Underactive thyroid (hypothyroidism)

Your health experts: Mr Robert Hardy, Consultant Surgeon and Dr Ade Adeniyi, Bupa Clinics GP
Content editor review by Dr Kristina Routh, December 2020
Next review due December 2023

If you have an underactive thyroid (hypothyroidism), it means your thyroid gland isn’t producing enough thyroid hormones.

Common symptoms of an underactive thyroid include tiredness, weight gain and depression. Treatment with replacement thyroid hormones usually works well to improve your symptoms.

An image showing the location of the thyroid gland and surrounding structures

About underactive thyroid

Your thyroid gland is in your neck, in front of your windpipe. It produces two main hormones called thyroxine (T4) and triiodothyronine (T3).

Thyroid hormones affect tissues throughout your body, helping to control your metabolism. Your metabolism is the combination of all the chemical processes that happen in your body including those that change the food you eat into energy. Too little thyroid hormone slows down your body’s metabolism, which can cause a wide range of symptoms. See our section on symptoms for more information.

Around one or two out of every 100 people develop an underactive thyroid. Women are up to 10 times more likely than men to develop this condition.

Symptoms of underactive thyroid

The symptoms of an underactive thyroid usually develop slowly and gradually. They can be quite vague and can be caused by many other conditions. This can sometimes make it hard to diagnose. You might not get any symptoms at all.

If you do get underactive thyroid symptoms, they may include:

  • putting on weight
  • having less of an appetite for food
  • feeling the cold
  • dry skin
  • feeling weak and tired and having no energy
  • a puffy face
  • muscle or joint pain
  • losing your hair, which may become generally sparse (but usually grows back with treatment)
  • a hoarse voice
  • constipation
  • a slow heartbeat
  • if you’re a woman, your periods may be heavy, irregular or stop completely

Having an underactive thyroid can affect your mental health too. You may become forgetful and find it difficult to concentrate. And you might feel emotionally up and down, and become depressed.

If you have any of these symptoms, contact your GP.

Diagnosis of underactive thyroid

Your GP will ask about your symptoms and examine you. They may also ask you about your medical history.

If your GP thinks you may have an underactive thyroid, they’ll ask you to have one or more blood tests. This is to check the level of thyroid hormones in your body. There are two main hormones measured in blood tests.

  • TSH (thyroid-stimulating hormone). This is made by your pituitary gland to control your thyroid gland. It tells your thyroid to make more hormones if the levels of thyroid hormone in your blood get too low.
  • FT4 – free T4 (thyroxine). This hormone is made by your thyroid gland.

You may hear these blood tests referred to as ‘thyroid function tests’. If you have an underactive thyroid, you’ll probably have a TSH level above normal and a free T4 level below normal.

If the results of the blood tests show that you have an underactive thyroid, you’ll usually be treated by your GP. But sometimes, your GP will refer you to an endocrinologist – a doctor who specialises in treating hormone problems such as thyroid conditions.

Subclinical (borderline) hypothyroidism

Some people have a mildly underactive thyroid, with few or no symptoms. Doctors call this subclinical hypothyroidism. If you have this, your blood test will show that your TSH is raised but your free T4 is normal. This is fairly common, especially in older people. Some people with this condition go on to get symptoms of underactive thyroid as the years go by.

If you have subclinical hypothyroidism, your GP will offer to monitor your thyroid hormones every year to check whether your thyroid is becoming less active. See our treatment section for more information about treating borderline underactive thyroid.

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Treatment of underactive thyroid

The aim of treatment of an underactive thyroid is to ease your symptoms and prevent any complications. This means getting your thyroid hormone levels back to normal. Once your treatment is working well, you should feel much better. You might need to adjust your treatment every now and then as your condition progresses. If your symptoms continue to affect your quality of life, talk to your doctor.

Some people with just a mildly underactive thyroid (subclinical hypothyroidism) will benefit from having treatment too. This depends on your age, your blood test results, whether you’ve had treatment for thyroid problems in the past, and your symptoms. If you’re not having treatment, your GP will usually check your thyroid hormone levels every year. If you develop symptoms, your GP may suggest then that you start having treatment.


If you have an underactive thyroid, your doctor may prescribe you a hormone replacement medicine called levothyroxine (thyroxine). You take this once a day as a tablet. If you’re over 65 or have heart disease, you’ll start by taking a small amount of levothyroxine. This will be increased every six weeks to give your body chance to adjust gradually.

After you start taking levothyroxine, your GP will take a blood sample every four to six weeks to check that your TSH level is returning to normal. They may need to adjust your dose, depending on the results of these tests.

