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Underactive thyroid (hypothyroidism) Q&As

Published by Bupa's health information team, March 2009.

Answers to questions about underactive thyroid (hypothyroidism)

This section contains answers to common questions about this topic. Questions have been suggested by health professionals, website feedback and requests via email.

 

What blood tests are used to diagnose an underactive thyroid? What are normal thyroid levels?

To diagnose an underactive thyroid, your doctor will ask you to take a blood test to measure the thyroid-stimulating hormone (TSH) and free thyroxine (FT4) levels in your blood. If your TSH level is higher than normal and your FT4 level lower than usual, a diagnosis of hypothyroidism will be made.

Explanation

TSH is a hormone produced by a part of your brain called the pituitary gland. TSH stimulates your thyroid gland to create more thyroid hormones to make sure that there is a constant supply in your bloodstream. The typical level of TSH in the blood is between 0.4 and 4.5mU/L.

The hormones secreted by your thyroid gland are called thyroxine (T4) and triiodothyronine (T3). Together, these hormones control the speed at which your body burns energy and how quickly reactions happen in your body. To test the levels of these hormones in your body, your doctor will look at the FT4 level and FT3 levels in your blood. FT refers to the amount of free thyroxine (FT4) or free triiodothyronine (FT3) hormones in your bloodstream. The usual range for FT4 is between 9.0 and 25pmol/L, for FT3 this is 3.5 to 7.8pmol/L.

A diagnosis of hypothyroidism will be made if your TSH level is 10mU/L or more, and your FT4 level is below the usual range of 9.0 to 25pmol/L. This is also often referred to as overt hypothyroidism, and is caused by your thyroid gland not producing enough thyroid hormones.

An underactive thyroid can develop slowly, so you may not have symptoms straightaway. This is called subclinical hypothyroidism. If you have subclinical hypothyroidism, you will have a TSH level above the normal range (usually between 5 and 10mU/L), but your FT4 level will be normal. Your doctor will ask you to come back for another blood test in three to six months to see if these levels have changed before starting you on treatment.

An underactive thyroid can sometimes be caused by problems with your pituitary gland or hypothalamus rather than the thyroid itself. This is called secondary hypothyroidism. If this is the case, your TSH level will be low or low-normal in the range and your FT4 level will be below the normal range.

Some medicines can affect the way your thyroid functions, for example lithium (a treatment for bipolar disorders) and amiodarone (used for people with certain heart problems). Both of these medicines can cause an underactive thyroid. If this happens, your TSH level will be above the normal range and your FT4 level will be low or low-normal of the usual range.

If you have any questions or concerns about underactive thyroid or blood tests to diagnose the condition, talk to your GP.

Further information

Sources

  • Hyperthyroidism guidance. Clinical Knowledge Summaries. www.cks.library.nhs.uk, accessed 9 September 2008
  • UK Guidelines for the use of thyroid function tests. The Association for Clinical Biochemistry BTA, British Thyroid Foundation, 2006. www.british-thyroid-association.org, accessed 9 September 2008
  • Vaidya B, Pearce SHS. Clinical review: management of hypothyroidism in adults. BMJ, 2008; 337:a801
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How long will it take to feel better once I've started on thyroxine?

Once your thyroid levels are back to normal you may start to feel better straightaway, or it may take several months - it varies from person to person.

Explanation

The aim of treatment with thyroxine (levothyroxine or L-thyroxine) is to return the thyroid-stimulating hormone (TSH) and free thyroxine (FT4) levels in your blood back to within the normal range. For TSH this is between 0.4 and 4.5mU/L, and for TF4 this is between 9.0 and 25pmol/L.

Usually your GP will start you on a low dose of thyroxine, between 50 and 100 micrograms. However, you may be started on an even lower dose if you are over the age of 50, have a heart condition or have severe hypothyroidism. Your dose will be gradually increased in steps of between 25 and 50 micrograms until the levels of TSH and FT4 in your bloodstream are normal. Your GP will ask you to take a blood test every two to three months so that he or she can monitor and adjust your dose as necessary. For most people, a maintenance dose of 75 to 150 micrograms of thyroxine is needed.

Your symptoms will begin to improve as the TSH and TF4 levels in your bloodstream return to normal. The length of time this takes will depend on how severe your hypothyroidism was when you were diagnosed and the dose of thyroxine you are taking. It can sometimes take several months before you start feeling better after your thyroid levels are stable.

If you have been taking thyroxine for several months and still don't feel any better, you should see your GP. You may have another condition that is also causing your symptoms.

If you have any questions or concerns about hypothyroidism or thyroxine, talk to your GP.

Further information

Sources

  • Vaidya B, Pearce SHS, Clinical review: management of hypothyroidism in adults. BMJ, 2008; 337:a801.
  • Hyperthyroidism guidance. Clinical Knowledge Summaries. www.cks.library.nhs.uk, accessed 9 September 2008
  • Joint Formulary Committee, British National Formulary. 55th ed. London: British Medical Association and Royal Pharmaceutical Society of Great Britain, 2008: 376
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Is it safe to take thyroxine during pregnancy?

Yes, it is safe to take thyroxine replacement medicine (levothyroxine or L-thyroxine) during your pregnancy. However, you will need to be closely monitored during your pregnancy to ensure your thyroid keeps functioning normally.

Explanation

During pregnancy it's important that your thyroid keeps functioning properly. This is because in the first trimester (up to 12 weeks) of pregnancy, your baby's thyroid won't have developed. Your baby will rely on the thyroid hormone in your bloodstream. Your baby needs thyroid hormones to help their nervous system develop.

If you have an underactive thyroid, your GP will need to increase your dose of thyroxine to ensure that you have enough thyroid hormones for you and your baby. It's helpful to have your thyroid function tested (by having a thyroid function blood test (TFT)) before you conceive. If this isn't possible, your GP will arrange for you to have a blood test once your pregnancy is confirmed. It's important that you see your GP as soon as you think you are pregnant.

Usually your GP will increase your dose of levothyroxine by about 25 to 50 micrograms straightaway - the exact amount will depend on the current dose you are taking and the results of your blood test. You GP will adjust your medicine until your TSH level is in the low to normal range (0.4 to 2.0mU/L) and your FT4 level is in the upper part of the normal range (normal FT4 range is 9.0 and 25pmol/L).

Your thyroid will be frequently checked during your pregnancy. In the first trimester, your GP may ask you to take blood tests until he or she is sure that your thyroid is functioning normally. After this, blood tests will be taken during your second trimester (13 to 26 weeks), your third trimester (27 to 40 weeks) and then two to four weeks after your baby is born.

If you have only been very recently diagnosed with an underactive thyroid before becoming pregnant or you have subclinical hypothyroidism, you may need to have blood tests more frequently.

Some woman may be referred by their GP to an endocrinologist for regular check ups throughout their pregnancy. An endocrinologist is a doctor who specialises in the treatment of conditions relating to the endocrine system (hormones).

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

Further information

Sources

  • UK guidelines for the use of thyroid function tests. The Association for Clinical Biochemistry BTA, British Thyroid Foundation, 2006. www.british-thyroid-association.org, accessed 9 September 2008
  • Hyperthyroidism guidance. Clinical Knowledge Summaries. www.cks.library.nhs.uk, accessed 9 September 2008
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Related topics

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This information was published by Bupa's health information team and is based on reputable sources of medical evidence. It has been peer reviewed by Bupa doctors. The content is intended for general information only and does not replace the need for personal advice from a qualified health professional.

Publication date: March 2009

 

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