Underactive thyroid (hypothyroidism)

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

If you have an underactive thyroid (hypothyroidism), it means your thyroid gland isn’t producing enough thyroid hormones. Around one or two out of every hundred people develop an underactive thyroid. Women are up to 10 times more likely than men to develop it.

About underactive thyroid

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

Too little thyroid hormone slows down your body’s metabolism. Your metabolism is the combination of chemical processes that happen in your body, 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.

Symptoms of underactive thyroid

Symptoms of an underactive thyroid can develop slowly and gradually. 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. You might not get any symptoms at all.

If you do get underactive thyroid symptoms, the main ones include:

  • putting on weight
  • less of an appetite to eat
  • feeling the cold
  • dry skin
  • feeling weak and tired and having no energy
  • a puffy face
  • stiff muscles
  • losing your hair – this is usually all of your hair rather than patches and usually goes away once you get treatment for an underactive thyroid
  • slowing down, mentally and physically
  • a hoarse voice
  • constipation
  • a feeling of something being stuck in your throat
  • a slow heart beat
  • in women, irregular periods

Having an underactive thyroid can affect your mental health too. You might feel emotionally up and down, and you might feel depressed if you have the condition.

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

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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 – a normal reading is in the range of 0.5 to 5mIU/L but this can vary between laboratories 
  • FT4 - free T4 (thyroxine)

If you have an underactive thyroid, you`ll 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 sometimes, your GP will refer you to a specialist – usually an endocrinologist – a doctor 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. Once your treatment is working well, you should feel much better. You might need to adjust your treatment every now and then as your condition progresses. If your symptoms continue to affect your quality of life, talk to your doctor.

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 months to a year. 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 until your level of thyroid-stimulating hormone returns to normal. Your GP will take a blood sample every four to six 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’ll probably need to take levothyroxine for the rest of your life. 

Causes of underactive thyroid

In the UK, the most common cause of an underactive thyroid is autoimmune thyroiditis, which is known as Hashimoto's thyroiditis. This is when your immune system attacks your thyroid gland, damaging it and stopping it producing enough thyroid hormone. If you’ve had your thyroid gland removed or radioactive iodine treatment for an overactive thyroid, this can cause an underactive thyroid too. 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 – some 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 quality of life suffers and it affects things like your work life, relationships and social life, for example.

Other complications of underactive thyroid include:

  • heart problems, such as chest pain (angina) or myxoedema – this is a rare but serious condition that can affect your heart – it can be life-threatening
  • osteoporosis – this can happen as a result of treatment
  • fertility problems

If you’re pregnant, an underactive thyroid can increase your chance of getting pre-eclampsia. It can also affect your baby’s development – they might be born too soon, for example. It can also lead to miscarriage. But you’ll be monitored regularly to aim to prevent these happening.

Diet and underactive thyroid

You don’t need to follow any special diet if you have an underactive thyroid, just aim to eat a healthy diet. You may worry about gaining weight, as this can happen with an underactive thyroid, but speak to your doctor about this. You can also find out more about weight after treatment in our frequently asked question (FAQ) below: Will I lose weight when I start treatment?

After treatment for underactive thyroid

Treatment for underactive thyroid raises the levels of thyroid hormone in your body. It’s important to make sure that these don’t become too high because you could develop symptoms such as:

  • chest pain
  • a racing heartbeat (palpitations)
  • anxiety
  • shaking (tremors)
  • changes to your bowel movements – you might go more often

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

Frequently asked questions

  • 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, sometimes longer.

    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.

  • Yes, you can take levothyroxine when you’re pregnant. You’ll need to have regular checks every six weeks 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.

  • 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.

    It’s understandable to worry about your weight if you have an underactive thyroid, as many people put weight on. 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 keep your weight healthy, it’s important to be active and eat the right foods. See Related information for tips and advice on healthy eating and exercise.

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

  • You’re classed as having borderline underactive thyroid if you have a raised 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 it goes up as you get older.

    Your doctor may suggest that you take the underactive thyroid medicine called levothyroxine if your TSH level is above 10mIU/L. This is because you may be more likely to develop an 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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Related information

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    • Hypothyroidism. NICE Clinical Knowledge Summaries., last revised April 2016
    • Overview of thyroid function. The MSD Manuals., last full review/revision April 2018
    • Hypothyroidism. Medscape., updated 25 February 2018
    • Cambridge dictionary: metabolism. Cambridge University Press., accessed 23 July 2018
    • Hair loss and thyroid disorders. British Thyroid Foundation., accessed 30 July 2018
    • Hypothyroidism. PatientPlus., last checked 7 September 2015
    • Thyroid function tests. British Thyroid Foundation., revised 2015
    • Thyroid. Oxford handbook of endocrinology and diabetes. Oxford Medicine Online., published March 2014
    • Levothyroxine sodium. NICE British National Formulary., reviewed 20 June 2018
    • Hypothyroidism. Medscape., updated 25 February 2018
    • 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 2014; 24(12):1670–751. doi: 10.1089/thy.2014.0028
    • Hypothyroidism. The MSD Manuals., last full review/revision April 2018
  • Reviewed by Rachael Mayfield-Blake, Freelance Health Editor, July 2018.
    Expert reviewer Mr Robert Hardy, Consultant Surgeon
    Next review due July 2021

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