What is an underactive thyroid?

From underactive thyroid symptoms to thyroid tests and treatments, here's all you need to know about hypothyroidism.

Do you have symptoms of an underactive thyroid, or have you just received a diagnosis and would like to learn more? We look at the causes, symptoms, testing, and treatments for hypothyroidism.

In this article, we cover:

What is an underactive thyroid?

An underactive thyroid (known as hypothyroidism) is where the thyroid gland doesn’t make enough of the hormones, thyroxine (T4) and triiodothyronine (T3) [1].

Thyroid hormones affect every cell in your body. If levels are too low, your body’s functions slow down, including your metabolism, and you may experience unwanted symptoms. 

Hypothyroidism is more common in women than men and the risk increases with age. Around one in 20 women over 60 have the condition [2].

Causes of hypothyroidism

The cause of an underactive thyroid can be primary, secondary, or tertiary:

  • Primary hypothyroidism — when the thyroid gland is unable to produce enough thyroid hormone, typically due to damage. An autoimmune disease often causes this damage, but other culprits include thyroid cancer and thyroid surgery.
  • Secondary and tertiary (also known as central) hypothyroidism— the thyroid functions normally, but there may be a problem with the pituitary gland or hypothalamus. Typically, there are reduced amounts of thyrotrophin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH), which causes the thyroid to make too few hormones. These conditions are much rarer than primary hypothyroidism.

Causes of hypothyroidism:

  • Autoimmune conditions — the most common cause of hypothyroidism is an autoimmune thyroid disease called Hashimoto’s thyroiditis. This happens if the immune system mistakenly triggers the release of anti-thyroid antibodies, which attack and destroy healthy thyroid tissues.
  • Thyroid treatments — such as surgical removal of all or part of the thyroid gland and radioactive iodine therapy.
  • Some cancer treatments — radiation therapy that targets the head or neck and certain chemotherapy drugs can damage the thyroid gland.
  • A congenital condition — some babies are born with hypothyroidism because their thyroid didn’t develop properly in the womb. This condition (known as congenital hypothyroidism) is uncommon. It’s usually picked up by blood spot tests, routinely used in the UK to screen babies soon after birth [3].
  • Iodine deficiency or excess.
  • Thyroiditis — inflammation of the thyroid gland.
  • Certain medicines including:
  • Lithium — a medicine sometimes used to treat certain mental health conditions
  • Amiodarone — a treatment for an irregular heartbeat (arrhythmia)
  • Interferons — used to treat some types of cancer and hepatitis C [4]

Is an underactive thyroid hereditary?

Having a close relative with a thyroid condition can increase your risk of developing one. Approximately 65% of your thyroid hormone production is attributed to genetics [5]. However, it’s not yet fully understood how hypothyroidism is inherited.

Researchers have linked several inherited genes to Hashimoto’s disease. If you have a close relative with the condition, it’s estimated that you’re nine times more likely to develop it too [6].

However, environmental factors, such as exposure to toxins and eating certain foods, can also negatively affect thyroid health. If you have hypothyroidism, it’s possible that genetic and environmental factors played a part — not just one or the other.

Signs and symptoms of an underactive thyroid

An underactive thyroid shares symptoms with several other conditions, making it easy to mistake the early warning signs of hypothyroidism for something else. Many of the symptoms are also common with ageing, and women and doctors often mistake hypothyroidism for menopause [7].

Typically, symptoms develop slowly, so you may not realise you have a thyroid problem for several years. The intensity of the initial symptoms may indicate the severity of the condition.

Common symptoms of hypothyroidism include:

  • Feeling tired all the time
  • Weight gain or difficulty losing weight
  • Sensitivity to the cold
  • Dry hair and skin
  • Muscle aches
  • Depression
  • Slow movements and thoughts
  • Low libido
  • Constipation
  • Brain fog including forgetfulness and difficulty concentrating

How can I lose weight with an underactive thyroid?

An underactive thyroid can make it difficult to lose weight, but focusing too much on cutting calories and shedding pounds might not be the best approach for your wellness.

Make sure you eat a healthy, balanced diet, with plenty of fresh fruit and veg, whole grains, and lean protein. These foods can help support your thyroid function and boost your overall health and wellbeing. Healthy weight loss may be a natural consequence of a new healthy eating approach.

Your diet should also give you enough of the micronutrients that are essential for thyroid health, such as iodine and selenium. Check your cholesterol levels too, as people with hypothyroidism often have raised levels of bad cholesterol.

It's key for you and your doctor to regularly monitor your thyroid to make sure you're on the right dose of thyroxine. This can have a big impact on your symptoms, including your weight.


