid gland, located in the neck, does not produce enough hormones.

Common signs of an underactive thyroid are tiredness, weight gain and feeling depressed.

Medically known as hypothyroidism, an underactive thyroid is not usually serious. It is easily treated by taking hormone tablets to replace the hormones that your thyroid isn't making.

The thyroid produces a hormone called thyroxine, which controls how much energy your body uses. When the thyroid does not produce enough thyroxine, many of the body's functions slow down.

An underactive thyroid cannot be prevented. Most cases of underactive thyroid are caused either by your immune system attacking your thyroid or a damaged thyroid.

When to see your GP

See your GP and ask to be tested for an underactive thyroid if you have symptoms including:

  • tiredness
  • weight gain
  • depression
  • being sensitive to the cold
  • dry skin and hair
  • muscle aches

Symptoms of an underactive thyroid are often confused for something else, by patients and doctors. Symptoms also usually begin slowly and you may not notice them for several years. The only accurate way to find out if you have a thyroid problem is to get a blood test to measure your hormone levels.

Who can it affect?

Both men and women can have an underactive thyroid. However, it's more common in women It is estimated that it affects 15 in every 1,000 women and 1 in 1,000 men. One in 4,500 babies are born with an underactive thyroid (called congenital hypothyroidism).


Underactive thyroid is usually not serious, and taking hormone-replacement tablets, called levothyroxine, will raise your thyroxine levels. You will usually need treatment for the rest of your life. However, with careful management, you should be able to lead a normal, healthy life.

If it is not treated, an underactive thyroid can lead to complications, including swelling of the thyroid (a condition called goitre), heart disease, mental health problems and infertility.


Page last reviewed: 13/07/2011

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
  • weight gain
  • constipation
  • depression
  • tiredness
  • slowness in body and mind
  • muscle aches and weakness
  • muscle cramps
  • dry and scaly skin
  • brittle hair and nails
  • heavy or irregular 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
  • dull facial expressions and a puffy-looking face
  • thinned or partly missing eyebrows
  • a slow heart rate
  • deafness
  • anaemia


Constipation is when you pass stools less often than usual, or when you are having difficulty going to the toilet because your stools are hard and small.

Page last reviewed: 13/07/2011

An underactive thyroid (hypothyroidism) happens 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.

Immune system

Most cases of underactive thyroid happen when the immune system, which normally fights infection, attacks the thyroid gland. Doctors describe this as an autoimmune reaction. 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.

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, coeliac disease and vitiligo.

Treatment for overactive thyroid

An underactive thyroid can also be a side effect of treatment for an overactive thyroid, a condition where the thyroid gland produces too much hormone.

Treatment for an overactive thyroid, medically known as hyperthyroidism, can involve medication, radiotherapy or surgery, all of which can cause your thyroid to become underactive.

Other rare causes

A lack of iodine in your diet may cause an underactive thyroid. This is because your body needs iodine to make thyroxine. However, severe iodine deficiency is virtually unknown in Ireland.

A viral infection or some drugs used to treat other conditions, such as depression and heart disorders, can cause the thyroid to stop working properly.

A baby may be born with an underactive thyroid if the gland does not develop properly in the womb. However, this is usually picked up during neonatal screening.

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.

If you have a deficiency, it means you are lacking in a particular substance needed by the body.
Pituitary gland
The pituitary gland is a pea-sized gland in the centre of the head, which hangs below the brain and produces hormones.
Thyroid gland
The thyroid gland in the throat makes hormones to help control growth and metabolism (the process that turns the food we eat into energy).

Page last reviewed: 13/07/2011

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 in the blood.

A high level of TSH and a low level of thyroxine hormone in the blood could mean you have an underactive thyroid.

If your test shows raised TSH but normal thyroxine, 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.


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 amiodarone or lithium medication
Sample of blood
During a blood test, a sample of blood is taken from a vein using a needle, so it can be examined in a laboratory.
Thyroid gland
The thyroid gland in the throat makes hormones to help control growth and metabolism (the process that turns the food we eat into energy).

How thyroid hormones work 

  • When thyroxine levels fall, thyroid-stimulating hormone (TSH) is released in the blood to stimulate the production of thyroxine.

  • When thyroxine levels are too high, TSH production drops to allow thyroxine levels to return to normal.

Medical terms

Hypothyroidism: an underactive thyroid.

Thyroxine: a hormone produced by the thyroid, also called T4.

Overt hypothyroidism: where you have clear symptoms of an underactive thyroid.

Subclinical hypothyroidism: where your symptoms are mild or barely noticeable.

Levothyroxine: tablets containing the hormone thyroxine, used to treat an underactive thyroid.

Page last reviewed: 13/07/2011

An underactive thyroid (hypothyroidism) is usually treated by taking hormone-replacement tablets called levothyroxine.

Levothyroxine replaces the thyroxine hormone which your thyroid does not make enough of.

A blood test measuring your levels of thyroid-stimulating hormone (TSH) will indicate whether you are taking an appropriate dose of levothyroxine

If the test detects high levels of TSH, it means you have an underactive thyroid and your doctor may advise you to take levothyroxine.

You may start on a low dose of levothyroxine, which may be increased gradually depending on how your body responds.

You will initially have regular blood tests until the correct dose of levothyroxine is reached. This can take a little while to get right.

If your symptoms are mild

If testing detects high levels of TSH but you do not have any symptoms or they are very mild, you may not need any treatment.

Your GP will usually monitor your hormone levels every few months and may start you on levothyroxine if you develop symptoms.

Some people start to feel better soon after beginning treatment, while others can take several months.

An underactive thyroid is a lifelong condition, so you will probably need to take levothyroxine for the rest of your life.

Levothyroxine does not usually have any side effects as the tablets simply replace a missing hormone.

Once you are taking the correct dose, you will usually have a blood test once a year to monitor your TSH levels.

If you are pregnant or planning to get pregnant

If you are pregnant and have an underactive thyroid, you should see a specialist. You are more likely to need more hormone-replacement tablets and more frequent check-ups to monitor your hormone levels.

When to call your doctor

If you are being treated for underactive thyroid, call your doctor if:

  • you develop chest pain or palpitations (an awareness of your heart beating)
  • your symptoms get worse or do not improve
  • you develop new symptoms

Page last reviewed: 13/07/2011

Several complications can occur if you have an underactive thyroid that is not treated.

An underactive thyroid can be caused by a problem with the immune system, the body's natural defence system, which can attack the body's own cells, including the thyroid.

This disorder may raise your risk of developing other immune conditions, although this is unusual.

These conditions include:

  • B12 deficiency
  • vitiligo (patches on the skin caused by pigment loss)
  • kidney failure
  • premature failure of the ovaries or early menopause
  • goitre
  • heart disease

Coma risk

In very rare cases, a severe underactive thyroid may lead to a life-threatening condition known as myxedema coma.

Warning signs include:

  • low body temperature
  • shallow breathing
  • low blood pressure
  • low blood sugar
  • unresponsiveness

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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