What is an underactive thyroid?

Everything you need to know about an underactive thyroid (hypothyroidism), including symptoms, causes, and treatment.

In this guide we discuss:

What is an underactive thyroid?

An underactive thyroid, also known as hypothyroidism, is where the thyroid does not produce enough hormones [1].

The thyroid gland is a small butterfly-shaped gland positioned in the neck. It creates hormones that regulate the heartbeat, digestive system, and many other bodily functions. These hormones are triiodothyronine (T3) and thyroxine (T4), and without them, the body begins to slow down.

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].

Some babies can be born with hypothyroidism, so the UK uses blood spot tests to screen babies at five days old.

Causes of hypothyroidism:

  • Immune system – autoimmune thyroid disease is where the immune system attacks the thyroid gland, reducing its ability to produce hormones. The common name for this is Hashimoto’s thyroiditis.
  • Response to treatments – some cancer treatments that target the neck and head can damage the thyroid glands. Replacement hormones may be needed if an overactive 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) or amiodarone (used to treat heart problems) [3].

Primary hypothyroidism

In primary hypothyroidism, the problem lies with the gland itself – usually because it has been damaged and cannot produce enough 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. The thyroid gland can also be damaged by 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) to cause the thyroid to produce more hormones.

Hereditary conditions

There is a strong family link in the development of thyroid disorders – if a close family member already has thyroid disease (or any autoimmune condition), then your risk of developing one is raised. The risk also increased if you already have another autoimmune condition such as lupus, rheumatoid arthritis, or pernicious anaemia.

Symptoms of an underactive thyroid

The symptoms typically associated with an underactive thyroid include putting on weight, feeling cold and lacking energy, brain fog and constipation. Dry skin and hair, hair loss and muscle aches and pains are also common.

Low thyroid hormones result in mental and physical processes slowing down. These symptoms are often like other conditions and develop slowly, 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

The intensity in which signs and symptoms appear indicates the severity of the condition. However, many early symptoms are common with ageing, often leading to women and doctors mistaking hypothyroidism for menopause [4].

Have a look at the full range of thyroid disease causes, symptoms, and risk factors.

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, swelling, and a slow heart rate [5]. Tell your doctor if you have any thyroid or autoimmune conditions in your family.

It generally causes high levels of thyroid-stimulating hormone (TSH) and low levels of thyroxine (T4) [6]. However, there is a spectrum in thyroid disorders, and so other tests may be necessary.

GPs in the UK will normally test for TSH, but not always free thyroxine (FT4), which is the most plentiful thyroid hormone produced by the thyroid gland. If TSH is above the normal range (over 4.2 mIU/L) and FT4 is low, it indicates that the thyroid is beginning to struggle to produce hormones. Most doctors will begin treatment once TSH is over 10 mIU/L. Some doctors will consider treating a TSH below 10 mIU/L if it is accompanied by symptoms.

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.

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) levels to be normal. It has minimal side effects when taken in the right dose. Dosages are dependent on weight and blood results. Doctors will take regular blood tests to measure the correct dose and monitor hormone levels annually. Once treatment has begun and hormones are restored, symptoms should improve within several weeks.

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 can too little.

Have a look at our section on diagnosing and managing a thyroid condition.

What are the health risks of an underactive thyroid?

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

The condition causes the heart to slow down, and with time, can weaken it or cause permanent damage [7].

Hypothyroidism also commonly occurs with other autoimmune diseases such as:

Hypothyroidism and pregnancy

It is possible to have a healthy pregnancy with hypothyroidism. However, if it is not being treated, it could cause some complications.

Uncontrolled hypothyroidism in pregnancy can cause miscarriage, pre-eclampsia, and developmental problems in the newborn child. If you plan to get pregnant and have an underactive thyroid, check that your hormone levels are normal. As soon as you are pregnant, visit your GP to discuss arranging antenatal care. Your levothyroxine dose will likely increase and your target TSH level will be lower than before [8].

What next steps do I take if I think I have hypothyroidism?

If you have any concerns or are experiencing any symptoms related to an underactive thyroid, visit your doctor. You can also have a look at 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.

How to get medichecked for a thyroid condition

  1. Thyroid Function Blood Test – a comprehensive test that looks for TSH, FT4 and FT3 - the more biologically active form of FT4. This test helps you to take a more in-depth look at your thyroid hormones.
  2. Thyroid Function with Antibodies Blood Test – with this test, you can tell if your thyroid hormones are functioning within the healthy range. You can also detect any abnormalities caused by an autoimmune disease. Even if your thyroid function is currently normal, elevated antibodies may increase your risk of developing a thyroid disorder in the future.
  3. Advanced Thyroid Function Blood Test – this test includes the thyroid hormones, thyroid antibodies, and nutritional markers that can support thyroid function. If your nutritional markers are low, this can cause symptoms like a thyroid disorder. For example, low iron (ferritin), low vitamin D and low B vitamins can cause fatigue and low energy.



  1. nhs.uk. 2022. Underactive thyroid (hypothyroidism). [online] Available at: <https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/> [Accessed 4 March 2022].
  2. Nice.org.uk. 2022. Context | Thyroid disease: assessment and management | Guidance | NICE. [online] Available at: <https://www.nice.org.uk/guidance/ng145/chapter/Context> [Accessed 4 March 2022].
  3. British Thyroid Foundation. 2022. Hypothyroidism. [online] Available at: <https://www.btf-thyroid.org/hypothyroidism-leaflet> [Accessed 4 March 2022].
  4. Koumourou, R (2004) Running on Empty: Hypothyroidism, Introduction to an Underactive Thyroid Gland. Melbourne: GN + EJ Ridgway. P. 213.
  5. Healthline. 2022. What You Need to Know About Hypothyroidism. [online] Available at: <https://www.healthline.com/health/hypothyroidism/symptoms-treatments-more> [Accessed 4 March 2022].
  6. British Thyroid Foundation. 2022. Hypothyroidism. [online] Available at: <https://www.btf-thyroid.org/hypothyroidism-leaflet> [Accessed 4 March 2022].
  7. Koumourou, R (2004) Running on Empty: Hypothyroidism, Introduction to an Underactive Thyroid Gland. Melbourne: GN + EJ Ridgway. P. 213.
  8. British Thyroid Foundation. 2022. Hypothyroidism. [online] Available at: <https://www.btf-thyroid.org/hypothyroidism-leaflet> [Accessed 4 March 2022].
  9. Fröhlich, E. and Wahl, R., 2017. Thyroid Autoimmunity: Role of Anti-thyroid Antibodies in Thyroid and Extra-Thyroidal Diseases. Frontiers in Immunology, 8.
  10. Testing.com. 2022. Thyroid Antibodies - Testing.com. [online] Available at: <https://labtestsonline.org/tests/thyroid-antibodies#:~:text=In%20general%2C%20the%20presence%20of,increase%20in%20autoimmune%20disease%20activity> [Accessed 4 March 2022].


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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