What causes hypothyroidism?

Iodine deficiency hypothyroidism

Worldwide, iodine deficiency is the most common cause of hypothyroidism but it is a rare cause in the developed world. (Your body needs iodine to make thyroxine. However, it is rare for a diet in western countries to lack sufficient iodine.) In Nepal the iodine deficiency hypothyroidism is decreasing after the government implemented the iodinized salt.

Autoimmune thyroiditis – the common cause in developed countries

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 gland’s 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, Sjögren’s syndrome.

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

Surgery or radioactive treatment to the thyroid gland

These are common causes 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 and include:

  • A side-effect to some medicines. For example, amiodarone and lithium.
  • Other types of thyroiditis (thyroid inflammation) caused by various rare conditions.
  • A pituitary gland problem is a rare cause. The pituitary gland that lies just under 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. 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 to stimulate the thyroid gland to make more thyroxine. 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 needs 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 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. TPO antibodies are required to confirm is the hypothyroidism was due to Hashimoto thyroiditis.

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