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Underactive thyroid (hypothyroidism)

Underactive thyroid (hypothyroidism) means your thyroid gland isn’t producing enough thyroid hormones for your body's needs. This slows down your metabolism, leading to symptoms such as feeling extremely tired and gaining weight.

Underactive thyroid is more common in older people and women are more likely to be affected than men. Sometimes, it’s possible for children and babies to be affected too.

Your thyroid gland is in your neck, in front of your windpipe. Your thyroid gland releases two main hormones (chemical messengers). These control how quickly your body uses its energy stores and how sensitive you are to other hormones. The two main hormones are known as thyroxine (T4) and triiodothyronine (T3).

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How a thyroid gland can become underactive
Having an underactive thyroid slows down the body's metabolism, leading to symptoms including tiredness and putting on weight
An image showing the location of the thyroid gland and surrounding structures


  • Symptoms Symptoms of underactive thyroid

    Symptoms of underactive thyroid can include:

    • extreme tiredness
    • weight gain, but with a poor appetite
    • feeling the cold easily
    • dry and/or pale skin
    • coarse, thinning hair and brittle nails
    • weak and sore muscles
    • a hoarse, croaky or deeper voice
    • puffy eyes
    • memory loss and poor concentration
    • constipation
    • heavy, irregular or prolonged menstrual periods, if you’re a woman
    • tingling in your hands and arms
    • depression
    • poor sex drive
    • slow heart rate
    • enlarged thyroid gland (goitre), which causes a lump to develop in your throat

    Remember, these symptoms aren’t always caused by an underactive thyroid. But if you have them, it’s best to see your GP.

  • Diagnosis Diagnosis of underactive thyroid

    Your GP will ask about your symptoms and examine you. They may also ask you about your medical history and take a blood test to check your thyroid function. The blood test will measure your levels of thyroid-stimulating hormone (TSH). TSH is made in the pituitary gland in your brain and controls the production of T4 and T3.

    If your blood tests confirm you have underactive thyroid, your GP may refer you to an endocrinologist. This is a doctor who specialises in conditions of the endocrine system.

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  • Treatment Treatment of underactive thyroid

    Treatment aims to return your levels of thyroid hormones to normal. If you have an underactive thyroid but don’t have any symptoms (known as subclinical hypothyroidism), you may not need any treatment. Your GP will usually monitor how your thyroid hormone levels change every few months. If you develop symptoms, your GP will suggest that you start having some treatment.


    Your doctor may prescribe you a hormone replacement medicine called levothyroxine. This is taken daily as a tablet, but it can take some time to get the dose right.

    Usually, you will start on a low dose that is gradually built up every few months. You will need regular blood tests to check your levels of thyroid hormones. Your doctor will adjust your dose according to the results of your blood tests. When your dose is being adjusted, it can take several months before you feel better and your symptoms ease. It’s important to get the dose right. Too much levothyroxine can cause symptoms of an overactive thyroid, whereas too little won’t ease your symptoms of an underactive thyroid.

    Once the correct dose of levothyroxine has been found, you will usually have a thyroid function test yearly to monitor your thyroid hormone levels. You will need to take this treatment for the rest of your life.

    Blood tests

    If you’re receiving treatment for an underactive thyroid, you’ll need regular blood tests. This helps to manage your condition and ensure you’re receiving the right dosage of medicine. Blood tests measure your level of TSH in your blood and will usually be:

    • every six to eight weeks at the beginning of treatment and until your correct dose of medication is established
    • every year when you’re on long-term medication

    Be aware that results from different labs can vary so aren’t always comparable. Plus, there are different ranges used if you’re pregnant. If you have any questions about your blood test results and what they mean, speak to your doctor.

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  • Causes Causes of underactive thyroid

    In the UK, the two most common causes of underactive thyroid are as follows.

    • Autoimmune thyroiditis (Hashimoto's thyroiditis). Your immune system produces antibodies that attack your thyroid gland. This prevents your thyroid gland from producing enough thyroid hormones.
    • Surgery or radioiodine treatment for an overactive thyroid (hyperthyroidism) or thyroid cancer. If you have treatment that destroys thyroid tissue, it can lead to hypothyroidism.

    Other, less common causes of underactive thyroid include the following.

    • Congenital hypothyroidism. This is when babies are born with an underdeveloped thyroid gland. In the UK, a heel-prick blood sample is taken between six and 14 days after your baby is born to test for congenital hypothyroidism.
    • Certain medicines. Some medicines can affect the normal functioning of your thyroid gland and cause underactive thyroid. Examples include lithium carbonate (for bipolar disorder) and amiodarone (for heart rhythm problems).
    • Disorders of the pituitary gland. This gland is involved in the regulation and production of thyroid hormones and having any problems with it can affect the normal functioning of your thyroid.
    • Iodine deficiency. Your body needs iodine to make thyroid hormones. People not having enough iodine in their diet is the leading cause of underactive thyroid in the world, but is uncommon in the UK.
  • Complications Complications of underactive thyroid

    If you’re a woman, an underactive thyroid can interfere with your menstrual cycle, causing heavy, irregular or prolonged periods. This may lead to anaemia and also affect your ability to ovulate, which can make it more difficult to get pregnant.

