Underactive thyroid (hypothyroidism): why it happens and how to deal with it

13th May, 2022 • 12 min read

Are you feeling low and more tired than normal? Or perhaps you’re putting on weight for no obvious reason, and your skin has become very dry. These symptoms could mean you have a condition called underactive thyroid, also known as hypothyroidism.

You’re more likely to get it if you’re a woman. In the UK, for example, research suggests it affects about 15 in every 1,000 women – compared with just 1 in 1,000 men.

The symptoms of an underactive thyroid can be frustrating, especially if they start to affect your wellbeing and social life. Maybe you can’t fit into your favourite clothes anymore, or you don't feel up to enjoying a night out with friends. You might also be struggling to sleep at night, because you’re having to take daytime naps just to feel more energised.

We don’t want having an underactive thyroid to stop you living your life. So here’s everything you need to know about the condition, including the signs to look out for, and what you and your doctor can do about it.

What is an underactive thyroid?

Having an underactive thyroid means that the thyroid gland in your neck isn’t making and releasing enough thyroid hormones into your bloodstream. This is opposite of an

overactive thyroid
(hyperthyroidism), when your thyroid gland makes too much of these hormones.

Your thyroid hormones help to control your body’s metabolism, affecting everything from how you use energy to your breathing rate and the way your heart beats. So without enough of these hormones to go around, many of your bodily functions begin to slow down. This is why you can put on weight, because your body is burning fewer calories than normal.

If an underactive thyroid isn’t treated, it may lead to other health issues over time. But most of the time, it’s diagnosed before it becomes severe, and can be easily treated.

Read more about your

thyroid gland and how it works

Find useful information on other areas of thyroid health with our

complete Guide

Who’s more likely to get an underactive thyroid?

While anyone can develop an underactive thyroid, some factors make it more likely.

As well as being more common in women, you may be more at risk of an underactive thyroid if:

  • you’re over 60
  • you have a family history of thyroid problems
  • you’ve been
    within the last 6 months

Your thyroid is also more likely to be underactive if you have certain health problems, including:

What are the signs and symptoms of an underactive thyroid?

Underactive thyroid symptoms can develop slowly, and you may not notice them at first. Common symptoms include:

  • feeling more tired than normal
  • putting on weight
  • feeling low or
  • slow thoughts and movements – sometimes known as ‘brain fog’
  • being more sensitive to the cold
  • trouble pooing (
  • muscle aches, cramps or feelings of weakness
  • dry and scaly skin
  • brittle hair and nails
  • loss of sex drive (
    low libido
  • pain, numbness and tingling in your hand and fingers (
    carpal tunnel syndrome

Remember that lots of these symptoms can also be signs of other conditions – they may not mean you have an underactive thyroid. So it’s important to

see a doctor for a diagnosis

If you’re worried about your symptoms but you’re not sure if you need to see a doctor, try using our

Smart Symptom Checker
to help you work out your best next step.

Underactive thyroid symptoms in women

Having an underactive thyroid causes an imbalance of hormones, which can affect you differently if you’re a woman, especially if it’s not treated early.

As well as the symptoms listed above, other symptoms may include:

What causes an underactive thyroid?

There are several reasons why your thyroid might stop making enough hormones.

The most common causes of an underactive thyroid are:

  • autoimmune
    – thyroiditis is inflammation of your thyroid gland, and autoimmune thyroiditis happens when your immune system mistakenly attacks the cells in your thyroid gland. There are several types, but Hashimoto’s disease is the most common
  • thyroid treatment or surgery – having treatments or surgery on your thyroid gland or neck can lead to an underactive thyroid. This includes
    radioactive iodine treatment
    , which is often used to treat an overactive thyroid

Other causes include:

  • iodine deficiency
    – your body needs iodine to make thyroid hormones. Iodine deficiency isn’t common in the UK and US, but in some countries it’s the most common cause of an underactive thyroid. You can get iodine from foods such as seafood, seaweed, plants grown in iodine-rich soil, and iodised salt (table salt in the US)
  • some medicines – including lithium, used to treat
    bipolar disorder
    ; amiodarone, used to treat an irregular heartbeat (
    ); and interferons, used to treat certain types of
    hepatitis C
    . Speak to your doctor if you think your medication might be causing thyroid problems
  • other types of thyroiditis – including postpartum thyroiditis, which can happen after you give birth, and subacute thyroiditis, which can be caused by a viral infection. Read more about types of thyroiditis
  • pituitary gland disorders – your pituitary gland in your brain makes thyroid stimulating hormone (TSH), which tells your thyroid gland how much hormone to make. So a problem with this gland can affect how your thyroid gland works, though this is a rare cause of underactive thyroid

What are the long-term complications of having an underactive thyroid?

If an underactive thyroid goes untreated for a long time, it can lead to other health problems. These can include:

  • goitre
    – a swelling of your thyroid gland that causes an often painless lump in your throat. If it gets very large, it can affect your swallowing or breathing
  • mental health problems – low mood and depression are common early symptoms, and these can become more severe over time
  • heart problems – low levels of thyroid hormones can lead to
    high cholesterol
    , and also increase your risk of
    heart disease
    heart failure
  • infertility
    – as mentioned above, low levels of thyroid hormone can stop ovulation from happening, which can lead to fertility problems. Having an autoimmune disorder can also affect your fertility
  • pregnancy and birth problems – an untreated underactive thyroid can increase your risk of
    , going into labour early,
    , heavy bleeding after birth (postpartum haemorrhage) or having a baby with a birth defect

How to cope with an underactive thyroid

The symptoms of an underactive thyroid can have a big impact on your daily life, which can be frustrating and make you feel low.

