What is an overactive thyroid?

An in-depth look at an overactive thyroid (hyperthyroidism) and how it can be treated.

An overactive thyroid, also known as hyperthyroidism, is where the thyroid produces too many thyroid-related hormones [1].

The thyroid is a small butterfly-shaped gland positioned in the neck. It creates hormones that regulate the heartbeat, digestive system, and many other bodily functions. These hormones are triiodothyronine (T3) and thyroxine (T4), and without them, the body begins to slow down.

In this blog we discuss:

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.

The causes of hyperthyroidism are:

  • Graves’ disease – a condition where the immune system attacks and damages the thyroid.
  • Thyroiditis – inflammation of the thyroid gland.
  • Incorrect medicine dosages – if people overmedicate with thyroid hormone replacement, it 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

An overactive thyroid causes an increase in metabolism. In some cases, the symptoms are subtle and go unnoticed, but 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

Have a look at the full range of thyroid disease causes, symptoms, and risk factors.

How is hyperthyroidism diagnosed?

Doctors can diagnose this condition with a short physical examination or blood tests. They will ask about any symptoms and may perform an ultrasound scan of the thyroid gland.

A blood test is a very accurate method of diagnosing an overactive thyroid. In most people with the condition, this will show low levels of the thyroid-stimulating hormone (TSH) and high levels of thyroxine (T4).

With diagnosed hyperthyroidism, another blood test is used to measure TSH receptor antibodies to see if it is being caused by Graves’ disease [2]. A Cholesterol Home Blood Test may also be useful as low cholesterol levels can indicate an elevated metabolic rate.

Read more about diagnosing and managing a thyroid condition.

How is hyperthyroidism treated?

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

Treatments for hyperthyroidism include:

  • Beta-blockers – this medication is prescribed to treat very mild cases of hyperthyroidism and helps to relieve some of the symptoms.
  • Medication – antithyroid medicines are usually the first response to hyperthyroidism, particularly for children and pregnant women. They are 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.
  • Radioiodine – radiotherapy is used to destroy cells in the thyroid, which reduces its ability to produce thyroid hormones. Radioiodine is very effective, safe and has minimal side effects.
  • Surgery – the thyroid gland can be partially or entirely removed during an operation. This option is usually considered if the medication does not work or if the thyroid gland is swollen. Levothyroxine will need to be taken after surgery [3].
  • A balanced diet - along with a few specific foods, a balanced diet can support thyroid health. Hyperthyroidism can cause bones to weaken, so having enough calcium is beneficial. 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].

Have a look at our blog: what foods are good for thyroid health? for more information.

What are the health risks of hyperthyroidism?

An overactive thyroid can cause complications if it is not treated appropriately or early enough. Some of the related health risks are:

  • Eye problems – 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 struggle to pump efficiently if hyperthyroidism is not treated.
  • Mental health problems – hyperthyroidism can cause anxiety and hyperactivity.
  • Weight loss and loss of bone – the increase in metabolic rate increases the body's energy requirements, which uses energy stored in body fat. The calcium stored in bones can also decrease, resulting in osteoporosis.
  • Pregnancy complications – untreated hyperthyroidism can cause preeclampsia, premature birth, and miscarriage.
  • A thyroid storm – the term given to a sudden and life-threatening flare-up of symptoms [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 an overactive thyroid

If you have any concerns or are experiencing any symptoms related to an overactive thyroid, visit your doctor. Need help choosing a test? Look at our range of Thyroid Blood Tests or read our Thyroid Buying Guide.

If you think that you may have hyperthyroidism, it is important to see your doctor as the condition can deteriorate rapidly.

Blood tests

  1. Thyroid Function Blood Test - a comprehensive test that looks for TSH, FT4 and FT3 - the more biologically active form of FT4. This test helps you to take a more in-depth look at your thyroid hormones.
  2. Thyroid Function with Antibodies Blood Test - see if your thyroid hormones are functioning within the healthy range. You can also detect any abnormalities caused by an autoimmune disease. Even if your thyroid function is currently normal, elevated antibodies may increase your risk of developing a disorder in the future.
  3. Advanced Thyroid Function Blood Test - this test includes the thyroid hormones, thyroid antibodies, and nutritional markers that can support thyroid function. If your nutritional markers are low, this can cause symptoms like a thyroid disorder. For example, low iron (ferritin), low vitamin D and low B vitamins can cause fatigue and low energy.

Unsure which blood test is right for you? Head over to our Thyroid Blood Test Buying Guide, or try our test finder



  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.

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