Overactive thyroid (hyperthyroidism)

Expert reviewer Mr Robert Hardy, Consultant Surgeon
Next review due August 2021

If you have an overactive thyroid (hyperthyroidism, or thyrotoxicosis), it means your thyroid gland is making too much thyroid hormone. This can cause symptoms, such as feeling weak and emotionally up and down, but treatment is available.

About overactive thyroid

Your thyroid gland is in your neck, in front of your windpipe. It produces two hormones called thyroxine (T4) and triiodothyronine (T3).

Your metabolism is the combination of chemical processes that happen in your body, such as those that change the food you eat into energy. Too much thyroid hormone speeds up your body's metabolism. When your metabolism speeds up, you may lose weight even though you’re eating more, and have other symptoms such as a fast heart rate.

An image showing the location of the thyroid gland and surrounding structures

Symptoms of overactive thyroid

Overactive thyroid symptoms can vary – you might just have mild symptoms, or they can be severe. Symptoms can also be different depending on your age. You might even think they’re because an existing health problem has got worse.

How bad your symptoms are doesn’t always reflect how severe your condition is. For example, you may have mild symptoms but have a severely overactive thyroid. The symptoms you have can also depend on what’s causing your overactive thyroid.

The main symptoms of overactive thyroid include:

  • feeling tired with weak muscles
  • feeling hot, and less able to cope with heat
  • losing (or sometimes gaining) weight
  • feeling hungry and eating more
  • diarrhoea
  • sweating more than usual
  • feeling emotionally up and down, restless and irritable
  • having a lump in your throat (this is called a goitre and can be caused by a large thyroid gland)
  • feeling anxious
  • shaking (tremors)
  • a fast or irregular heart rate
  • feeling breathless
  • losing your hair – this is usually all of your hair rather than patches and usually goes away once you get treatment for an overactive thyroid
  • in women, having irregular or no periods
  • in men, less interest in having sex (a reduced libido) and swollen breast tissue (gynaecomastia)

If you have any of these symptoms, contact your GP.

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Diagnosis of overactive thyroid

Your GP will ask about your symptoms and examine you. They may ask you about your medical history, and if anyone in your family has an overactive thyroid gland.

If your GP thinks you may have an overactive thyroid, they`ll ask you to have one or more blood tests. This is to check the level of thyroid hormones in your body. The main hormones measured in blood tests are:

  • TSH – thyroid-stimulating hormone – a normal reading is in the range of 0.5 to 5mIU/L but this can vary between laboratories
  • FT4 – free T4 (the active part of thyroxine)
  • FT3 – free T3 (the active part of triiodothyronine)

If you have an overactive thyroid, you will have a low TSH level with a high FT4 level and a high FT3 level. The exact levels and types of hormones measured may vary depending on the laboratory and your situation. What’s classed as a normal, high or low level for each of the hormones may vary from one area to the next.

If the results of the blood tests show you have an overactive thyroid, your GP will refer you to see a specialist. This will be an endocrinologist, a doctor who specialises in treating thyroid conditions.

Treatment of overactive thyroid

Treatment for overactive thyroid depends on what has caused it and how severe your symptoms are. Any treatment you have aims to lower your thyroid hormones levels and ease your symptoms.


When you’re first diagnosed as having an overactive thyroid, your doctor may prescribe beta-blockers. These help to ease symptoms such as a fast heartbeat (palpitations) or shaking. You may need to take these until your thyroid hormone levels come down. If you can’t take a beta-blocker, you may be given a medicine called a calcium channel blocker.

Your doctor may prescribe you antithyroid drugs, either one called carbimazole, or propylthiouracil. These help to reduce the amount of hormone your thyroid gland produces. These medicines usually work quite quickly, but it can take two to three weeks until you get the full benefit. You’ll be asked to take these medicines for around 18 months. You may be able to stop taking them, although in about half of people their symptoms come back when they stop. If this happens, you may need to try a different treatment.

If you’re pregnant, your doctor may change your medicines after your first trimester (the first three months of your pregnancy). This is to make sure your baby develops properly and is healthy.

Radioiodine treatment

Radioactive iodine aims to destroy your thyroid gland, so that it stops producing hormones. You take radioactive iodine as a drink. It takes around three to four months to work fully.

Your doctor might suggest radioiodine treatment if:

  • you can’t take the medicines for an overactive thyroid
  • medicines for thyroid disease haven’t worked

Once the radioactive iodine has worked, your body won’t produce any more thyroid hormones. This means you’ll need to take levothyroxine (thyroid hormone replacement) for the rest of your life.

You can’t have radioactive iodine treatment if you’re pregnant or breastfeeding. You should also use contraception for at least six months after your treatment.


Another option for treating an overactive thyroid is to have all or most of your thyroid gland removed. It’s not commonly used as a treatment. But your doctor may suggest it if:

  • medicines and radioactive iodine treatments haven’t worked, or you can’t take them
  • you need treatment very quickly
  • you’re pregnant, or want to become pregnant in the next six months

After the operation, your body won’t produce any more thyroid hormones. This means you might develop an underactive thyroid – but this can be treated with medicines.

Monitoring your thyroid levels

During and after treatment for an overactive thyroid, you may need to have regular blood tests and check-ups. This is to measure the amount of thyroid hormones in your body and to check that treatment has worked. How often you’ll need these will depend on what caused your condition and what treatment you had. Your doctor will tell you what you need to do.

