Underactive thyroid (hypothyroidism)

How your thyroid gland can become underactive


About underactive thyroid

If you have underactive thyroid, it means your thyroid gland isn’t producing enough thyroid hormone.

Your thyroid gland is in your neck, in front of your windpipe. Your thyroid gland releases hormones (chemical messengers) to control how quickly your body uses its energy stores and how sensitive your body is to other hormones.

       The position of the thyroid gland and surrounding structures

Underactive thyroid is more common in older people, and women are more likely to be affected than men.

Symptoms of underactive thyroid

Symptoms of underactive thyroid can include:

  • extreme tiredness
  • weight gain, but with a poor appetite
  • feeling the cold easily
  • dry and/or pale skin
  • coarse, thinning hair and brittle nails
  • weak and sore muscles
  • a hoarse, croaky or deeper voice
  • puffy eyes
  • memory loss and poor concentration
  • constipation
  • heavy, irregular or prolonged menstrual periods, if you’re a woman
  • tingling in your hands and arms
  • depression
  • poor sex drive
  • slow heart rate
  • enlarged thyroid gland (goitre), which causes a lump to develop in your throat

These symptoms aren’t always caused by underactive thyroid but if you have them, see your GP.

Complications of underactive thyroid

In women, underactive thyroid can interfere with your menstrual cycle, leading to heavy, irregular or prolonged periods. This may lead to anaemia and also affect your ability to ovulate, making it more difficult for you to get pregnant.

Underactive thyroid in pregnancy can increase the risk of having a miscarriage, stillbirth, pre-eclampsia, premature birth, or a baby with low birthweight or impaired cognitive function.

Causes of underactive thyroid

In the UK, the two most common causes of underactive thyroid are as follows.

  • Autoimmune thyroiditis (Hashimoto's thyroiditis). Your immune system produces antibodies that attack your thyroid gland. This prevents your thyroid gland from producing enough thyroid hormones.
  • Surgery or radioiodine treatment for an overactive thyroid (hyperthyroidism) or thyroid cancer. If you have treatment that destroys thyroid tissue, it can lead to hypothyroidism.

Other, less common causes of underactive thyroid include the following.

  • Congenital hypothyroidism. This is when babies are born with an underdeveloped thyroid gland. In the UK, a heel-prick blood sample is taken between six and 14 days after birth to test for congenital hypothyroidism.
  • Certain medicines. Some medicines can affect the normal functioning of your thyroid gland and cause underactive thyroid. Examples include lithium carbonate (for bipolar disorder) and amiodarone (for heart rhythm problems).
  • Disorders of the pituitary gland. This gland is involved in the regulation and production of thyroid hormones and having any problems with it can affect the normal functioning of your thyroid.
  • Iodine deficiency. Your body needs iodine to make thyroid hormones. Not having enough iodine in the diet is the leading cause of underactive thyroid worldwide, but is uncommon in the UK.

Diagnosis of underactive thyroid

Your GP will ask about your symptoms and examine you. He or she may also ask you about your medical history and take a blood test to check your thyroid function.

If blood tests confirm you have underactive thyroid, your GP may refer you to an endocrinologist (a doctor who specialises in identifying and treating conditions of the endocrine system).

Treatment of underactive thyroid

Treatment aims to return your levels of thyroid hormones to normal. If you have underactive thyroid but don’t have any symptoms (known as subclinical hypothyroidism), you may not need any treatment. Your GP will usually monitor how your thyroid hormone levels change every few months. If you develop symptoms, your GP will suggest that you start treatment.


Your doctor may prescribe thyroid hormone replacement medicine (levothyroxine). This is taken daily as a tablet, but can take some time to get the dose right. Usually you will start on a low dose that is gradually built up every few months. You will need regular blood tests to check your levels of thyroid hormones. Your doctor will adjust your dose according to the results of your blood tests.

Once the correct dose of levothyroxine has been established, you will usually have a thyroid function test once a year to monitor your thyroid hormone levels. You will need to take this treatment for the rest of your life.

Special considerations

If you’re pregnant or breastfeeding

If you’re taking levothyroxine, your thyroid hormone levels should be closely monitored during and after pregnancy. As soon as you know you’re pregnant, you should have a thyroid blood test. Your requirements of levothyroxine will usually be higher during pregnancy, so your dose will need to be carefully adjusted according to your blood results.

What blood tests are used to diagnose an underactive thyroid?


To assess your thyroid function, your GP will take a sample of your blood and check your levels of thyroid-stimulating hormone, and the hormones released by your thyroid gland (thyroxine and triiodothyronine).


Thyroid-stimulating hormone is produced by a part of your brain called the pituitary gland. It stimulates your thyroid gland to produce the thyroid hormones thyroxine and triiodothyronine. Together, these hormones control how quickly your body uses its energy stores (metabolism) and how sensitive your body is to other hormones.

To assess your thyroid function, your GP will take a sample of your blood and measure the levels of thyroid-stimulating hormone, thyroxine and triiodothyronine in your blood.

If the results show you have high levels of thyroid-stimulating hormone with low levels of thyroxine, it means you have an underactive thyroid.

How long will it take for me to feel better once I've started taking levothyroxine for underactive thyroid?


You should expect a gradual improvement, feeling back to normal about four to six months after starting levothyroxine treatment. Once your thyroid hormone levels are back to normal, you may start to feel better fairly quickly, but this varies from person to person.


Treatment with levothyroxine aims to return your levels of thyroid hormones to normal.

If you’re over 18, your doctor may start you on a levothyroxine dose of 50 to 100 micrograms (μg) once a day. You will be monitored regularly and your dose will be adjusted in steps of 25 to 50μg every three to four weeks according to your response. The usual maintenance dose is 75 to 150μg once a day.

Your symptoms will begin to improve as the hormone levels in your bloodstream return to normal. The length of time this takes will depend on how severe your underactive thyroid was when you were diagnosed and on the dose you start on. It can sometimes take several months after your thyroid levels are stable before you start feeling better.

You will be closely monitored by your doctor, especially in the first six months of treatment. It’s important to let your doctor know if you don’t feel any better, or if you have any side-effects.

Side-effects to look out for include diarrhoea, vomiting, chest pains, irregular heartbeat, flushing, weight loss, headaches and muscle cramps. Having side-effects may be a sign that the dose of levothyroxine you’re taking is too high, so it’s important to tell your doctor so that your dose can be reduced.

Is it safe to take levothyroxine for underactive thyroid during pregnancy?


Yes, it's safe to take levothyroxine during pregnancy. You will need to be closely monitored during your pregnancy to ensure your thyroid hormone levels are at a healthy level.


During pregnancy, it's important that you have enough thyroid hormones in your bloodstream. This is because in the first trimester (first 12 weeks) of pregnancy, thyroid hormones help to develop your baby’s brain and nervous system.

If you have an underactive thyroid, you must see your GP as soon as you think you’re pregnant. Your doctor will usually increase your dose of levothyroxine to ensure you have enough thyroid hormones for you and your baby. The increase will depend on the current dose you're taking and the results of your blood test.

Your thyroid hormone levels will be regularly checked during your pregnancy. If you have only been diagnosed with underactive thyroid very recently before becoming pregnant, or you have subclinical hypothyroidism (which means you don't have any symptoms but your thyroid hormone levels are disrupted), you may need to have blood tests more frequently. Your GP may refer you to an endocrinologist (a doctor who specialises in the treatment of conditions relating to your endocrine system) for regular check-ups throughout your pregnancy.

If you have any questions or concerns about underactive thyroid and pregnancy, talk to your GP.