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Underactive thyroid (hypothyroidism)

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

Too little thyroid hormone slows down your body’s metabolism. Your metabolism is the combination of chemical processes that happen in yourbody, such as those that change the food you eat into energy. When your metabolism slows down, it can lead to symptoms such as feeling tired and cold, and putting on weight.

Your thyroid gland is in your neck, in front of your windpipe. It produces two main hormones called thyroxine (T4) and triiodothyronine (T3). Women are around eight times more likely than men to develop an underactive thyroid. If you’re middle-aged, you’re also more likely to develop it.

Thyroid disease
Dr Watkinson talks about thyroid disease

Symptoms of underactive thyroid

Symptoms of an underactive thyroid can develop slowly and gradually, sometimes over several years. The symptoms of underactive thyroid can also be quite vague and can be caused by many other conditions or situations, such as feeling tired. This can sometimes make it hard to diagnose.

The main symptoms of underactive thyroid include:

  • feeling cold / less able to cope with cold
  • gaining weight
  • feeling tired with stiff muscles
  • dry skin and hair loss
  • heavy, irregular or no periods
  • slowing down, mentally and physically
  • a deep and raspy voice
  • constipation
  • cramps
  • tingling in your hands and arms

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

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

Diagnosis of underactive thyroid

Your GP will ask about your symptoms and examine you. They may also ask you about your medical history.

If your GP thinks you may have an underactive thyroid, they will 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
  • FT4 - free T4 (thyroxine)
  • FT3 - free T3 (triiodothyronine)

If you have an underactive thyroid, you will usually have a TSH level above normal and a free T4 level below normal. The exact levels of hormones measured may vary between different laboratories. So 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 that you have an underactive thyroid, you’ll usually be treated by your GP. But in some circumstances, your GP may refer you to a specialist – usually an endocrinologist – who specialises in treating thyroid conditions.

Treatment of underactive thyroid

The aim of treatment is to ease your symptoms and prevent any complications. This means getting your thyroid hormone levels back to normal.

If you have a mildly underactive thyroid (called subclinical hypothyroidism), you may not need any treatment. If you’re not having treatment, your GP will usually check your thyroid hormone levels every six to 12 months. If you develop symptoms, your GP may suggest that you start having treatment.


Your doctor may prescribe you a hormone replacement medicine called levothyroxine (thyroxine). You take this once a day as a tablet.

Usually, you start by taking a small amount of levothyroxine and this is gradually increased. Your GP will take a blood sample every three to four weeks and then change the amount, depending on the results.

It can take a while for your hormone levels to get back to normal, so it may be a few months until you start to feel better. Once your thyroid levels have returned to normal, you’ll usually have a blood test once a year to check your thyroid hormone levels. You will need to take levothyroxine for the rest of your life.

Causes of underactive thyroid

In the UK, the most common cause of underactive thyroid is autoimmune thyroiditis (Hashimoto's thyroiditis). This is when your immune system attacks your thyroid gland, damaging it and stopping it producing enough thyroid hormone. If you have had your thyroid gland removed or radioactive iodine treatment for an overactive thyroid, this can cause an underactive thyroid. Radiotherapy for thyroid cancer can also lead to an underactive thyroid.

Other causes of underactive thyroid include:

  • some medicines, such as lithium and amiodarone
  • an infection caused by a virus (thyroiditis)
  • pregnancy – around one in 20 women develop an underactive thyroid in the six months after their baby is born

Complications of underactive thyroid

The symptoms of an underactive thyroid, such as physical and mental tiredness and lack of energy, can have a serious effect on your day-to-day life. You may find that your work life, relationships and social life suffer.  

If you’re pregnant, an underactive thyroid can mean your baby may be born early or underweight. It can also lead to miscarriage.

Other complications include:

  • heart problems, such as chest pain (angina) and an irregular and fast heartbeat (atrial fibrillation)
  • myxoedema – this is a rare but serious condition that can affect your heart – it can be life-threatening

FAQ: How soon will I feel better when taking levothyroxine?

Most people make a full recovery and feel completely better once their hormone levels get back to normal.

But don’t worry if it takes a few months for your symptoms to go after starting treatment – this is to be expected. Some people find that they don’t feel completely better until about six months after their hormone levels get back to normal.

Around one in every 20 people with an underactive thyroid will still have some symptoms, even though their hormone levels are normal. If this happens, your GP may arrange for you to have further tests to see whether something else is causing your symptoms.

