Underactive thyroid (hypothyroidism)


Underactive thyroid means your thyroid gland isn’t producing enough thyroid hormone for your body’s needs. This slows down your body's metabolism, leading to symptoms such as extreme tiredness and weight gain.

Video: 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 hormone.

Your thyroid gland is in your neck, in front of your windpipe. Your thyroid gland releases hormones (chemical messengers) to control how quickly your body uses its energy stores and how sensitive your body is to other hormones.

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.

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

These symptoms aren’t always caused by underactive thyroid but if you have them, see your GP.

Complications of underactive thyroid

In women, underactive thyroid can interfere with your menstrual cycle, leading to heavy, irregular or prolonged periods. This may lead to anaemia and also affect your ability to ovulate, making it more difficult for you to get pregnant.

Underactive thyroid in pregnancy can increase the risk of having a miscarriage, stillbirth, pre-eclampsia, premature birth, or a baby with low birthweight or impaired cognitive function.

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 birth 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. Not having enough iodine in the diet is the leading cause of underactive thyroid worldwide, but is uncommon in the UK.

Diagnosis of underactive thyroid

Your GP will ask about your symptoms and examine you. He or she may also ask you about your medical history and take a blood test to check your thyroid function.

If blood tests confirm you have underactive thyroid, your GP may refer you to an endocrinologist (a doctor who specialises in identifying and treating conditions of the endocrine system).

Treatment of underactive thyroid

Treatment aims to return your levels of thyroid hormones to normal. If you have 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 treatment.


Your doctor may prescribe thyroid hormone replacement medicine (levothyroxine). This is taken daily as a tablet, but 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.

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

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, so your dose will need to be carefully adjusted according to your blood results.


Produced by Krysta Munford, Bupa Health Information Team, May 2012.

For answers to frequently asked questions on this topic, see FAQs.

For sources and links to further information, see Resources.

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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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