What is an underactive thyroid (hypothyroidism)?

From underactive thyroid symptoms to thyroid tests and treatments, find out all you need to know about hypothyroidism.

Have you been diagnosed with an underactive thyroid? Or do you just want to know more about hypothyroidism? You’ve come to the right place.

In this blog, we discuss:

What is an underactive thyroid?

An underactive thyroid (also known as hypothyroidism) is where the thyroid gland doesn’t produce enough of the thyroid hormones T3 and T4 [1]. Thyroid hormones affect every cell in your body, and without them, your body’s metabolism starts to slow down, and you may start to experience some unwanted symptoms.

What causes an underactive thyroid?

An underactive thyroid can occur in men and women at any age but is more common in women over 60. One in 20 women over 60 has hypothyroidism [2].

Causes of hypothyroidism:

  • Autoimmune conditions – autoimmune thyroid disease is where the immune system attacks the thyroid gland, affecting hormone production. The most common type causing an underactive thyroid is called Hashimoto’s thyroiditis.
  • Response to treatments – some cancer treatments that target the neck and head can damage the thyroid gland. Replacement hormones are needed if an overactive thyroid gland is removed.
  • Certain medicines – some medicines and supplements could cause hypothyroidism. That includes medicines with large amounts of iodine, lithium (used to treat mental health conditions), or amiodarone (used to treat heart problems) [3].
  • Hereditary thyroid conditions – there is a strong family link in the development of thyroid disorders. Some babies can be born with hypothyroidism, so the UK uses blood spot tests to screen babies at five days old [4].

The cause of your hypothyroidism may be either primary or secondary.

Primary vs. secondary hypothyroidism

Primary hypothyroidism – in primary hypothyroidism, the problem lies with the thyroid gland itself – usually because it has been damaged and cannot produce enough thyroid hormones.

The most common reason for a damaged thyroid is an autoimmune disease called Hashimoto's thyroiditis. An autoimmune disease causes the immune system to attack and destroy cells and tissues, and in this case, the thyroid. The thyroid gland can also be damaged by thyroid cancer or during treatments for an overactive thyroid.

Secondary hypothyroidism - in secondary hypothyroidism, the thyroid functions normally, but there is a problem with the pituitary gland. The pituitary gland fails to produce enough thyroid-stimulating hormone (TSH) which is needed to trigger the release of thyroid hormones from the thyroid. This condition is much rarer than primary hypothyroidism.

Symptoms of an underactive thyroid

Low levels of thyroid hormones can slow down mental and physical processes. The symptoms of an underactive thyroid often develop slowly, and so could go unnoticed for years.

Common symptoms include:

  • Fatigue/tiredness
  • Weight gain
  • Sensitivity to the cold
  • Dry hair and skin
  • Muscle aches
  • Depression
  • Slow movements and thoughts
  • Low libido
  • Constipation
  • Brain fog and concentration issues

The intensity with which signs and symptoms appear may indicate the severity of the condition. However, many early symptoms are common with ageing, often leading to women and doctors mistaking hypothyroidism for menopause [5].

How is hypothyroidism diagnosed?

Typically, hypothyroidism is diagnosed with a consultation, physical examination, and blood tests.

Doctors will evaluate your medical history and check for physical signs such as dry skin, slow reflexes, thyroid swelling, and a slow heart rate [5]. Tell your doctor if you have any thyroid or autoimmune conditions in your family, as this may leave you at a higher risk of developing a thyroid condition.

A blood test is a very accurate method of diagnosing an underactive thyroid. In most cases, an underactive thyroid causes high levels of thyroid-stimulating hormone (TSH) and low levels of thyroxine (T4) [6]. However, in other cases, such as in central hypothyroidism (though very rare), your TSH levels may be low alongside low levels of T4. So, your blood tests should always be interpreted alongside your symptoms and history.

Check out our range of Thyroid Blood Tests or read our Thyroid Blood Test Buying Guide if you need help choosing which test is most suitable for you.

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How can an underactive thyroid be treated?

An underactive thyroid is a lifelong condition, but symptoms can be treated with daily hormone medication. Lifestyle changes may also help you to control symptoms.

Levothyroxine is a hormone replacement medication that raises thyroxine (T4) to normal levels. It has minimal side effects when taken at the right dose, which will be adjusted according to your blood test results. You might need frequent monitoring at first, but once hormone levels are stable, this can be reduced to an annual check-up. As hormones are restored to normal levels, symptoms should start to improve.

