What is an overactive thyroid (hyperthyroidism)?

From symptoms to complications, learn everything you need to know about an overactive thyroid.

Men and women can have an overactive thyroid, though it is more common in women between 20 and 40 years old.

From this article, you will learn about the symptoms and treatment for an overactive thyroid and how thyroid blood tests work. 

In this blog, we discuss: 

What is hyperthyroidism?

Hyperthyroidism (also known as an overactive thyroid), is where the thyroid produces too much of the thyroid hormones [1].  

Thyroid hormones, T4 and T3, all play a part in several bodily functions, from metabolism to heat regulation. So, if the body starts producing more of these hormones, you may experience symptoms.  

What causes an overactive thyroid?

An overactive thyroid can occur in both men and women at any age. But, it is more common in women between 20 and 40 years old. 

Causes of hyperthyroidism include: 

  • Graves’ disease – a condition where the immune system attacks and damages the thyroid. 
  • Thyroiditis – inflammation of the thyroid gland. 
  • Incorrect dose of medication – overmedicating with thyroid hormone replacement, can cause hyperthyroidism. 
  • Lumps on the thyroid – extra tissue called nodules that grow on the thyroid can lead to an overproduction of thyroid hormones. 

Symptoms of an overactive thyroid

In some cases, the symptoms of hyperthyroidism are subtle and go unnoticed. In others, they come on very quickly and severely. 

Common symptoms of an overactive thyroid include: 

  • Anxiety and irritability 
  • Mood swings 
  • Tiredness 
  • Heat sensitivity and sweating 
  • Swelling in your neck (goitre) 
  • Heart palpitations and arrhythmia 
  • Weight loss 
  • Increased appetite 
  • Nausea and loose bowels 

How is hyperthyroidism diagnosed?

A hyperthyroidism diagnosis is based on your symptoms and blood test results. Your doctor may also request an ultrasound scan of the thyroid gland to help work out the underlying cause. 

A blood test to measure thyroid hormones is a very accurate method of diagnosing an overactive thyroid. Results show low levels of thyroid-stimulating hormone (TSH) and high levels of thyroxine (T4) in most people with the condition. 

Another blood test may be used if you are diagnosed with an overactive thyroid to measure TSH receptor antibodies. This will indicate if it is likely caused by Graves’ disease [2].  

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How is hyperthyroidism treated?

An overactive thyroid is treatable. The doctor’s approach will depend on the cause and severity of hyperthyroidism. 

Treatments for hyperthyroidism include: 

  1. Beta-blockers – to help relieve some of the symptoms. 
  2. Medication –  used to stop the thyroid from producing high levels of hormones. The tablets should be taken daily and only stopped if advised to do so by a doctor. 
  3. Radioiodine – this is very effective, safe and has minimal side effects. 
  4. Surgery – this may be considered if the medication does not work or if the thyroid gland is swollen and causing symptoms of compression. Removing all or part of the thyroid may also be the preferred option if there is a possibility or suspicion of thyroid cancer. Levothyroxine will need to be taken after surgery [3]. 
  5. A balanced diet – a balanced diet can support thyroid health, but is not a standalone treatment. Hyperthyroidism can cause bones to weaken, so getting enough calcium is important. Calcium is found in foods such as cheese, yoghurt, spinach, soybeans, and nuts (especially almonds). Taking vitamin D and calcium supplements during treatment can strengthen bones [4]. 

Read our blog: what foods are good for thyroid health? for more information. 

What are the health risks associated with hyperthyroidism?

An overactive thyroid can cause complications if it is not treated appropriately or early enough.  

Complications of hyperthyroidism include: 

  • Thyroid eye disease – the immune system can attack the tissues around the eyes and cause irritation, double vision, and bulging eyes. 
  • Heart problems – the heart can develop an irregular rhythm and may struggle to pump efficiently if hyperthyroidism is not treated. 
  • Mental health problems – hyperthyroidism can cause anxiety and hyperactivity. 
  • Weight loss and loss of bone – increased metabolic rate increases the body's energy requirements, which use energy stored in body fat. The calcium stored in bones can also decrease, resulting in osteoporosis. 
  • Pregnancy complications – untreated hyperthyroidism can increase the risk of preeclampsia, premature birth, and miscarriage. 
  • A thyroid storm – the term given to a sudden and life-threatening flare-up of symptoms and usually occurs when hyperthyroidism is undertreated [5]. 

