What causes hypothyroidism?

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Autoimmune thyroiditis - the common cause

The most common cause is due to an 'autoimmune disease' called autoimmune thyroiditis. 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 that attach to your own thyroid gland which affect the glands function. 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. Again, an annual blood test to screen for hypothyroidism is usually advised for people with this condition.
An enlarged thyroid gland (diffuse goitre).
A past history of Graves' disease, or thyroiditis following childbirth.
A personal or family history of other autoimmune disorders. For example, vitiligo, pernicious anaemia, Addison's disease, Type 1 diabetes, premature ovarian failure, coeliac disease, Sjogren's syndrome.

Some people with autoimmune thyroiditis also develop a swollen thyroid gland (goitre). Autoimmune thyroiditis with a goitre is called Hashimoto's disease.People with autoimmune thyroiditis have a small increased risk of developing other autoimmune conditions such as vitiligo, pernicious anaemia, etc.