A balanced diet can support thyroid health. The thyroid needs iodine to function, which is found in fish, shellfish, dairy, eggs, and fortified plant milk. Take care with iodine quantities, as having too much can be as harmful to the thyroid as too little.

Have a look at our five ways to manage your thyroid condition.

What are the health risks of an underactive thyroid?

An underactive thyroid can cause problems if it’s not treated appropriately or early enough.

Complications of hypothyroidism include [7]:

  • causing the heart to slow down, and with time, can weaken it or cause permanent damage [5]
  • increased risk of heart disease
  • nerve damage
  • goitre
  • infertility
  • pregnancy complications

Hypothyroidism and pregnancy

Provided hypothyroidism is well controlled with treatment, it’s very possible to have a healthy pregnancy. Though thyroid hormone levels may affect your chances of falling pregnant.

Untreated hypothyroidism during pregnancy can increase your risk of certain complications, such as miscarriage, pre-eclampsia, premature birth, and birth defects. These risks can be minimised with specialist input from an endocrinologist – often, your target TSH level will be lower than before, and you may require a higher dose of levothyroxine [6].

Can an underactive thyroid get better by itself?

This depends. In most cases, hypothyroidism will not get better by itself, but there are certain situations where a thyroid condition can self-resolve. You can read more in our blog: can thyroid conditions go away by themselves?

Though often a life-long condition, hypothyroidism can be well managed with medication and the right lifestyle changes.

Is there a diet for an underactive thyroid?

Many people look to diet and lifestyle to see if they can support their thyroid function. The best advice is to make sure your diet is healthy and balanced. Add plenty of fresh fruit and vegetables, wholegrains, and lean protein.

What vitamins can support thyroid function?

Some vitamins and minerals are particularly important for thyroid function.

Micronutrients that can support thyroid function include:

• selenium
• iron
• B Vitamins
vitamin D

Please speak to your GP before taking any vitamin supplements, especially iodine. In some cases, taking too many supplements such as iodine can do more harm than good.

You can find out more information on food and your thyroid condition in our blog: what foods are good for thyroid health?

Goitrogenic foods and thyroid health

Some foods are goitrogenic, which means that they can inhibit thyroid hormone production, but this only really applies if you’re eating these foods in extremely large quantities and are iodine deficient.

Most goitrogenic foods also happen to be very nutritious, so generally it’s not advised to cut them out of your diet. There is also some evidence that they might help protect against thyroid cancer.

Goitrogenic foods include:

  • broccoli
  • spinach
  • bok choy
  • swede
  • turnips
  • soy products
  • peanuts
  • strawberries
  • peaches


What next steps can I take?

If you are experiencing any symptoms related to an underactive thyroid, visit your doctor. You can also test your thyroid function, hormones, and antibodies with our range of Thyroid Blood Tests.

Not sure what test is right for you? Take a look at our Thyroid Blood Test Buying Guide.

You can also find more information about your thyroid, including the links to insomnia and mental health, in our Thyroid Hub.



  1. Underactive thyroid (no date) NHS choices. NHS. Available at: https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/ (Accessed: April 27, 2023).
  2. Context: Thyroid disease: Assessment and management: Guidance (no date) NICE. Available at: https://www.nice.org.uk/guidance/ng145/chapter/Context (Accessed: April 27, 2023).
  3. Hypothyroidism (no date) British Thyroid Foundation. Available at: https://www.btf-thyroid.org/hypothyroidism-leaflet (Accessed: April 27, 2023).
  4. Congenital hypothyroidism (no date) NHS choices. NHS. Available at: https://www.gosh.nhs.uk/conditions-and-treatments/conditions-we-treat/congenital-hypothyroidism/#:~:text=In%20the%20UK%2C%20all%20babies,tests%20to%20confirm%20the%20diagnosis (Accessed: April 27, 2023).
  5. Koumourou, R (2004) Running on Empty: Hypothyroidism, Introduction to an Underactive Thyroid Gland. Melbourne: GN + EJ Ridgway. P. 213.
  6. Hypothyroidism (no date) British Thyroid Foundation. Available at: https://www.btf-thyroid.org/hypothyroidism-leaflet (Accessed: April 27, 2023).
  7. Hypothyroidism (no date) NHS choices. NHS. Available at: https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/complications/ (Accessed: April 27, 2023).


