Underactive Thyroid – A Guide

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A Guide to Hypothyroidism which means a low level of thyroid hormone (called thyroxine, or T4) in the blood. The level of thyroxine is low because the thyroid gland does not make enough of it. This is why the condition is also known as underactive thyroid.

Symptoms of underactive thyroid
Causes of underactive thyroid
Diagnosing underactive thyroid
Treating underactive thyroid
Complications of underactive thyroid


The role of the 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 to turn food into energy to keep the body going.

Normally, the levels of thyroxine and triiodothyronine in the blood are carefully controlled so that these processes happen at a stable rate.

However, when the thyroid gland does not produce enough thyroxine, the body’s metabolism slows down.

This may cause symptoms such as:

  • weight gain,
  • muscle aches, and
  • tiredness (See Symptoms for a full list).

Hypothyroidism can be classed as either:

  • subclinical, when the levels of thyroid hormone are still within normal limits and symptoms are mild or not even noticeable, or
  • overt, where you have symptoms.

Why does it happen?

The most common cause of hypothyroidism is an autoimmune reaction, which means your body’s immune system attacks the thyroid gland, causing inflammation (thyroiditis). For more information, see Causes.

Hypothyroidism can also be a side effect of medical treatment for hyperthyroidism (an overactive thyroid) or for thyroid cancer.

These two types of hypothyroidism account for well over 90% of all cases.

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Who is affected

Hypothyroidism can occur in anyone, but is most common in women and occurs more often with increasing age.

It affects 15 in every 1,000 women and one in 1,000 men.

Congenital hypothyroidism (where it is present at birth) occurs in about one in 4,500 live births in the UK.


Hypothyroidism is usually not serious, and taking thyroxine tablets will restore the lack of thyroxine in your blood and cure your symptoms. However, you should work with your doctor to fully control any issues.

Symptoms of underactive thyroid

Symptoms of hypothyroidism tend to come on slowly, and you may not notice themfor several years.

Possible early symptoms

  • being sensitive to cold,
  • constipation,
  • depression,
  • tiredness,
  • slowness in body and mind,
  • muscle aches andweakness,
  • muscle cramps,
  • dry and scaly skin,
  • brittle hair and nails,
  • heavy or irregular periods, and
  • weight gain.

The elderly may develop memory problems and/or depression. Children may have restricted growth.

Late symptoms (if left untreated)

It is unlikely that you would have many of the later symptoms of hypothyroidism as the condition is often diagnosed and treated at an early stage.

Later symptoms 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, and
  • anaemia.

Causes of underactive thyroid

Hypothyroidism occurs when your thyroid gland does not produce enough of the hormone thyroxine (T4).

Autoimmune reaction

The most common cause of hypothyroidism is an autoimmune reaction, which means your body’s immune system makes antibodies that attack the cells of the thyroid gland, causing inflammation (swelling).

This inflammation, or thyroiditis, can result in a damaged thyroid gland that is not able to make enough thyroxine.

Hashimoto’s thyroiditis is an example of autoimmune hypothyroidism. It is associated with goitre (a swollen thyroid gland) and runs in families.

Autoimmune hypothyroidism is more common in people who already have another autoimmune disorder, such as type 1 diabetes or vitiligo.

Treatment for hyperthyroidism

The second most common cause of hypothyroidism is treatment for hyperthyroidism (an overactive thyroid) with medication, radioactive iodine or surgery. Surgery for hyperthyroidism completely removes the thyroid, so people having this operation will always need thyroxine.

Drug treatment for hyperthyroidism (carbimazole or propylthiouracil) can cause your thyroid gland to become underactive, so you will need to either take a reduced dose of this or take thyroxine at the same time.

