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:
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 of hypothyroidism are less common and include:
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:
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.