Causes of overactive thyroid

Lots of things can cause overactive thyroid. The most common cause is Graves’ disease, which is an autoimmune disease. This means that your body starts attacking your thyroid gland and it responds by producing too much hormone. Other causes of an overactive thyroid include:

  • conditions which produce swellings (nodules) on your thyroid gland
  • taking some medicines, such as amiodarone and lithium

Complications of overactive thyroid

Most people with an overactive thyroid recover well after treatment. But some people may develop complications, some of which can be serious or life-threatening. The main complications are listed below.

  • Heart problems, such as heart failure or an irregular and fast heartbeat (atrial fibrillation).
  • Sight problems – if you have Graves’ disease, you may develop a condition called thyroid eye disease. Find out more about this in our frequently asked questions (FAQs).
  • Thyroid storm – this is a severe condition that needs emergency treatment. The symptoms include a fever, heart problems and restlessness.
  • If you’re pregnant, an overactive thyroid could affect your baby if you don’t get it under control – they may be born early or underweight. It can also lead to miscarriage.

Diet and overactive thyroid

You don’t need to follow any special diet if you have an overactive thyroid, just aim to eat healthily. After you have had treatment you might want to consider what and how much you eat. See Related information for tips and advice about healthy eating and portion sizes. While your thyroid was producing too much hormone, you may have felt hungry and eaten more to keep your weight up. After your treatment, you may need to eat less to avoid putting on weight.

After treatment for overactive thyroid

Treatment for overactive thyroid lowers the levels of thyroid hormone in your body. It’s important to make sure that these don’t become too low because you could develop hypothyroidism. The main symptoms of hypothyroidism are:

  • putting on weight
  • feeling the cold
  • dry skin and hair
  • lack of energy
  • a puffy face

If you develop any of these symptoms, contact your GP.

Frequently asked questions

  • The iodine used for this treatment is radioactive. It’s very safe for you to have, but it takes a few days to a week for it to completely leave your body. During this time, you’ll need to take some precautions to make sure that you, and the people around you, are safe.

    After you’ve taken radioactive iodine, most of it leaves your body in your urine. Some also leaves your body in your sweat, tears, poo and saliva. This takes a few days to a week and as time goes on, the amount of radioactive iodine in your body gets less and less.

    When your body is getting rid of the radioactive iodine, it’s important to be careful about what contact you have with other people, and for how long. Your doctor will give you some specific advice about this. You may be asked to:

    • drive home from hospital alone rather than use public transport if possible, or sit far away from other passengers if you have to use it
    • only travel for short periods on public transport in the first week after your treatment
    • limit your contact with children and pregnant women
    • stay more than an arm’s length away from other people
    • sleep alone
    • stay away from busy places such as cinemas, pubs and restaurants
    • wash your hands, crockery and cutlery thoroughly
    • flush the toilet twice after using it

    How long you need to follow the precautions will depend on how much radioiodine you were given. Your doctor will be able to give you more advice.

    If you’re pregnant or breastfeeding, you shouldn’t have radioactive iodine. This is because radiation can pass through the placenta and into breast milk, so it may harm your baby. Ideally, wait six months after having your treatment before you try to get pregnant. Men should wait at least four months before trying for a baby.

    If you have any concerns or questions about radioiodine treatment, talk to your doctor.

  • If you have Graves’ disease, you could develop problems with your eyes. Around a quarter of all people with Graves’ disease develop a condition called Graves’ ophthalmopathy, also known as Graves' orbitopathy, or thyroid eye disease. It’s more common in people who smoke.

    The main symptoms of thyroid eye disease are:

    • bulging or staring eyes
    • swollen and red eyelids
    • watery eyes
    • pain behind your eye, which gets worse in the morning and when you move your eyes
    • red and irritated eyes
    • sensitivity to light

    Most people who have thyroid eye disease have mild eye problems, which get better with treatment. But it’s possible to develop more serious problems.

    If your eyes and the tissues around them become very swollen, it can affect your optic nerve. This can cause you to lose your sight. So, if you have any of these symptoms it’s important to see your GP as soon as you can. Your GP may refer you to an ophthalmologist (a doctor who specialises in eye health, including eye surgery).

    The type of treatment you have for thyroid eye disease depends on your symptoms and how severe they are. Treatments include eye drops, medicines, such as steroids, and surgery.

  • If you have an overactive thyroid, it can affect your periods and your ability to get pregnant. Your periods can be irregular and lighter. Some thyroid conditions, such as Graves’ disease, are also linked to conditions like endometriosis and polycystic ovary syndrome. These conditions can make getting pregnant even more difficult.

    If you have an overactive thyroid and you want to get pregnant, speak to your doctor. If at all possible, it’s best to plan your pregnancy and work with your doctor to manage your thyroid levels. If you’re taking carbimazole, you’ll need to switch to another antithyroid drug, propylthiouracil, before you start trying for children. If you get pregnant, once you reach your second trimester, you might need to switch back to carbimazole.

    You can take medicines to keep your thyroid hormone levels under control while you’re pregnant. But this needs careful monitoring to make sure that you and your baby stay well.

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  • Reviewed by Rachael Mayfield-Blake, Freelance Health Editor, August 2018.
    Expert reviewer Mr Robert Hardy, Consultant Surgeon
    Next review due August 2021