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Underactive thyroid (hypothyroidism)

Hypothyroidism occurs when your thyroid gland doesn't make enough thyroid hormones. This slows down your body's metabolism, leading to symptoms such as tiredness and putting on weight.

In the UK, about 19 in every 1,000 women and 1 in 1,000 men will develop hypothyroidism at some point in their lives. Once diagnosed, treatment is usually straightforward.

  • About the thyroid gland
  • Symptoms
  • Causes
  • Diagnosis
  • Treatment
  • Special considerations
  • Further information
  • Sources
  • Related topics

About the thyroid gland

The thyroid gland is an endocrine gland. This means that it secretes hormones into your bloodstream. Hormones are chemicals produced by the body to help regulate how your cells and organs work. They are sometimes called chemical messengers.

The thyroid gland is located in the neck in front of your windpipe. It is about 2cm wide and 4cm high.

What are the thyroid hormones?

The thyroid gland secretes two hormones: thyroxine (also called T4) and triiodothyronine (also called T3). Together, these hormones regulate your body's metabolism. They control how quickly your body burns energy and how quickly reactions in your body happen.

Your rate of metabolism affects lots of things, such as how much you weigh, and how much you sleep. Thyroxine and triiodothyronine speed up the body's metabolism, causing processes in your body to happen faster.

The production of the thyroid hormones is controlled by another hormone called thyroid stimulating hormone (TSH). This is made by an endocrine gland in the brain called the pituitary gland.


Hypothyroidism usually develops gradually. The symptoms are mild, and you may not even notice them at first. This is called subclinical hypothyroidism.

If hypothyroidism develops, it causes a general slowing down of your body's functions. Some of the symptoms include:

  • feeling tired and sleeping a lot
  • feeling the cold easily
  • dry and/or pale skin
  • coarse, thinning hair and brittle nails
  • sore muscles, slow movements and weakness
  • a hoarse or croaky voice
  • a change in facial expression
  • depression
  • problems with memory and concentration
  • weight gain
  • constipation
  • fertility problems and increased risk of miscarriage
  • heavy, irregular or prolonged menstrual periods
  • a slow heart rate

You may also have swelling of the thyroid gland in your neck. This is called a goitre.

Occasionally, hypothyroidism gets better without treatment. In general, however, the symptoms get progressively worse if it isn't treated, and it becomes more and more difficult to function normally.

Overactive and underactive thyroid

The opposite condition to hypothyroidism is hyperthyroidism. This happens when the thyroid is overactive and produces too much thyroid hormones. This speeds up your body's metabolism, leading to symptoms such as weight loss and anxiety.

For more information on hyperthyroidism, please see Related topics.


Some people have a higher chance of developing hypothyroidism than others.

  • Hypothyroidism is more common in older people.
  • Women are more likely to be affected than men.
  • Autoimmune hypothyroidism is more likely if you have another autoimmune disorder such as type 1 diabetes mellitus, vitiligo and Addison's disease.
  • Some medicines can affect the normal functioning of the thyroid gland. These include lithium carbonate (for bipolar disorder) and amiodarone (for heart rhythm problems).

Specific causes of hypothyroidism are described below.

Iodine insufficiency

Your body needs an element called iodine to make thyroid hormones. Iodine deficiency in the diet is the leading cause of hypothyroidism worldwide. However, this isn't common in the UK because iodine is added to salt during the manufacturing process.

Autoimmune thyroiditis

Autoimmune thyroiditis is the most common cause of hypothyroidism in the UK. It is an autoimmune disease. This means that it's caused by antibodies from your immune system attacking your body. Rather than attacking foreign organisms such as bacteria or viruses system, antibodies destroy your thyroid gland cells. This prevents the gland from producing enough thyroid hormones.

Hashimoto's thyroiditis is the most common type of autoimmune hypothyroidism. It can make the thyroid gland swell up. This may be visible as a lump on the neck (goitre). If no goitre is present, the condition may be called atrophic thyroiditis or primary myxoedema.

Treatment for hyperthyroidism

Some treatments for hyperthyroidism, such as surgery, antithyroid drugs, or radioiodide therapy can cause hypothyroidism.

Other causes

Congenital hypothyroidism is a condition babies are born with. It means the thyroid gland fails to develop or it doesn't produce enough thyroid hormones. Babies in the UK are screened for this when they are six to eight days old. A blood sample is taken from a prick on the heel. With treatment, babies who have congenital hypothyroidism develop normally.

Disorders of the hypothalamus (the part of the brain which links the nervous system with the endocrine system) and pituitary gland (a gland in the brain) can also lead to hypothyroidism. This is because they are involved in the regulation and production of thyroid hormones.


Many of the symptoms of hypothyroidism can be caused by other conditions. However, if you have some or all of the symptoms listed above you should visit your GP. He or she will discuss your symptoms with you, perform a physical examination and may request some blood tests.

Hypothyroidism can be diagnosed by monitoring the levels of TSH and thyroid hormones in your blood. A diagnosis of autoimmune hypothyroidism is usually confirmed by the presence of particular antibodies in the blood.

When levels of TSH and thyroid hormones are difficult to interpret, other causes of hypothyroidism may be suspected, such as involvement of the hypothalamus or pituitary gland.


Hypothyroidism can be treated by taking thyroxine in tablet form. It can take some time to get the dose right. Usually you start on a low dose that is gradually built up every few months. Your doctor will adjust your dose according to the results of your blood tests.

You will usually feel much better once you are taking thyroxine. Side-effects are unusual because a missing hormone is simply being replaced. However, if you take too much replacement thyroxine, you may develop symptoms related to an overactive thyroid, such as anxiety and weight loss.

If you have subclinical hypothyroidism, which means you don't have any symptoms but your thyroid hormone levels are disrupted, you may not need any treatment at first. Your doctor will usually monitor how your thyroid hormone levels change every few months. If you develop symptoms, your doctor will begin treatment.

Once the correct dose of thyroxine replacement has been established, you will usually have a thyroid function test every year to check the levels of your thyroid hormones.

Special considerations

The thyroid gland can change during pregnancy. In the first half of pregnancy it is normal for the total amount of thyroid hormone to be slightly increased. If you have hypothyroidism you will need more frequent check ups during and after pregnancy, as your thyroxine requirements tend to be increased as these times.

Further information

British Thyroid Foundation

British Thyroid Association


  • Hypothyroidism Guidance. Clinical Knowledge Summaries. www.cks.library.nhs.uk, accessed 24 February 2007
  • Signs and Symptoms of Hypothyroidism. British Thyroid Foundation. www.btf-thyroid.org, accessed 24 February 2007

Related topics

Overactive thyroid