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Diagnosing a thyroid condition

Your doctor will check for signs of an underactive thyroid, like thyroid swelling (goitre) or feeling tired all the time. They will also ask you about your medical history, including any family history of thyroid or autoimmune disease.

A blood test is the only accurate way to confirm a diagnosis of hypothyroidism. This measures your thyroid hormone levels and is known as a thyroid function test. In most cases, an underactive thyroid causes:

  • High levels of TSH and
  • Low levels of T4 [8]

In rare cases, such as central hypothyroidism, your TSH and T4 levels may both be low. So, your doctor will always interpret your blood test results alongside your symptoms and history.

If your TSH level is high, but your T4 is normal, you may be more likely to develop hypothyroidism in the future. In this case, your GP may recommend regular testing to monitor your levels.


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Treatment options

An underactive thyroid is a lifelong condition, but it can be treated with daily hormone replacement medication called levothyroxine.

Levothyroxine replaces missing thyroxine hormone and helps bring T4 to normal levels. Taken at the right dose, determined by your blood test results, it usually has no side effects.

If your doctor prescribes levothyroxine, you’ll have regular blood tests until you find the right dose. Finding the right dose may take time. Once you’ve found it, you’ll usually just need an annual blood test to monitor your hormone levels.

When your hormones return to normal levels, your symptoms should start to improve.

Can I boost my thyroid hormones naturally?

You can take steps to improve your thyroid health with diet and lifestyle changes — these can complement your treatment and help improve your symptoms.

Natural ways to improve your thyroid health:

Can an underactive thyroid get better by itself?

In most cases, hypothyroidism will not get better by itself, but there are situations where certain thyroid conditions can self-resolve. These include some types of subclinical hypothyroidism like subacute thyroiditis, which is usually triggered by a viral infection.

Health risks of hypothyroidism

Untreated hypothyroidism can lead to other serious health problems.

Complications of hypothyroidism include:

  • Heart problems — there are several reasons why hypothyroidism is linked to heart problems. For example, people with an underactive thyroid often develop high levels of low-density lipoprotein (LDL), known as bad cholesterol. This increases the risk of cardiovascular disease including heart attacks.
  • Nerve damage — if left untreated for a long time, hypothyroidism can damage the peripheral nerves, causing pain and numbness in the arms and legs.
  • Goitre — a swelling of the thyroid gland, which may cause problems with breathing or swallowing.
  • Infertility — low levels of thyroid hormone can interfere with ovulation, which can impact fertility. Some causes of hypothyroidism, such as autoimmune disease can also limit fertility.
  • Pregnancy complications — e.g. an increased risk of pre-eclampsia and premature birth.
  • Myxoedema coma — a rare, life-threatening condition that can happen in severe cases of hypothyroidism and requires emergency hospital treatment.

Thyroid function tests

You can check your thyroid function, hormones, and antibodies with our range of Thyroid Blood Tests. These tests help you investigate symptoms, rule out other possible causes such as vitamin deficiencies, and monitor your thyroid hormone levels with regular testing.

Pay attention to any niggling symptoms or health concerns you think may be linked to an underactive thyroid. Hypothyroidism is highly treatable and early detection can help prevent future health complications. You can find more information about your thyroid in our Thyroid Hub.


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1. NHS Choices (2021) Underactive thyroid (hypothyroidism). Available at: https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/ (Accessed: 11 March 2024).

2. NICE (2023) Thyroid disease: assessment and management. Available at: https://www.nice.org.uk/guidance/ng145/chapter/Context (Accessed: 11 March 2024).

3. NHS Great Ormond Street Hospital for Children NHS Foundation Trust (2015) Congenital hypothyroidism. 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: 11 March 2024).

4. NHS Choices (2021) Underactive thyroid (hypothyroidism) causes. Available at: https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/causes/ (Accessed: 12 March 2024).

5. Babić Leko, M. et al. (2021) Environmental factors affecting thyroid-stimulating hormone and thyroid hormone levels, MDPI. Available at: https://www.mdpi.com/1422-0067/22/12/6521 (Accessed: 11 March 2024).

6. Bothra, N. et al. (2017) Hashimoto's thyroiditis: relative recurrence risk ratio and implications for screening of first-degree relatives, Clinical Endocrinology (Oxford), 87 (2), p. 201-206. doi: 10.1111/cen.13323. Epub 2017 May 31. PMID: 28273382.

7. Koumourou, R. (2004) Running on Empty: Hypothyroidism, Introduction to an Underactive Thyroid Gland. Melbourne: GN + EJ Ridgway, p. 213.

8. British Thyroid Foundation (2021) Your guide to hypothyroidism. Available at: https://www.btf-thyroid.org/hypothyroidism-leaflet (Accessed: 11 March 2024).


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.
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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