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.

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 may 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.

What causes hypothyroidism?

Autoimmune thyroiditis - the common cause
The most common cause is due to an 'autoimmune disease' called autoimmune thyroiditis (also called 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.

Autoimmune thyroiditis is more common than usual in people with:

• A family history of hypothyroidism caused by autoimmune thyroiditis.
• Down's syndrome. Hypothyroidism 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.
• Turner's syndrome.
• An enlarge thyroid gland (diffuse goitre).
• A past history of Grave's disease, of de Quervains thyroiditis, or thyroiditis following childbirth.
• A personal or family history of other autoimmune disorders. For example, vitiligo, pernicious anaemia, etc.
• Primary pulmonary hypertension.
• Multiple sclerosis.

People with autoimmune thyroiditis have a small increased risk of developing other autoimmune conditions such as vitiligo, pernicious anaemia, Addison's disease, Type 1 diabetes, premature ovarian failure, coeliac disease, Sjorgren's syndrome.

Surgery or radioactive treatment to the thyroid gland
This has become a common cause of hypothyroidism in the UK due to increasing use of these treatments for other thyroid conditions.

Other causes
Other causes of hypothyroidism are less common an include:

• 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.)
• Some children are born with an underactive thyroid gland (congenital hypothyroidism).

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.

How is hypothyroidism treated?

The treatment is to take levothyroxine (thyroxine) tablets each day. This replaces the thyroxine which your thyroid gland is not making. Most people feel much better soon after starting treatment. Ideally, take the tablet on an empty stomach (before breakfast). This is because some foods rich in calcium or iron may interfere with the absorption of levothyroxine from the gut. (For the same reason, don't take levothyroxine tablets at the same time of day as calcium or iron tablets.)

What is the dose of levothyroxine?
Most adults need between 50 and 200 micrograms daily. A low dose is prescribed at first and is gradually increased over a period of time. Blood tests are usually taken every 2-3 months, and the dose may be adjusted accordingly. The blood test measures TSH (see above). Once the blood TSH level is normal it means you are getting the correct amount of levothyroxine from the tablets. It is then common practice to check the TSH blood level once a year. As you get into your 70s and 80s, you may need a reduced dose of levothyroxine.

Missed a tablet?
Everyone forgets to take their tablets from time to time. Don't worry as it is not dangerous to miss the odd forgotten levothyroxine tablet. However, you should try to take thyroxine regularly each morning for maximum benefit.

How long is the treatment for?

For most people, treatment is for life. Occasionally, the disease process reverses. This is rare apart from the following:

• Children. Sometimes hypothyroidism is a temporary condition in older children. (This is not so for children who are born with an underactive thyroid.)
• Pregnancy. Some women develop thyroid imbalance after having a baby. If it occurs, it typically happens about two months after the birth. Often this lasts just a few weeks and corrects itself. Treatment is needed only in a small number of cases. However, afterwards it is wise to have a yearly blood test as there is an increased risk of developing autoimmune thyroiditis and long-term hypothyroidism in the future.

Are there any side-effects or problems from treatment?

Usually not. Levothyroxine tablets replace the body's natural hormone, so side-effects are uncommon. However, if you have angina, you may find that your angina pains become worse when you first start levothyroxine. Tell a doctor if this happens.

If you take too much levothyroxine it can lead to symptoms of an over-active thyroid, such as palpitations, diarrhoea, irritability, or flushing. This is why the initial treatment dose is small and gradually built up, and why you need blood tests to check that you are taking the correct dose.

Other medicines may interfere with the action of levothyroxine. For example, carbamazepine, phenytoin, and rifampicin. If you start any of these medicines, or change the dose, then you may need to alter the dose of the levothyroxine. Your doctor will advise. Also, if you take warfarin, the dose may need to be altered if you have a change in your dose of levothyroxine.

Free prescriptions

If you have hypothyroidism, you are entitled to free prescriptions. This is for all your medicines, whether related to the hypothyroidism or not. Ask your pharmacist for details and for a form to fill in to claim this benefit.

In summary

• Hypothyroidism is common.
• Symptoms develop gradually. They may be confused with other conditions.
• Treatment with levothyroxine tablets is usually easy and effective.
• Treatment is usually for life.
• Have a blood test once a year if you take levothyroxine tablets.