Other rare causes

  • Lack of iodine in the diet. Severe iodine deficiency may lead to hypothyroidism, because your body needs iodine to make thyroxine. However, severe iodine deficiency is virtually unknown in the UK.
  • Reaction of the thyroid to viruses or drugs. Inflammation (swelling) of the thyroid gland may also occur in response to a viral infection or to drugs such as amiodarone or lithium.
  • Abnormal thyroid development. A baby may be born with hypothyroidism if their thyroid gland does not develop properly in the womb (called congenital hypothyroidism). However, congenital hypothyroidism is usually picked up during neonatal screening.
  • A problem with the pituitary gland. The pituitary gland regulates the thyroid, so damage to the pituitary may lead to hypothyroidism.

Pregnancy – It is normal for your body to need additional thyroxine during pregnancy, so it is important to have your thyroid level checked before trying to get pregnant. If you are pregnant and have hypothyroidism, you will need to see a specialist. You are more likely to need additional thyroxine and more frequent check-ups to monitor your hormone levels.

Normal control of thyroid hormones

  • When levels of thyroid hormones fall, the pituitary gland (at the base of the brain) releases thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to produce more thyroid hormones.
  • When levels of thyroid hormones are too high, the pituitary gland stops producing TSH and levels of thyroid hormones fall back to normal levels.

Diagnosing underactive thyroid

Hypothyroidism is straightforward for doctors to detect, so many cases are diagnosed and treated at an early stage.

A diagnosis of hypothyroidism will be based on your symptoms and the results of a blood test, which can assess how well your thyroid gland is working. This blood test is known as a thyroid function test.

Thyroid function test

A thyroid function test assesses your blood in two different ways to confirm a diagnosis of hypothyroidism. Your GP will take a sample of your blood and test it for levels of:

  • Thyroid-stimulating hormone (TSH). A raised level of this indicates an underactive thyroid.
  • Thyroxine (T4). A low level of this confirms hypothyroidism.

Also, if particular antibodies are detected in the blood, this usually indicates that you have autoimmune hypothyroidism. For more information, see Causes.

If your thyroid function test confirms a diagnosis of hypothyroidism, your GP should be able to tell you whether you have overt (fully developed) hypothyroidism, or subclinical hypothyroidism (where your symptoms are mild, or absent).

If you have subclinical hypothyroidism, your blood may have high levels of TSH, but normal levels of thyroxine.

For more information on testing, go to Lab Tests Online: thyroid function tests.


Your GP may refer you to an endocrinologist (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), or
  • are taking amiodarone or lithium medication.

Treating underactive thyroid

Hypothyroidism is treated with tablets called levothyroxine (often referred to as thyroxine).

The aim is to cure the symptoms of hypothyroidism by replacing the lack of thyroid hormones in your blood.

If you are diagnosed with subclinical hypothyroidism (where there are mild or no symptoms), you may not need any treatment for your condition. You may need to discuss the need for treatment with a specialist. Your GP will monitor your level of thyroid hormones every few months and will only give you treatment if it starts to fall below the normal level.

Judging the correct dose

A blood test to measure levels of thyroid-stimulating hormone (TSH) will help to establish the correct dose of thyroxine for you. High TSH levels indicate a low level of thyroid hormone.

If there are other complicating issues, your GP or specialist may start you on a low dose and increase this gradually.

Your dose will be adjusted according to blood test results, which will be taken regularly until the correct dose has been reached (this may take a little while to get right).

Once you are on the correct dose, you will usually have a thyroid function test every year to monitor your hormone levels.

Complications of underactive thyroid

There are several possible complications that can occur if you have hypothyroidism. These are outlined below and are most likely to occur if the condition is left untreated.


In very rare cases, severe hypothyroidism may lead to myxedema coma (loss of brain function). Symptoms include:

  • low body temperature,
  • shallow breathing,
  • low blood pressure,
  • low blood sugar, and
  • unresponsiveness.

Other problems

Hypothyroidism is an autoimmune disease, which means your body’s immune system attacks its own cells. As a result, you are at risk of developing other immune conditions, although this is unusual. These conditions include:

  • B12 deficiency,
  • vitiligo (patches of skin pigment loss),
  • kidney failure, and
  • premature failure of the ovaries or early menopause.