What is an overactive thyroid (hyperthyroidism)?

Thyroid Health

Everything you need to know about hyperthyroidism


Emily Condon
BSc (Hons) Biological Sciences

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What is hyperthyroidism?

Hyperthyroidism, or an overactive thyroid, occurs when the thyroid makes too much of the hormones, thyroxine (T4) and triiodothyronine (T3). An excess of thyroid hormones speeds up metabolism in the body.

What can cause hyperthyroidism?

Many things can lead to hyperthyroidism, with Grave’s disease which is an autoimmune disease being the most common. In Grave’s disease, the body’s immune system creates antibodies that target the thyroid and increase the production of both T3 and T4. These antibodies are called thyroid-stimulating immunoglobulins (TSIs). TSIs mimic the role of the thyroid stimulating hormone (TSH) which instructs the thyroid to produce hormones. Grave’s disease can run in families, so having a close family member with Grave’s disease increases an individual's risk of development. Like many autoimmune diseases, women are at a greater risk of developing Grave’s disease compared to men. 

Other possible causes of hyperthyroidism include:

  • Thyroiditis which is the inflammation of the thyroid gland. Caused by either a problem with the immune system or a viral infection, inflammation causes thyroid hormones that are stored in the gland to leak into the bloodstream. 
  • Certain medications including amiodarone, lithium, interferon alpha and interleukin-2 can affect the production of thyroid hormones. 
  • A toxic nodular goitre (TNG) is an enlarged thyroid with independently functioning thyroid nodules, capable of increasing the levels of thyroid hormones produced. TNG is often caused by low iodine levels in the body. The thyroid requires iodine to produce hormones. 
  • Low iodine levels encourage the thyroid to grow in size, absorb as much iodine as possible and create an excess amount of thyroid hormones. Excessively high levels of iodine can also result in hyperthyroidism. 

What are the symptoms of hyperthyroidism?

Hyperthyroidism causes an increase in metabolic rate which causes symptoms such as:

  • Fatigue
  • Unexplained weight loss
  • Feeling anxious
  • Rapid heartbeat and heart palpitations
  • Difficulty sleeping 
  • Sensitivity to heat
  • Itchy red skin
  • Fine hair and hair loss
  • An enlarged thyroid gland 

How is hyperthyroidism diagnosed and treated?

Often hyperthyroidism symptoms mimic the symptoms of many other conditions, because of this, hyperthyroidism is often misdiagnosed. A blood test to measure thyroid hormone and antibody levels is the best way to diagnose hyperthyroidism.

There are many ways to treat an overactive thyroid:

  • Medications such as thioamides, which are a group of drugs used to treat hyperthyroidism, work by reducing the levels of thyroid hormones produced. Methimazole and propylthiouracil which belong to this drug class, are the most commonly used. 
  • Radioiodine treatment can be used to treat hyperthyroidism. Radioactive iodine is taken in by the thyroid and destroys the thyroid cells, which ultimately reduces the levels of hormones produced and decreases the size of the enlarged gland.
  • Surgery can also be used to treat hyperthyroidism and involves either removing a small section of the thyroid or the whole gland. After surgery, an individual may be required to take synthetic thyroid hormones to ensure they do not develop hypothyroidism. If the entire thyroid is removed, then lifelong thyroid hormone replacement therapy is needed to maintain sufficient hormone levels in the body. 

How are thyroid blood tests affected by hyperthyroidism?

In the most common forms of hyperthyroidism, it is the thyroid gland which is responsible for producing too much T3 and thyroxine. In these conditions, the pituitary gland is functioning normally so TSH production will be reduced. This gives the result below:

TSH: Low
T3: High or high normal
T4: High

There are rare forms of hyperthyroidism in which the pituitary gland malfunctions and produces too much TSH. So although the thyroid gland is healthy, it is overstimulated by TSH and produces too much T3 and t4. This gives the result below:

TSH: High
T3: High
T4: High

If you think that you may have hyperthyroidism, it is important to see your doctor promptly as the condition can worsen over days to weeks.

Learn more about thyroid disease:

What is the thyroid?

What is thyroid disease?

What are the risk factors for thyroid disease?

What is hypothyroidism (an underactive thyroid)?

How is thyroid disease diagnosed?

Reverse T3 - what is it and do I need to test for it?

Reverse T3 - results explained

What are the symptoms of thyroid disease?

Thyroid FAQs

Medicheck your thyroid

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