An underactive thyroid is what happens when your thyroid gland doesn't produce enough hormones. It's also known as hypothyroidism. Underactive thyroid symptoms include tiredness, weight gain, and difficulty concentrating. You can check your thyroid function with a blood test.

What is an underactive thyroid?

Your thyroid gland is found at the front of your neck. It produces two hormones that help control your metabolism — the chemical processes that occur in your body that keep you alive, like breathing and breaking down food into energy. These hormones are called triiodothyronine (T3) and thyroxine (T4).

If you don’t make enough of these hormones you have an underactive thyroid — also known as hypothyroidism.

You can check your thyroid hormone levels at home by using a thyroid blood test. This will check your thyroid-stimulating hormone levels, which is responsible for producing T3 and T4. 

Woman touching thyroid gland on front of neck

What is the thyroid-stimulating hormone (TSH)?

TSH is produced by the pituitary gland in your brain. Its role is to regulate the production of T3 and T4. It’s located at the base of your brain near the hypothalamus, which also helps to regulate hormones as well as body temperature.

Sometimes your body won’t produce enough circulating thyroid hormones. When this happens, your body sends a signal to the hypothalamus to produce thyrotropin-releasing hormone (TRH) which stimulates the pituitary gland to produce more T3 and T4.

This means your TSH levels will be high when your thyroid hormone levels are too low. So a blood test showing high TSH levels is a sign of an underactive thyroid.

Problems with your thyroid function are most commonly caused by issues in the thyroid gland itself, rather than your hypothalamus or pituitary gland in your brain.

What are the symptoms of an underactive thyroid?

The most common underactive thyroid symptoms include:

  • fatigue
  • weight gain
  • difficulty concentrating
  • dry skin
  • brittle hair and nails
  • feeling sensitive to the cold
  • depression
  • constipation
  • muscle aches, cramps, or weakness
  • low sex drive (libido)
  • irregular or heavy periods

Underactive thyroid symptoms are similar to other conditions and can develop over time, so you might not notice them for years.

If you experience any underactive thyroid symptoms, you should see your GP for advice. If an underactive thyroid is left untreated, it can lead to an increased risk of heart disease.

What causes an underactive thyroid?

Autoimmune thyroiditis

Autoimmune thyroiditis is the most common cause of an underactive thyroid.

Usually, your body’s immune system makes antibodies that fight bacteria and viruses. Autoimmune thyroiditis causes your body to produce antibodies that damage your thyroid gland. This means your thyroid gland stops functioning properly and your T4 levels drop — leading to an underactive thyroid.

Autoimmune thyroiditis is linked to other autoimmune conditions, including:

  • coeliac disease — when your immune system damages your gut if you eat gluten
  • Addison’s disease — when your adrenal glands (on top of your kidneys) don’t produce enough hormones
  • pernicious anaemia — when your immune system mistakenly attacks cells in your stomach and you can’t absorb enough vitamin B12
  • vitiligo — a skin condition that affects your skin pigmentation
  • type 1 diabetes

Other related thyroid diseases are:

  • Hashimoto’s disease — a common cause of autoimmune hypothyroidism, (usually causes swelling at the front of your neck, called goitre) 
  • atrophic autoimmune hypothyroidism — a very rare thyroid disease similar to autoimmune hypothyroidism, but without goitre

Surgery or treatment to the thyroid gland

An underactive thyroid can also occur as a side effect of treatment to a previously overactive thyroid gland.

Postpartum thyroiditis

In rare cases, your thyroid gland can become inflamed — usually within the first 6 months after giving birth. This can cause a temporary increase in your thyroid hormone levels (thyrotoxicosis), resulting in symptoms of an overactive thyroid.

After a few weeks, your thyroid gland uses all of the hormone supply. This leads to low thyroid hormone levels and symptoms of an underactive thyroid — but not all women will experience both of these.

Postpartum thyroiditis isn't usually permanent, and your thyroid gland should go back to normal 12 months after giving birth. But very rarely, you might develop a permanent problem with your thyroid gland.

Iodine deficiency

Your body needs a nutrient called iodine to make T4. So a lack of iodine in your diet means your T4 levels drop, leading to hypothyroidism.

Iodine-rich foods include:

  • fish — like cod and tuna, seaweed, shrimp, and other seafood
  • dairy products — like milk, yoghurt, and cheese
  • iodized salt (table salt)

Iodine deficiency is the most common cause of hypothyroidism worldwide — but it's not common in the UK.


Some medications used to treat other illnesses can cause hypothyroidism as a side effect.

The medication most commonly associated with thyroid disease is amiodarone — it's used to treat heart abnormalities.

How to test for an underactive thyroid

You can monitor your thyroid health regularly by using an at-home thyroid blood test.

In some cases, your GP might refer you to an endocrinologist (a specialist in hormone disorders) if you:

  • are younger than 16
  • are pregnant or are trying to conceive
  • have just given birth
  • have heart disease 
  • are taking medication that affects thyroid hormone levels — like amiodarone or lithium

What is the treatment for an underactive thyroid?

The most common underactive thyroid treatment is a medication called Levothyroxine — a hormone replacement used to raise your T4 levels. It’s usually taken orally as a tablet or a drinkable liquid. 

Levothyroxine starts working as soon as you take it, but your symptoms might not improve for a few weeks. Your GP will carry out regular blood tests to see how well the medication is working and will adjust your dosage if necessary. 

If you’re pregnant or are thinking about conceiving, it’s important to let your doctor know as Levothyroxine and your thyroid function need to be closely monitored during pregnancy. 

Levothyroxine isn’t suitable for some people — your GP will be able to advise if this is the right treatment for you. 

What is subclinical hypothyroidism?

Subclinical hypothyroidism (SCH) occurs when your TSH levels are above the upper end of the normal range (0.5mU/L - 4.5mU/L), but your T4 levels are normal.

Most people with subclinical hypothyroidism will go on to develop hypothyroidism in the future. Your risk of developing hypothyroidism is increased with the presence of thyroid peroxidase (TPO) antibodies, which can be a sign of autoimmune thyroid disease. This is when your immune system produces abnormal TPO antibodies that mistakenly attack healthy tissue.

Current UK guidelines suggest treating a person under the age of 70 with levothyroxine if their TSH level is greater than 10mU/L — even if they don't have any symptoms.

If your TSH is between 4 and 10mU/L, treatment should be considered if you have symptoms of hypothyroidism.

The symptoms of hypothyroidism can be very non-specific — like feeling tired all of the time. So it’s important for your doctor to rule out other causes of your symptoms before diagnosing subclinical hypothyroidism. 

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