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Underactive thyroid (hypothyroidism)

Key points

  • Hypothyroidism means you have an underactive thyroid. This is caused by your thyroid gland not producing enough thyroid hormones.
  • The symptoms of underactive thyroid include feeling extremely tired, gaining weight, feeling the cold, thinning hair, weak muscles and an enlarged thyroid gland.
  • Underactive thyroid is treated with thyroid hormone replacement medicine called levothyroxine. When you’re first diagnosed, it may take a while for your symptoms to ease until your GP finds the right dosage of medicine for you.
  • Thyroid function tests can help manage your thyroid disorder and ensure you’re receiving the right dosage of medication.

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.

How your thyroid gland can become underactive

About underactive thyroid

If you have underactive thyroid, it means your thyroid gland isn’t producing enough thyroid hormones.

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).

Illustration showing the position of the thyroid gland and surrounding structures

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.

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.

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.

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.

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.

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.

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.

Reviewed by Alice Rossiter, Bupa Health Information Team, May 2014.

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  • This information was published by Bupa's Health Information Team and is based on reputable sources of medical evidence. It has been reviewed by appropriate medical or clinical professionals. Photos are only for illustrative purposes and do not reflect every presentation of a condition. The content is intended only for general information and does not replace the need for personal advice from a qualified health professional. For more details on how we produce our content and its sources, visit the about our health information page.

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