Underactive thyroid

Last updated:

Reviewed by:

Dr Bryony Henderson

Medically reviewed

An underactive thyroid (or hypothyroidism) is when your thyroid gland does not produce enough hormones. Although it can have serious complications if left untreated, treatment is very effective and most people will live a normal life.

What is an underactive thyroid?

Your thyroid is a small gland, shaped like a butterfly, that’s found at the front of your neck just below the larynx. When your thyroid is underactive, it produces insufficient levels of the two thyroid hormones, thyroxine (T4) and triiodothyronine (T3).

Thyroid hormones play an important role in regulating energy, metabolism, body temperature and weight, muscle strength, appetite, growth and the reproductive system. It also helps keep your heart, brain and kidneys healthy.

When your thyroid is functioning well, your thyroid hormones are in balance. If you have an underactive thyroid, you may develop common symptoms like weight gain, depression and tiredness.

What are the symptoms of an underactive thyroid?

Since thyroid hormones have a number of different functions, the symptoms of an underactive thyroid can be very generalised and often confused for something else. It may also go unnoticed for a number of years as symptoms can be subtle. Some common symptoms to look out for include: 

  • Weight gain

  • Tiredness 

  • Feelings of depression

  • Dry skin 

  • Thin hair and brittle nails 

  • Constipation 

  • Muscle aches and weakness

  • Loss of libido (sex drive) 

  • Pain, numbness and tingling sensation in hands and fingers 

  • Irregular or heavy periods for women

Symptoms will vary from person to person. Elderly people will often suffer from memory problems, children may experience slower growth than usual and teenagers may go through puberty earlier than expected. 

A more dangerous complication can involve the heart, such as risks of angina or a heart attack. Other symptoms that can develop if left untreated include a slow heart rate, a hoarse voice or hearing loss.

What causes an underactive thyroid?

Autoimmune conditions are the most common cause of an underactive thyroid in the UK. This is when your immune system attacks the thyroid gland resulting in damage to the gland and less hormones being produced. The most common type is known as Hashimoto’s disease. Although the cause is not clear, it can run in families and is more common in people with type 1 diabetes and vitiligo. 

Having thyroid treatment, as a result of an overactive thyroid or thyroid cancer is another common cause. Iodine deficiency can cause an underactive thyroid too, but this is less common. 

There are certain drugs that have been linked with an underactive thyroid including lithium (used for bipolar), amiodarone (used for irregular heartbeats) and interferons (sometimes used to treat cancer or hepatitis C).

An underactive thyroid in women

Women are 5 to 10 times more likely to be affected by hypothyroidism than men.  Scientists do not know exactly why this is the case, but autoimmune conditions, such as hypothyroidism, are more common in women than men. 

One theory is that there may be an interaction between female and thyroid hormones. Although hypothyroidism can happen at any time, it is more common during and after the menopause. As the symptoms of the menopause and hypothryoidism are so similar, women often remain undiagnosed.

An underactive thyroid in pregnancy

Similarly, changes to the menstrual cycle can affect your fertility and pregnancy. Having low thyroid hormone levels is important to be aware of if you're pregnant as it can cause health problems to both the mother and baby. If you do have an underactive thyroid, you will generally be monitored by a specialist during your pregnancy. 

Screening for thyroid disease in pregnant women is carried out if the woman is over 30 or has any medical history of thyroid problems. If you have any concerns about your thyroid or related symptoms, speak to a doctor.

How to test for an underactive thyroid

You should speak to a GP if you think you have symptoms of an underactive thyroid. Your GP will ask about the symptoms and request a blood test to look at your thyroid function (TFT) if needed. 

These examples below may mean you have or are at risk of developing hypothyroidism.

  • High TSH and low T4 hormone – could indicate an underactive thyroid.

  • High TSH but normal T4 hormone – may put you at risk of developing an underactive thyroid. This is sometimes termed ‘subclinical hypothyroidism.'

How to treat an underactive thyroid

An underactive thyroid can be treated by replacing the thyroid hormones in your body with a drug called levothyroxine. You will initially be started on a low dose and then it is increased gradually depending on how your body responds.

Your GP or doctor will carry out regular blood tests to monitor your hormone levels and find the correct dose. It is uncommon for people to suffer with side effects from levothyroxine and these will typically only occur if the dosage is too high. Possible side effects are sweating, chest pain, headaches and diarrhoea. 

If a blood test suggests you have an underactive thyroid but you don’t have any symptoms, you may not require drug treatment. Instead, your GP may order blood tests at regular intervals  to monitor your levels.

If you think you are experiencing symptoms of an underactive thyroid, it’s important to speak to a doctor.

Last updated:
Reviewed by:
Dr Bryony Henderson

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