Underproduction of thyroid hormones, causing many body functions to slow down
- May be present from birth but most common over the age of 40
- More common in females
- Sometimes runs in families
- Dietary iodine deficiency is a risk factor in developing countries
In hypothyroidism, the thyroid gland does not produce enough thyroid hormones. These hormones are important in metabolism (the chemical reactions constantly occurring in the body). A deficiency causes many of the body’s functions to slow down. The condition may be present from birth, in which case treatment is begun before symptoms appear, but it is more common in people over the age of 40, particularly women.
What are the causes?
Hypothyroidism that is present from birth has no known cause. In adults, a common cause of hypothyroidism is thyroiditis. The most common type of thyroiditis that leads to hypothyroidism is an autoimmune disorder known as Hashimoto’s thyroiditis, which tends to run in families. In Hashimoto’s thyroiditis, the body produces antibodies that attack the thyroid gland, damaging it permanently. Other forms of thyroiditis may lead to temporary or permanent hypothyroidism. Treatments for an overactive thyroid gland that involve radioactive iodine or surgery (see Hyperthyroidism) can lead to permanent hypothyroidism.
Rarely, hypothyroidism is due to an underproduction of thyroid-stimulating hormone (TSH) by the pituitary gland, often as the result of a pituitary tumour. TSH stimulates the thyroid gland to secrete its own hormones.
Insufficient dietary iodine, essential for the production of thyroid hormones, can also cause hypothyroidism.
What are the symptoms?
The symptoms of hypothyroidism vary in severity and usually develop slowly over months or years. They may include:
Hoarseness of the voice.
Intolerance to cold.
Swelling of the face, puffy eyes, and dry, thickened skin.
Generalized hair thinning.
In women, heavy menstrual periods.
Some people with hypothyroidism develop a swelling in the neck due to an enlarged thyroid (see Goitre).
What might be done?
Blood tests to measure the levels of thyroid hormones and antibodies that act against the thyroid gland are used to diagnose hypothyroidism. In the UK, all newborns are tested for hypothyroidism shortly after birth (see Blood spot screening tests).
Babies with an underactive thyroid are treated promptly with synthetic thyroid hormones, which need to be taken for life (see Drugs for hypothyroidism). Lifelong hormone treatment may also be needed if permanent hypothyroidism is diagnosed in adulthood, in which case symptoms should improve about 3 weeks after starting treatment. Hormone treatment must be monitored regularly to ensure that the correct dose is maintained. If hypothyroidism is the result of a pituitary disorder, this will need to be investigated and treated.
Temporary hypothyroidism does not usually need to be treated. Supplements can be taken to correct iodine deficiency.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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