Published by Bupa's Health Information Team, April 2010.
This factsheet is for people who have an underactive thyroid (hypothyroidism), or who would like information about it.
Hypothyroidism occurs when the thyroid gland doesn't make enough thyroid hormones. This slows down the body's metabolism, leading to symptoms including tiredness and putting on weight.
Your thyroid is an endocrine gland. This means it secretes hormones into your bloodstream. Hormones are chemicals produced by your body to help regulate how your cells and organs work. They are sometimes called chemical messengers.
Your thyroid gland is in your neck, in front of your windpipe. It's about 2cm wide and 4cm high.
The position of the thyroid gland and surrounding structures
Your thyroid gland makes two hormones: thyroxine (also called T4) and triiodothyronine (also called T3). Together, these hormones regulate your body's growth and metabolism. Your metabolism is how quickly your body burns energy and how quickly reactions in your body happen. It affects lots of things, including how much you weigh and how much you sleep.
The production of thyroid hormones is controlled by another hormone called thyroid stimulating hormone (TSH). This is made by an endocrine gland in your brain called the pituitary gland.
Hypothyroidism is when your thyroid is underactive and doesn't produce enough thyroid hormones. Not having enough thyroid hormones slows down your body's metabolism.
Hypothyroidism usually develops gradually. The symptoms are mild, and you may not even notice them at first. This is called subclinical hypothyroidism.
Hypothyroidism causes a general slowing down of your body's functions. Some of the symptoms include:
You may also have swelling of your thyroid gland. This is called a goitre.
If you have some or all of these symptoms, see your GP.
Very rarely, untreated hypothyroidism can lead to coma - called myxoedema coma.
Some people are more likely to develop hypothyroidism than others. Hypothyroidism is more common in older people, and women are more likely to be affected than men.
Specific causes of hypothyroidism are described below.
Autoimmune thyroiditis is the most common cause of hypothyroidism in the UK. It's an autoimmune disease. This means that it's caused by antibodies from your immune system attacking your body. Rather than attacking foreign bodies such as bacteria or viruses, antibodies destroy your thyroid gland cells. This prevents your thyroid gland from producing enough thyroid hormones.
Hashimoto's thyroiditis is the most common type of autoimmune hypothyroidism. It can make your thyroid gland swell up. This may be visible as a lump on your neck (goitre). If you don't have a goitre, the condition may be called atrophic thyroiditis or primary myxoedema.
Around one newborn in 4,000 is born with congenital hypothyroidism. This means the thyroid gland fails to develop or it doesn't produce enough thyroid hormones. Babies in the UK are screened for this before they are 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 your hypothalamus (the part of your brain that links your nervous system to your endocrine system) and pituitary gland can also lead to hypothyroidism. This is because these parts of your brain are involved in the regulation and production of thyroid hormones.
Your body needs a substance called iodine to make thyroid hormones. Not having enough iodine in the diet is the leading cause of hypothyroidism worldwide, although it's a rare cause in the UK. Some countries now add iodine to salt during the manufacturing process, but this doesn't currently happen in the UK.
Some treatments for hyperthyroidism (an overactive thyroid gland), such as surgery, anti-thyroid drugs and radioiodide therapy can cause hypothyroidism.
Some medicines can affect the normal functioning of your thyroid gland and cause hypothyroidism. These include lithium carbonate (for bipolar disorder) and amiodarone (for heart rhythm problems).
Your GP will ask about your symptoms, examine you and you may need to have a blood test.
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 your blood.
Occasionally, hypothyroidism gets better without treatment. In general, however, the symptoms get progressively worse if it isn't treated.
Hypothyroidism can be treated with a thyroxine replacement medicine, called levothyroxine. This is taken as tablets and it can take some time to get the dose right. Usually you will start on a low dose that is gradually built up every few months. Your GP will adjust your dose according to the results of your blood tests.
Once the correct dose of levothyroxine has been established, you will usually have a thyroid function test every year to check the levels of your thyroid hormones.
You will usually feel much better once you're taking levothyroxine. Side-effects are unusual because a missing hormone is simply being replaced. However, if you take too much, you may develop symptoms of 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 GP will usually monitor how your thyroid hormone levels change every few months. If you develop symptoms, your GP will suggest that you start treatment.
Your thyroid gland can change during pregnancy. In the first half of pregnancy it's normal for your total amount of thyroid hormone to be slightly higher than usual. If you have hypothyroidism you will need more frequent check ups during and after pregnancy, as your thyroxine requirements tend to be higher as this time.
For answers to frequently asked questions on this topic, see FAQs.
For sources and links to further information, see Resources.
See a Private GP in confidence to discuss any concerns you may have about your health or your family's health or call 0845 600 3458 quoting ref. HFS GP .
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This information was published by Bupa's Health Information Team and is based on reputable sources of medical evidence. It has been reviewed by appropriate medical or clinical professionals. Photos are only for illustrative purposes and do not reflect every presentation of a condition. The content is intended only for general information and does not replace the need for personal advice from a qualified health professional. For more details on how we produce our content and its sources, visit the About our Health Information page.
Publication date: April 2010
Updated November 2011
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