It’s important to read the patient information leaflet that comes with your medicine. If you have any queries about taking your medicine, you can ask your pharmacist for advice.

It can take a while for your hormone levels to get back to normal, so it may be a few months until you start to feel better. Once your thyroid levels have returned to normal, you’ll usually have a blood test once a year to check your thyroid hormone levels. You’ll probably need to take levothyroxine for the rest of your life.

Causes of underactive thyroid

In the UK, the most common cause of an underactive thyroid is autoimmune thyroiditis, which is known as Hashimoto's thyroiditis. This is when your immune system attacks your thyroid gland, damaging it and stopping it producing enough thyroid hormone.

On the other hand, you may have had your thyroid gland removed or had radioactive iodine treatment for an overactive thyroid. This can cause an underactive thyroid. Radiotherapy for thyroid cancer can also lead to an underactive thyroid.

Other causes of underactive thyroid include:

  • some medicines, such as lithium and amiodarone
  • an infection caused by a virus (thyroiditis)
  • pregnancy – some women develop an underactive thyroid in the first year after their baby is born

Complications of underactive thyroid

The symptoms of an underactive thyroid such as physical and mental tiredness and lack of energy can have a serious effect on your day-to-day life. You may find that your quality of life suffers and it affects things like your work life, relationships and social life.

Other complications of underactive thyroid include:

  • heart problems such as chest pain (angina), coronary heart disease and heart failure
  • fertility problems
  • myxoedema – this is a rare but serious condition (sometimes life-threatening), usually linked with a severe underactive thyroid which hasn’t been treated

Complications that can happen as a result of treatment for an underactive thyroid include osteoporosis and atrial fibrillation (an irregular heart rhythm). For information on things to watch out for when you’re taking levothyroxine, see our section: After treatment for underactive thyroid.

Underactive thyroid and pregnancy

If you’re pregnant, an underactive thyroid can increase your chance of getting pre-eclampsia. It can also affect your baby’s development – they might be born too soon, for example. Having an underactive thyroid can also lead to miscarriage. But you’ll be monitored regularly to aim to prevent these effects.

Diet and underactive thyroid

You don’t need to follow any special diet if you have an underactive thyroid, just aim to eat a healthy diet. You may worry about gaining weight because this can happen with an underactive thyroid, but speak to your doctor about this. You can find out more about weight after treatment in our FAQ: Will I lose weight when I start treatment?

After treatment for underactive thyroid

Treatment for underactive thyroid raises the levels of thyroid hormone in your body. It’s important to make sure that these don’t become too high because you could develop symptoms such as:

  • chest pain
  • a racing heartbeat (palpitations)
  • anxiety
  • shaking (tremors)
  • changes to your bowel movements – you might go more often

If you develop any of these symptoms, contact your GP.

Most people make a full recovery and feel completely better once their hormone levels get back to normal. But don’t worry if it takes a few months for your symptoms to disappear after starting treatment – this is to be expected. Some people find that they don’t feel completely better until about six months after their hormone levels get back to normal.

Around one in every 20 people with an underactive thyroid will still have some symptoms even when their hormone levels are normal. If this happens, your GP may arrange for you to have further tests to see whether something else is causing your symptoms.

Yes, you can take levothyroxine when you’re pregnant, and it’s important to have the right amount of thyroid hormones in your bloodstream during pregnancy. While you’re pregnant, your body usually needs more thyroid hormone. Because of this, your doctor may need to increase the amount of levothyroxine you take.

Thyroid hormones are important for your baby’s development right from the beginning of pregnancy. So, if you know you have an underactive thyroid, tell your GP as soon as you think you’re pregnant. You’ll need to have regular checks every six weeks during your pregnancy to make sure your thyroid hormones are at a healthy level.

After your baby is born, the amount of levothyroxine you need to take should go down again.

It’s understandable to worry about your weight if you have an underactive thyroid because many people do put weight on. Having an underactive thyroid means that your metabolism will be slower, so you’ll use less energy (calories). But this doesn’t seem to be the main cause of weight gain with an underactive thyroid. Much of the weight gain is because your body holds on to more water than usual, rather than because you’re storing fat.

When you start treatment, you can expect a small amount of weight loss as your body no longer holds onto water. But you may not see a large weight loss because your weight is determined by many factors over time, not just your thyroid hormones. If all your other symptoms of an underactive thyroid get better, but you still have excess weight, this probably isn’t due to your thyroid condition.

If you want to keep your weight healthy, it’s important to eat the right foods and be active. For tips and advice on healthy eating, exercise and how to lose weight safely, see our section: Related information.

If your weight changes a lot, see your doctor. They may need to change the amount of levothyroxine you take.

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