Thyroxine is a body chemical (hormone) made by the thyroid gland in the neck. It is carried round the body in the bloodstream. It helps to keep the body's functions (the metabolism) working at the correct pace. Many cells and tissues in the body need thyroxine to keep them going correctly.

Hypothyroidism is also known as an underactive thyroid. It occurs when the thyroid gland does not make enough thyroxine. This causes many of the body's functions to slow down. (In contrast, if you have hyperthyroidism, you make too much thyroxine. This causes many of the body's functions to speed up.)

What are the symptoms of hypothyroidism?

Many symptoms can be caused by a low level of thyroxine. Basically, everything 'slows down'. Not all symptoms develop in all cases.

• Symptoms that commonly occur include: tiredness, weight gain, constipation, aches and pains, feeling cold, dry skin, lifeless hair, fluid retention, mental slowing, and depression.
• Less common symptoms include: a hoarse voice, irregular or heavy menstrual periods in women, infertility, carpal tunnel syndrome (which causes pains and numbness in the hand), and memory loss or confusion in the elderly.
• If you have angina, you may find that your angina pains become more frequent.

However, all these symptoms can be caused by other conditions, and sometimes the diagnosis is not obvious. Symptoms usually develop slowly, and gradually become worse over months or years as the level of thyroxine in the body gradually falls.

Possible complications
If you have untreated hypothyroidism:

• You have an increased risk of developing heart disease. This is because a low thyroid level causes the blood lipids (cholesterol etc) to rise.
• If you are pregnant, you have an increased risk of developing some pregnancy complications. For example: pre-eclampsia, anaemia, premature labour, low birth weight, stillbirth, and serious bleeding after the birth.
• Hypothyroid coma (myxoedema coma) is a very rare complication.

With treatment, the outlook is excellent. Symptoms go, as do the risk of complications.

Who gets hypothyroidism?

About 1 in 50 women, and about 1 in 1000 men develop hypothyroidism at some time in their life. It most commonly develops in adult women, and becomes more common with increasing age. However, it can occur at any age and can affect anyone.

Hypothyroidism is more common in

• Women who have recently given birth.
• Women with a family history of hypothyroidism caused by autoimmune thyroiditis (see below).
• People with Down's syndrome. It develops in 1 in 3 people with Down's syndrome before the age of 25 years. Symptoms of hypothyroidism may be missed more easily in people with Down's syndrome. Therefore, some doctors recommend that all people with Down's syndrome should have an annual blood test to screen for hypothyroidism.
• People with Turner's syndrome.
• People who have had previous thyroid surgery.
• People who have had previous radiotherapy to the head or neck for any reason.

What causes hypothyroidism?

The common cause - autoimmune thyroiditis
The most common cause is due to an 'autoimmune disease' called autoimmune thyroiditis (Hashimoto's disease). The immune system normally makes antibodies to attack bacteria, viruses, and other 'germs'. If you have an autoimmune disease, the immune system makes antibodies against certain tissues of your body. With autoimmune thyroiditis, you make antibodies which attack your own thyroid gland. The thyroid gland is then not able to make enough thyroxine, and hypothyroidism gradually develops.

It is thought that something triggers the immune system to make antibodies against the thyroid. The 'trigger' is not known. People with autoimmune thyroiditis have a small increased risk of developing other autoimmune conditions including: pernicious anaemia, type 1 diabetes, or Addison's disease. Autoimmune thyroiditis also tends to run in some families. If you have this condition, tell your children, particularly daughters, to look out for the symptoms in their adult lives. Not all your children will develop the hypothyroidism, but the risk is higher than average.

Other causes
Other causes are uncommon and include:

• Surgery or radioactive treatment to the thyroid gland for other conditions.
• A side-effect to some medicines. (For example, amiodarone and lithium.)
• Other types of 'thyroiditis' (thyroid inflammation) caused by infection or other rare conditions.
• A pituitary gland problem is a rare cause. (The pituitary gland in the brain makes a hormone called thyroid stimulating hormone (TSH). This stimulates the thyroid gland to make thyroxine. If the pituitary does not make TSH, then the thyroid cannot make enough thyroxine.) 医学 全在.线提供www.med126.com
• Some children are born with an underactive thyroid gland (congenital hypothyroidism).
• Radiotherapy to the neck region for other conditions.

How is hypothyroidism diagnosed?

A blood test can diagnose hypothyroidism. A normal blood test will also rule it out if symptoms suggest that it may be a possible diagnosis. One or both of the following may be measured:

• Thyroid-stimulating hormone (TSH). This hormone is made in the pituitary gland in the brain. It is released into the bloodstream. It stimulates the thyroid gland to make thyroxine. If the level of thyroxine in the blood is low, then the pituitary releases more TSH to try and stimulate the thyroid gland even more. Therefore, a raised level of TSH means the thyroid gland is underactive and is not making enough thyroxine.
• Thyroxine (T4). A low level of T4 confirms hypothyroidism.

Note: Some people have a raised TSH but have a normal T4. This means that you are making enough thyroxine but the thyroid gland is needing extra 'stimulation' from TSH to make the required amount of thyroxine. In this situation you have an increased risk of developing hypothyroidism in the future. Your doctor may advise a repeat blood test every so often to see if you do eventually develop hypothyroidism.

Other tests are not usually necessary unless a rare cause of hypothyroidism is suspected. For example, tests of the pituitary gland may be done if both the TSH and T4 are low.

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