“Going through the process of working out what might be wrong, then adjusting to having a long-term condition and taking medicine, can be a big challenge,” agrees Dr Adiele Hoffman, Healthily Clinical Content Reviewer. “But support from your doctor and specialist charities can help you find ways to reset that work for you.”

You should always speak to your doctor if you think you have a thyroid problem, because medical treatment is the key way to get your symptoms sorted.

Good self-care and other coping strategies can also help – read our tips for

living and coping with a thyroid problem

If you’re struggling with getting a diagnosis or managing your symptoms, remember that help and support is available from charities and online groups, including:

When to see a doctor

Getting a diagnosis

If you have symptoms of an underactive thyroid, or you notice a lump in your neck, speak to a doctor as soon as you can.

They can help you get the right diagnosis – even if it’s not an underactive thyroid – and work out the best treatment for you.

How to prepare for your appointment

Before you see your doctor, it’s worth doing a bit of preparation to help you get the right diagnosis.

In the week before your appointment, try:

  • keeping a symptom diary – use a notebook or your phone to record what your symptoms are, anything that triggers them and if they affect you at certain times of day. Try to be specific: for example, if you’ve put on weight, make a note of how much and over how long
  • getting to know your family history – your doctor will want to know if any of your close relatives have thyroid problems or other autoimmune diseases, and the details if so
  • creating a list of medicines – if you take any medication, vitamins or supplements, you should tell your doctor
  • making a note of any questions you might want to ask and any concerns you have, so you don’t forget them

How is an underactive thyroid diagnosed?

Common tests used to diagnose an underactive thyroid include:

  • physical exam – your doctor will listen to your symptoms and may examine your neck to see if your thyroid gland is swollen
  • blood tests – the levels of thyroid hormones and certain antibodies in your blood can show if you have a thyroid problem. You may also need blood tests to rule out other possible causes of your symptoms, or to check for other conditions that can be associated with an underactive thyroid
  • scans – you might have an ultrasound scan, thyroid scan or radioactive iodine uptake test to give your doctor more information about your thyroid gland, for example if you have a goitre or
    thyroid nodule
  • biopsy
    – if there’s a lump or nodule in your neck, a tissue sample may be taken to check for abnormal cells (though thyroid cancer is rare)

Learn more about these

thyroid tests and scans

Doctor’s tip: it can take time to get a diagnosis

“People with thyroid problems often have a long list of symptoms, because the thyroid gland is important for so many different bodily processes,” says Dr Hoffman. “It can be hard to address all these at once, and work out what’s going on in just 1 appointment. So if you don’t get to the bottom of your symptoms the first time you see a doctor, don't be put off or disheartened. See them again, and be reassured that they’ll work with you to figure out your symptoms. It could be that you’ll get an overall diagnosis such as underactive thyroid – or it’s also possible that there could be several different things going on.”

Underactive thyroid treatment

If you have an underactive thyroid, you’ll need medication from your doctor to increase your levels of thyroid hormones. This is usually a tablet called levothyroxine, which you’ll need to take every day. Read more about

underactive thyroid treatment

Self-care measures to ease thyroid symptoms
can also be helpful, when used alongside medication.

Can an underactive thyroid be cured?

The good news is that once it’s diagnosed, an underactive thyroid is a manageable condition.

While the medication isn’t a cure, it will bring your levels of thyroid hormones back to normal, which should ease your symptoms.

You’ll probably need to take medication for the rest of your life. Your thyroid hormone levels will be monitored regularly with blood tests, and your dose might change over time.

When to call an ambulance

In rare cases, an underactive thyroid that’s severe and left untreated can lead to a potentially life-threatening condition called myxoedema. It’s very uncommon these days, due to access to effective treatment. But you should call an ambulance if you have these symptoms:

Your health questions answered

What is subclinical hypothyroidism?

“Subclinical hypothyroidism is sometimes known as ‘mild thyroid failure’. It doesn’t always cause symptoms, so may only be found during routine blood tests. It’s diagnosed if

thyroid blood tests
show that your levels of thyroid stimulating hormone (TSH) arehigh, but your thyroid hormones are still normal. It means your pituitary gland is working a bit harder to keep your thyroid hormone levels normal, and you could have a higher risk of developing an underactive thyroid in the future. Your doctor may recommend testing your TSH levels again in 3 months. If they're still abnormal, whether you need treatment will depend on several things, including how high your TSH levels are, your age, and whether you have any symptoms. In some cases, your doctor may just want to keep an eye on your thyroid hormones by doing blood tests around once a year or every 2-3 years.”

Can an underactive thyroid change to an overactive thyroid?

“It’s more common for people with an overactive thyroid to develop an underactive thyroid. This can be caused by overactive thyroid treatment – such as radioactive iodine – or as part of thyroiditis, which often has an overactive phase before an underactive phase. It’s very rare for an underactive thyroid to switch to an overactive thyroid, although it is possible.”

Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.