    An underactive thyroid in pregnancy can increase your risk of having a miscarriage, stillbirth, pre-eclampsia or a premature birth. It can also increase your risk of having a baby with a low birth weight or impaired cognitive function. If you have an underactive thyroid and are thinking about starting a family, or are pregnant, talk to your GP. They will be able to offer you advice, support and the right care throughout your pregnancy.

  • Considerations Special considerations

    If you’re pregnant or breastfeeding

    If you’re taking levothyroxine, your thyroid hormone levels should be closely monitored during and after pregnancy. As soon as you know you’re pregnant, you should have a thyroid blood test. Your requirements of levothyroxine will usually be higher during pregnancy. Your medication dose will need to be carefully adjusted to the right level for you according to your blood results.

  • FAQs FAQs

    What blood tests are used to diagnose an underactive thyroid?


    To check your thyroid function, your GP will take a sample of your blood and check your levels of thyroid-stimulating hormone (TSH). Your level of thyroxine (T4) may also be measured.


    TSH is produced by a part of your brain called the pituitary gland. It stimulates your thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Together, these hormones control how quickly your body uses its energy stores (metabolism) and how sensitive your body is to other hormones.

    To check your thyroid function, your GP will take a sample of your blood and measure the levels of TSH. If this is high, the level of thyroxine in your blood may also be measured.

    If the results show you have high levels of TSH with low levels of thyroxine, it means you have an underactive thyroid.

    How long will it take for me to feel better once I've started taking levothyroxine for underactive thyroid?


    You should expect a gradual improvement, feeling back to normal about four to six months after starting levothyroxine treatment. Once your thyroid hormone levels are back to normal, you may start to feel better fairly quickly, but this varies from person to person.


    Treatment with levothyroxine aims to return your levels of thyroid hormones to normal.
    If you’re over 18, your doctor may start you on a levothyroxine dose of 50 to 100 micrograms (μg) once a day. You will be monitored regularly and your dose will be adjusted in steps of 25 to 50μg every three to four weeks, depending on how to you respond to it.

    Your symptoms will begin to improve as the hormone levels in your bloodstream return to normal. The length of time this takes will depend on how severe your underactive thyroid was when you were diagnosed and the dose you started on. It can sometimes take several months after your thyroid levels are stable before you start feeling better, so don’t worry if your symptoms don’t ease straight away.

    You will be closely monitored by your doctor, especially in the first six months of treatment. It’s important to let your doctor know if you don’t feel any better or if you have any side-effects.

    Side-effects to look out for include diarrhoea, vomiting, chest pains, irregular heartbeat, flushing, weight loss, headaches and muscle cramps. Having side-effects may be a sign that the dose of levothyroxine you’re taking is too high, so it’s important to tell your doctor so that your dose can be reduced.

    Is it safe to take levothyroxine for underactive thyroid during pregnancy?


    Yes, it's safe to take levothyroxine during pregnancy. You will need to be closely monitored during your pregnancy to ensure your thyroid hormones are at a healthy level.


    During pregnancy, it's important that you have enough thyroid hormones in your bloodstream. This is because in the first trimester (first 12 weeks) of pregnancy, thyroid hormones help to develop your baby’s brain and nervous system.

    If you have an underactive thyroid, see your GP as soon as you think you’re pregnant. They will usually increase your dose of levothyroxine to ensure you have enough thyroid hormones for you and your baby. How much this is increased by will depend on the current dose you're taking and the results of your blood test.

    Your thyroid hormone levels will be regularly checked during your pregnancy. If you were diagnosed with an underactive thyroid very recently before becoming pregnant, your GP may refer you to an endocrinologist. This is a doctor who specialises in conditions of the endocrine system. They will keep a close eye on you throughout your pregnancy.

    If you have any questions or concerns about underactive thyroid and pregnancy, speak to your GP or endocrinologist.

  • Resources Resources

    Further information


    • Hypothyroidism. The Merck Manuals., published October 2013
    • Hypothyroidism. NICE Clinical Knowledge Summaries., published February 2011
    • Overview of thyroid function. The Merck Manuals., published September 2013
    • Your thyroid gland. British Thyroid Foundation., accessed 1 March 2014
    • Hypothyroidism. British Thyroid Foundation., published October 2011
    • Thyroid function tests guide. British Thyroid Foundation., published October 2011
    • UK guidelines for the use of thyroid tests. British Association of Endocrine and Thyroid Surgeons., published July 2006
    • Joint Formulary Committee. British National Formulary (online) London: BMJ Group and Pharmaceutical Press., accessed 1 February 2014 (online version)
    • Stagnaro-Green A, Abalovich M, Alexander E, et al. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid 2011; 21(10):1081–125. doi:10.1089/thy.2011.0087
    • Primary hypothyroidism. BMJ Best Practice., published 24 April 2013
    • Congenital hypothyroidism. British Thyroid Foundation., published August 2011
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