Hyperthyroidism (Overactive Thyroid)

What are the causes of hyperthyroidism?

There are various causes which include the following:

Graves’ disease

This is the most common cause. It can occur at any age, but is most common in women aged 20 to 40 years. It can affect anyone, but there is often a family history of the condition. There may also be family members with other autoimmune diseases (for example, diabetes, rheumatoid arthritis and myasthenia gravis). Graves’ disease is also an autoimmune disease. The immune system normally makes antibodies (tiny proteins that travel in the bloodstream) to attack bacteria, viruses and other germs. In autoimmune diseases, the immune system makes antibodies against tissues of the body. If you have Graves’ disease, you make antibodies that attach to the thyroid gland. These stimulate the thyroid to make lots of thyroxine. It is thought that something triggers the immune system to make these antibodies. The trigger is not known. In Graves’ disease the thyroid gland usually enlarges, which causes a swelling (goitre) in the neck. The eyes are also affected in about half of cases. If they are affected, the eyes are pushed forwards and look more prominent (proptosis). This can cause discomfort and watering of the eyes. Problems with eye muscles may also occur and lead to double vision. It is not clear why eye symptoms occur in some people who have Graves’ disease. They may be due to the antibodies affecting the tissues around the eye.

Thyroid nodules

This is a less common cause of hyperthyroidism. Thyroid nodules are lumps, which can develop in the thyroid gland. It is not clear why they develop. They are usually benign (noncancerous) but contain abnormal thyroid tissue. The abnormal thyroid tissue in the thyroid nodules does not respond to the normal controlling system which ensures that you make just the right amount of thyroxine. Therefore, if you have a thyroid nodule, you may make too much thyroxine.

Sometimes only one nodule forms. This is called a toxic solitary adenoma. This most commonly occurs in people aged between 30 and 50 years.The thyroid may become generally lumpy or nodular. This most commonly occurs in older people, and is called a toxic multinodular goitre. Note: the word toxic above, relating to adenomas or multinodular goitres, does not mean poisonous. It is just one of those medical words which refers to the hyperthyroidism.

Other causes

There are several other rare causes of hyperthyroidism. For example, some people who take amiodarone and lithium develop hyperthyroidism. There are various other rare conditions that result in excess thyroxine being made.

How is hyperthyroidism diagnosed?

A blood test can diagnose hyperthyroidism. 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 in a blood sample:

  • Thyroid-stimulating hormone (TSH). This hormone is made in the pituitary gland in the brain. It is released into the bloodstream. It stimulates the thyroid gland to make thyroxine. If the level of thyroxine in the blood is high, then the pituitary releases less TSH. Therefore, a low level of TSH means that your thyroid gland is overactive and is making too much thyroxine.
  • Thyroxine (T4/ T3). A high level of T4 confirms hyperthyroidism.

Sometimes the results of the tests are borderline. For example, a normal T4 but with a low TSH. Other tests are sometimes done to clarify the situation and the cause. For example, another blood test that measures T3 is sometimes helpful and an ultrasound scan of the thyroid or a thyroid scan may be done if you have a nodular goiter. Also, if tests are borderline, one option is to repeat the tests a few weeks later, as sometimes borderline tests are due to another illness. Other tests may be done if a rare cause of hyperthyroidism is suspected. 

In people with Graves’s disease a blood test may detect specific autoantibodies like TSH-R antibodies and TPO; which are commonly raised. However, TPO can also be raised in some people without Grave’s disease so this is not a specific test for Grave’s disease.

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