Overactive thyroid

Also known as hyperthyroidism or thyrotoxicosis

Key points about overactive thyroid

  • An overactive thyroid (hyperthyroidism) is when your thyroid gland makes too much thyroid hormone.
  • It can cause weight loss, anxiety, rapid heartbeat, sleep problems and low energy.
  • Common causes include Graves’ disease, thyroid nodules, thyroiditis and some medicines, eg, amiodarone.
  • If you have symptoms of an overactive thyroid, see your doctor for a diagnosis.
  • Possible treatments include medicines, radioiodine or surgery.
  • If an overactive thyroid is not treated, you could develop heart problems, eye problems, osteoporosis or a life-threatening condition called thyroid storm.
Hands feel for thyroid on young woman's throat
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See your GP, go to the nearest emergency department or call 111 for an ambulance if you or someone you care for has thyroid disease and experiences any of the following:

  • a very high temperature or fever
  • very fast heart rate or palpitations
  • shortness of breath
  • confusion or hallucinations
  • agitation
  • nausea (feeling sick) or vomiting (being sick)
  • fainting or loss of consciousness
  • feeling very unwell.

These symptoms could be a sign of very high thyroid hormone levels (known as thyroid storm, hyperthyroid crisis or thyrotoxic crisis). It is a medical emergency and needs immediate treatment.

Your thyroid is a small, butterfly-shaped gland in your neck. It produces 2 thyroid hormones: tri-iodothyronine (T3) and thyroxine (T4). Thyroid hormones help your body use energy, stay warm and keep your brain, heart, muscles and other organs working as they should.

 Image credit: Canva

Thyroid problems can affect anyone, but are more common in women than men.

An overactive thyroid can be caused by a number of conditions:

  • Graves’ disease. This is the most common cause. It is an autoimmune disease in which your body’s immune system mistakenly attacks your thyroid gland. Sometimes this condition runs in families.
  • Thyroid nodules. These are benign (non-cancerous) lumps that grow on your thyroid. They can produce extra thyroid hormones. Read more about thyroid nodules.
  • Too much iodine in your body. This can be caused by taking iodine supplements such as kelp or seaweed
  • Thyroiditis (inflammation of your thyroid gland). This can be caused by a virus or can happen soon after having a baby. 
  • Some medicines such as lithium or amiodarone can cause an overactive thyroid.
  • Thyroid cancer. This is rare, but a thyroid cancer can affect the production of thyroid hormones.

People with an overactive thyroid may have no symptoms at all, or symptoms may start off mild and then gradually get worse.

Symptoms may include:

  • losing weight, even though you are eating the same amount or more than usual
  • feeling nervous, anxious, moody, weak or tired
  • tremor (shaky hands)
  • palpitations (fast or ‘thumping’ heartbeat)
  • problems breathing
  • feeling hot and sweaty
  • having looser, more frequent, bowel movements (poo) than usual
  • skin problems such as a rash, itching or hair thinning
  • trouble sleeping
  • a swollen thyroid gland, also known as goitre
  • problems with your eyes or vision – this is more common if you have Graves’ disease
  • lighter or no menstrual periods.

The diagnosis of hyperthyroidism may not be obvious at first because symptoms may be mild. See your doctor if you have any of the symptoms noted above. Your doctor will ask you questions, including whether you are taking any medicines or supplements that may affect your thyroid gland. Your doctor will usually do a physical examination and arrange blood tests which may include:

  • a thyroid function test – this blood test measures your thyroid hormones (T3, T4) and thyroid stimulating hormone (TSH)
  • a test for TSH receptor antibody – if you have Graves’ disease, this antibody level can be high.

If blood tests show you have an overactive thyroid, you will be referred to an endocrinologist (a specialist hormone doctor).  Sometimes other tests are needed, such as a scan of your thyroid gland.

Treatment depends on the cause of your overactive thyroid, your age, the severity of your condition, other medical conditions that may be affecting your health and your own preference. Your doctor will discuss the best treatment option with you.

Possible treatment of an overactive thyroid include:

  • medicines such as carbimazole
  • radioactive iodine therapy
  • surgery.



How does it work?

  • Carbimazole helps to reduce the amount of thyroid hormones produced by your thyroid gland. It works best if your symptoms are mild.

How long does it take to work?

  • Carbimazole takes 4–8 weeks to start working.
  • This is because it doesn't affect the extra thyroid hormones that have already been made and stored, but reduces further production.

What is the dose?

  • The dose of carbimazole needed is different from person to person.
  • A high dose is usually given in the beginning and is then reduced as your thyroid hormone levels come down.

How long is a course of treatment?