FAQ: Is it safe to take levothyroxine when I’m pregnant?

Yes, you can take levothyroxine when you’re pregnant. You will need regular checks during your pregnancy to make sure your thyroid hormones are at a healthy level.

When you’re pregnant, it's important to have the right amount of thyroid hormones in your bloodstream. In the first trimester (first 12 weeks) of your pregnancy, thyroid hormones help your baby’s brain to develop. Later on in your pregnancy, low thyroid hormone levels can cause complications such as pre-eclampsia and heart problems.

If you have an underactive thyroid, see your GP as soon as you think you’re pregnant. While you’re pregnant, your body usually needs more thyroid hormone. Your doctor may need to increase the amount of levothyroxine you take. After your baby is born, the amount you need to take should go down again.

Your thyroid hormone levels will be regularly checked while you’re pregnant.

FAQ: Will I lose weight when I start treatment?

One of the main symptoms of an underactive thyroid is that you put on weight, even though you may feel less like eating. The weight doesn’t always come off when you start treatment.

Being overweight has been linked to having an underactive thyroid. But, if you have an underactive thyroid, you put on weight because your body holds on to more water than usual, rather than because you’re storing fat. Your metabolism also slows down, which means you don’t use energy in the same way.

There are only a small number of research studies that have looked at weight in people who have an underactive thyroid. These show that once treatment starts, any weight that you’ve already put on doesn’t always come off again.

If you want to lose the weight you put on, or keep your weight healthy, it’s important to be active and eat the right foods. The main things you can do are listed below.

  • Do at least 150 minutes (2½ hours) of moderate-intensity activity each week. You don’t need to do this all at once, you can break it down to sessions of 10 minutes or more. One way to do this is to do 30 minutes on at least 5 days a week.
  • Eat mostly vegetables, fruits, beans and pulses, wholegrains and fish.
  • Eat fewer foods and drinks high in saturated fat or sugar, such as fried foods, biscuits, savoury snacks and sweets.
  • Eat smaller portions.
  • Eat more wholegrain foods and foods that have a lot of fibre in them.
  • Have no more than 70g of red and processed meat a day.

If your weight changes a lot, see your doctor. They may need to change the amount of levothyroxine you take.

FAQ: I have borderline underactive thyroid. What is that?

You’re classed as having borderline underactive thyroid if you have raised a thyroid-stimulating hormone (TSH) level, but normal thyroid hormone levels in your blood. Doctors usually call this subclinical hypothyroidism. As many as one in 10 people have borderline underactive thyroid, and the likelihood of developing underactive thyroid goes up as you get older.

Your doctor may suggest that you take levothyroxine if your TSH level is above 10mU/L. This is because you’re more likely to develop underactive thyroid that needs treatment in the future.

Whether you have treatment or not, your doctor will do regular blood tests to check your thyroid hormone levels.


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    Further information


    • Primary hypothyroidism. BMJ Best Practice., last updated April 2016
    • Hypothyroidism. The MSD Manuals., last full review/revision May 2014
    • Stedman’s Medical Dictionary. Lippincott Williams & Wilkins., accessed 15 March 2016
    • Overview of thyroid function. The MSD Manuals., last full review/revision May 2014
    • Thyroid. Oxford handbook of endocrinology and diabetes (online). Oxford Medicine Online., published March 2014
    • Hypothyroidism. NICE Clinical Knowledge Summaries., last revised February 2011
    • The diagnosis and management of primary hypothyroidism. Royal College of Physicians, 2011..
    • Hyperthyroidism. Medscape., updated November 2015
    • Hyperthyroidism. PatientPlus., last checked December 2015
    • UK guidelines for the use of thyroid function tests. The Association for Clinical Biochemistry, British Thyroid Association, British Thyroid Foundation., published July 2006
    • Hypothyroidism guide. British Thyroid Foundation., revised 2015
    • Joint Formulary Committee. British National Formulary (online) London: BMJ Group and Pharmaceutical Press., accessed 23 March 2016
    • Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid December 2014; 24(12): 1670–1751.
    • Preventing excess weight gain. National Institute for Health and Care Excellence (NICE), 2015.
    • Start Active, Stay Active. A report on physical activity from the four home countries’ Chief Medical Officers. Department of Health, 2011.
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    Reviewed by Pippa Coulter, Specialist Health Editor, Bupa Health Content Team, May 2016
    Peer reviewed by Dr Jonathan Katz MD FRCP, Consultant Endocrinologist
    Next review due May 2019

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