If you are planning a pregnancy and have an overactive thyroid, it is important to check that your hormone levels are normal. As soon as you are pregnant, you should have a thyroid function test. Your doctor will likely adjust your medication and recommend more frequent blood tests. 

Next steps if you have symptoms of an overactive thyroid 

For an assessment of your thyroid health, look at our range of Thyroid Blood Tests or read our  Thyroid Blood Test Buying Guide



  1. nhs.uk. 2022. Overactive thyroid (hyperthyroidism). [online] Available at: <https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/> [Accessed 4 March 2022]. 
  2. British Thyroid Foundation. 2022. Hyperthyroidism. [online] Available at: <https://www.btf-thyroid.org/hyperthyroidism-leaflet> [Accessed 4 March 2022]. 
  3. Healthline. 2022. Hyperthyroidism: Causes, Symptoms, Treatment, Diagnosis & More. [online] Available at: <https://www.healthline.com/health/hyperthyroidism> [Accessed 4 March 2022]. 


What can cause an overactive thyroid (hyperthyroidism)?
There are many causes of hyperthyroidism, with Graves' disease being the most common. In Graves' disease, the immune system produces 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 thyroid-stimulating hormone (TSH) which triggers the thyroid to produce hormones.

Graves' disease often runs in families – having a close family member with the condition increases an individual's risk of developing an overactive thyroid. Like many autoimmune diseases, women are at a greater risk of developing Graves' disease than men.

Other causes of hyperthyroidism include:
  • Thyroiditis - 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) - 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.
  • Low iodine levels - encourage the thyroid to grow, absorb as much iodine as possible and create excess levels of thyroid hormones. Excessively high levels of iodine can also result in hyperthyroidism.
What are the symptoms of hyperthyroidism?
Hyperthyroidism causes the metabolic rate to increase, causing symptoms of fatigue, unexplained weight loss, feeling anxious, a rapid heartbeat and/or heart palpitations and difficulty sleeping. Other symptoms of Graves' disease are an enlarged thyroid gland (goitre) and a protuberance of one or both eyes (exophthalmos).
How is hyperthyroidism diagnosed and treated?
Symptoms of an overactive thyroid, especially in the early stages, might be missed or mistaken for other conditions. It’s not uncommon for some people to put their weight loss, rapid heartbeat, and sensitivity to heat down to stress or anxiety. A blood test to measure thyroid hormone and antibody levels is used to diagnose hyperthyroidism.

There are many ways to treat an overactive thyroid:
  • Medications - thioamides, a group of drugs used to treat hyperthyroidism, work by reducing the levels of thyroid hormones produced. Methimazole and propylthiouracil belong to this drug class are the most used.
  • Radioiodine treatment - radioactive iodine is taken in by the thyroid and destroys the thyroid cells, ultimately reducing the level of hormones produced and decreases the size of the enlarged gland.
  • Surgery - either removing a small section of the thyroid or the whole gland. After surgery, synthetic thyroid hormones may be needed to prevent hypothyroidism from developing. If the entire thyroid is removed, then lifelong thyroid hormone replacement therapy is needed to maintain 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 that is responsible for producing too much T3 and T4. In these conditions, the pituitary gland is functioning normally so TSH production will be reduced. Low TSH levels will be accompanied by high FT4 and high or high normal FT3 results.

There are rare forms of hyperthyroidism where the pituitary gland malfunctions and produces too much TSH. Although the thyroid gland is healthy, it is overstimulated by TSH and produces too much T3 and T4. In this case TSH, FT4 and FT3 results are all elevated.

Related tests

Thyroid Function Blood Test

The Thyroid Function Blood Test checks your thyroid stimulating hormone (TSH), free T3 (FT3) and free T4 (FT4) to see if you have the right level of thyroid hormones for a healthy metabolism

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Advanced Thyroid Function Blood Test

Get detailed insights into your thyroid function, including antibodies and nutrition, for optimum thyroid health

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