  • Carbimazole is usually taken for 12–18 months at first.
  • After this, in about half of cases, the condition will have settled down and the carbimazole can be stopped.
  • If the condition flares up again in the future, a further course may be needed, or an alternative treatment such as radioactive iodine may be recommended.
  • In about half of cases, carbimazole needs to be continued long term to control symptoms.
  • A different treatment may then be a better option if you do not want to take carbimazole long term.

Are there any risks?

  • In rare cases, carbimazole can cause a sudden drop in your white blood cells. This puts you at risk of picking up serious infections.
  • Signs of an infection include high temperature (fever), chills, tiredness, sore throat, sores in your mouth or on your skin, feeling unwell or other flu-like symptoms.
  • If you experience any of these symptoms while taking carbimazole, contact your doctor immediately.


Radioactive iodine therapy

How does it work?

  • Radioactive iodine therapy involves taking a drink, or swallowing a capsule, that contains radioactive iodine.
  • The treatment works by damaging the cells that produce the thyroid hormones in your thyroid gland.
  • The radiation dose is very small, so it is a very safe and effective treatment. 

How long does it take to work?

  • Radioactive iodine therapy doesn't work immediately.
  • Over a few weeks to months, radioactive iodine damages the cells in your thyroid gland, causing it to shrink in size.
  • For most people this then leads to your thyroid hormones returning to normal.

Are there any risks?

  • The main side effect is an underactive thyroid (hypothyroidism).
  • You will have several blood tests after treatment to monitor your thyroid hormone levels and can be given thyroid tablets if this happens.



What happens?

  • This is an operation in which all or part of your thyroid gland is removed. This is called a thyroidectomy.

Who is it for?

  • This is often the last resort and is considered if other therapies and medicines have not worked.
  • People also consider surgery if they are at a higher risk of getting side effects from other treatments.
  • Surgery is a good option if you have a large tumour (called goitre) causing problems in your neck.

Ongoing treatment

  • If your entire thyroid gland is removed, you will need thyroid hormone replacement, which means taking thyroid tablets for life. 

If hyperthyroidism goes untreated for a long time, it may lead to severe complications, which can be avoided with treatment.

These include:

  • heart problems such as atrial fibrillation or heart failure
  • eye problems
  • osteoporosis (brittle bones)
  • a thyroid storm (fever, delirium, rapid pulse or loss of consciousness) – this is an medical emergency that needs immediate treatment.

The NZ Thyroid Support Group provides support for people to meet, talk and share information about thyroid disorders. Visit their website(external link) or contact them by phone: 09 480 2680.

The following links provide further information about overactive thyroid. Be aware that websites from other countries may have information that differs from New Zealand recommendations.   

Overactive thyroid (hyperthyroidism)(external link) HealthInfo Canterbury, NZ
Hyperthyroidism(external link) DermNet NZ
Thyroid – hyperthyroidism(external link) Better Health Channel, Australia
Hyperthyroidism, Graves’ disease and thyroid eye disease(external link) The Australian Thyroid Foundation, Australia
Overactive thyroid gland(external link) Patient Info, UK
Overactive thyroid(external link) NHS, UK 


  1. Hyperthyroidism(external link) 3D Regional HealthPathways, NZ, 2021
  2. Campbell K, Doogue M. Evaluating and managing patients with thyrotoxicosis(external link) Australian Family Physician. 2012. 41(8):564-572.
  3. Management of thyroid dysfunction in adults(external link) BPAC, NZ, 2010
  4. Hyperthyroidism(external link) Patient Info, UK
  5. Treating an overactive thyroid (hyperthyroidism)(external link) Healthinfo Canterbury, NZ
  6. Carbimazole(external link) NZ Formulary

The following information on hyperthyroidism is taken from 3D Regional HealthPathways, NZ, accessed March 2021:

Red flags:

Hyperthyroidism and:

  • significantly unwell, especially if fever or dehydration
  • rapid atrial fibrillation or heart failure
  • psychosis
  • signs of severe and sight-threatening eye disease.

Clinical guidelines and resources

Hyperthyroidism(external link) 3D Regional HealthPathways, NZ, 2021
Carbimazole(external link) NZ Formulary
Hyperthyroidism(external link) Patient Info, UK
Management of thyroid dysfunction in adults(external link) BPAC, NZ, 2010
Campbell  K, Doogue M. Evaluating and managing patients with thyrotoxicosis(external link) Australian Family Physician. 2012. 41(8):564-572.

Continuing professional development

New diagnosis of hyperthyroidism in primary care(external link) BMJ Learning, UK

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Credits: Healthify editorial team. Healthify is brought to you by Health Navigator Charitable Trust.

Reviewed by: Dr Alice Miller, GP, FRNZCGP, Wellington

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