Why doesn't my doctor test for T3 (triiodothyronine)?
Although thyroxine (T4) is the most plentiful of the thyroid hormones, it is not biologically active and has no impact on your cells' metabolism. To be active, it must be converted to triiodothyronine (T3). This process takes place in the liver as well as other organs and most people convert T4 to T3 without difficulty, making testing for FT3 unnecessary for most people. However, a small number of people do not convert T4 to T3 easily and, for them, testing for FT3 is a useful investigation. This is especially the case if they are still experiencing symptoms of an underactive thyroid despite normal levels of TSH and FT4.
Why doesn't my doctor test for thyroid antibodies?
If your levels of thyroid hormones are normal you do not need to discuss a positive thyroid antibody result with your doctor unless you have been advised to do so by our doctors. Most cases of underactive thyroid are caused by Hashimoto's thyroiditis, an autoimmune condition. In this case, the thyroid gland is gradually destroyed, which diminishes its ability to produce thyroid hormones. In most cases of hypothyroidism, an autoimmune condition is assumed and does not require a confirmatory test.

A positive antibody result together with normal levels of thyroid hormones usually does not mean that you have a thyroid disorder. For every 100 healthy people without thyroid disease, between 10 and 20 will have a positive thyroid peroxidase antibody result and will not go on to develop a thyroid condition [9]. It is not necessary to test for thyroid antibodies routinely unless your endocrinologist has advised you to do so.
Why does Medichecks test for thyroid antibodies?
For someone without a diagnosis of thyroid disease, who may or may not be experiencing symptoms, knowing if their antibodies are raised can help them to understand whether there is a risk that they might develop an underactive thyroid in the future, given that elevated antibodies raise your future risk of thyroid disease [10].

People who have been diagnosed with thyroid disease and are undergoing treatment often monitor their thyroid antibodies along with their thyroid hormones. Autoimmune diseases often do not develop in a straight line; sometimes they flare up, which can cause a worsening of symptoms, while at other times they are stable or in remission.
How is an underactive thyroid treated?
An underactive thyroid is treated by replacing the thyroid hormone that is no longer being produced. For the vast majority, treatment is with levothyroxine, a synthetic form of thyroxine (T4). It can take several months to find the right dose of thyroxine.

For some people, especially if they do not convert T4 to T3 easily, treatment with thyroxine alone does not alleviate symptoms. Some people respond better by incorporating some T3 into their treatment by taking liothyronine (a synthetic form of T3) alongside their levothyroxine. Please be aware doctors may not be able to prescribe T3 and you may need to be referred to an endocrinologist.
What are the health risks of having an underactive thyroid?
Untreated or under-treated hypothyroidism carries serious health risks which include:
  • Obesity
  • Joint pain
  • Raised cholesterol
  • Increased risk of heart disease
  • Infertility
  • Increased risk of miscarriage
  • Increased risk of birth defects
  • Depression
  • Peripheral neuropathy (damage to your peripheral nerves)
  • Puffy face
Will an underactive thyroid get better by itself?
No, an underactive thyroid will not get better by itself because once the thyroid tissue is damaged, it will not be able to repair. Hashimoto's is not always a progressive disease, so people with this autoimmune disease may not need to increase their dose of thyroid replacement. However, most people with an underactive thyroid require thyroid replacement for the rest of their lives.
Is there a diet for an underactive thyroid?
Many people look to diet and lifestyle to see if they can support their thyroid function. In the case of the thyroid, some foods are called goitrogenic, which means that they can inhibit the thyroid to produce thyroid hormones.

Goitrogenic foods include vegetables from the cruciferous family like broccoli, spinach, bok choy, swedes and turnips, soy products and some fruits and plants like peanuts, strawberries, and peaches.

Many of these foods are nutritious, so it is best not to avoid them all. Their effect on your thyroid function can be reduced by cooking them beforehand. Some vitamins and minerals can support thyroid function, including iodine, selenium, iron, vitamin B12, folate and vitamin D.

Only consider taking iodine if you have a properly functioning thyroid; if you are already taking thyroxine, you do not need iodine. Do not take if you have an overactive thyroid as it can worsen your condition. Too much iodine can be as harmful to the thyroid as too little.

Have a look at our thyroid-friendly diet. for more information.

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