Thyroid blood test pregnancy

An undiagnosed thyroid disorder can make it difficult to conceive. It can also cause problems during pregnancy itself. Once the over- or under-active thyroid is under control, however, there is no reason why you should not have a successful pregnancy and a healthy baby.

Pregnancy and hypothyroidism

Before pregnancy

If your under-active thyroid (hypothyroidism) is untreated or under-treated then you are likely to find it more difficult to conceive. You may have longer or heavier periods, which can cause anaemia, or your periods may stop completely. Once you are taking medication (levothyroxine tablets) and your thyroid hormone levels are back to normal your chances of becoming pregnant should improve dramatically.

If you are planning a pregnancy you should let your doctor know and ideally have a blood test before you conceive. Experts in the field recommend that the TSH concentration, which is measured on the thyroid function blood test, should ideally be kept in the lower half of the reference range before pregnancy as this has been associated with a lower risk of miscarriage.

During pregnancy

As soon as you know you are pregnant and if you are already taking levothyroxine it is usually recommended that the dosage is increased immediately by 25-50mcg daily. You should then arrange to have a thyroid function test.

Even if your thyroid function test result is not ideal at the start of pregnancy, your risk of a pregnancy complication is only slightly higher than normal and you would still have a good chance of a successful pregnancy outcome. However, your levothyroxine treatment should be adjusted to normalise your thyroid function as soon as possible.

You should have regular blood tests throughout your pregnancy so that your dose can be adjusted if necessary.

If you are prescribed supplements containing iron, calcium or Gaviscon you should take these several hours before or after the levothyroxine since these can alter the absorption of levothyroxine.

After the baby is born

After the birth you will probably need to return to the dose of levothyroxine you were taking before the pregnancy. You should have a blood test to check your thyroid hormone levels a few weeks after the birth. It is safe to breast-feed while taking levothyroxine.

In the UK all babies have a heel-prick blood test to screen for hypothyroidism shortly after birth and treatment can be started very quickly if your baby needs levothyroxine. Hypothyroidism is extremely rare in new-born babies in the UK – only about one baby in every 3,500-4,000 is born with hypothyroidism.

Pregnancy and hyperthyroidism

Before pregnancy

The most common cause of an over-active thyroid (hyperthyroidism) is Graves’ disease. If it is untreated you may have lighter, irregular periods and find it difficult to conceive. After treatment, if you are planning to have a baby you should first have a blood test to check your thyroid condition. If you are not planning to get pregnant then use a contraceptive during and after treatment, as normal fertility can return extremely quickly.

If you are pregnant and have (had) Graves’ disease it is important to tell your obstetrician about your medical history. Even if your thyroid function has returned to normal you may still have Graves’ antibodies in your blood and these could affect you and/or your baby during or after your pregnancy. You should also tell the doctor looking after your thyroid that you are pregnant as soon as possible. You can expect to have regular check-ups.

In men, hyperthyroidism can cause a marked reduction in sperm count, resulting in infertility. The sperm count usually returns to normal once the thyroid condition has been treated.

During pregnancy

If you have active hyperthyroidism, you will still need to take antithyroid drugs during your pregnancy. These drugs cross the placenta so the lowest possible dose will be prescribed so that your baby is less likely to be affected. If you are already on Carbimazole (CMZ) when you conceive you should change to Propylthiouracil (PTU) as soon as possible. PTU is the drug of choice when trying to conceive (preconception) and in the first three months of pregnancy. If PTU is not available CMZ can be used. ‘Block and replace’ therapy (blocking the thyroid from working using CMZ or PTU and then preventing hypothyroidism with levothyroxine tablets) should not be used in pregnancy. Very rarely, antithyroid drugs can cause side effects, including agranulocytosis (lowering of the number of white blood cells) and severe liver impairment. (See: Your Guide to Antithyroid Drug Therapy to Treat Hyperthyroidism)

Thyroid surgery is rarely required. If needed it should ideally be performed during the middle three months of pregnancy. Radioactive iodine, another treatment for hyperthyroidism, is never used during pregnancy.

There are several complications to be alert to if you have (had) hyperthyroidism. There is, unfortunately, an increased risk of miscarriage in the early stages of pregnancy if your hyperthyroidism is not under control. If you are taking antithyroid drugs there is a very slight increased risk of the baby developing structural abnormalities so some patients choose to have definitive treatment of Graves’ disease with radioactive iodine or surgery before considering a pregnancy. Also, if the dose of antithyroid drugs is too high, the baby’s thyroid may become under-active and the baby may develop a goitre. When trying to conceive or during pregnancy, do not stop taking antithyroid drugs before speaking to your doctor. There is greater risk to the pregnancy from an untreated over-active thyroid gland than from taking antithyroid medication. Untreated hyperthyroidism can also lead to complications of high blood pressure in pregnancy, poor growth of the baby and premature delivery. You will require regular thyroid function tests in pregnancy to ensure you are on an appropriate dose. If you have been treated for Graves’ disease with radioactive iodine or surgery in the past, or need antithyroid drugs during pregnancy, you may have Graves’ antibodies (also known as thyroid-stimulating immunoglobulins or TSI), which can cross the placenta. On rare occasions these can cause temporary hyperthyroidism in the baby during pregnancy and after birth, but this is treatable. A simple blood test to measure the thyroid-stimulating hormone receptor antibodies in the mother can help predict whether the baby will be affected in this way.

After the baby is born

Hyperthyroidism can recur during the first year after the baby is born, so you should arrange to have your blood tested around three months after delivery and at intervals thereafter. If you stopped taking antithyroid drugs during your pregnancy you should check with your doctor if you notice any symptoms of hyperthyroidism.

Only small amounts of antithyroid drugs cross into breast milk. If you are on antithyroid drugs, you can breast-feed provided the dose is small but check first with your doctor. Antithyroid drugs are best taken in smaller doses over two or three times a day following a feed. If you plan to breast-feed for a long time your baby can have a blood test to check whether its thyroid is being affected.

Mothers with Graves’ disease who are not taking antithyroid drugs can safely breast-feed.

Post-partum thyroiditis

Probably the most common thyroid disease now seen in the UK is a temporary disorder called postpartum thyroiditis, which occurs especially in women with thyroid auto-antibodies. This usually shows up in the mother in the six months after the birth. Your thyroid may be a little swollen, but it is almost never painful. It usually starts with symptoms of an over-active thyroid (hyperthyroidism), which can resolve by itself but may develop into symptoms of an under-active thyroid (hypothyroidism). If it develops into hypothyroidism you may feel tired, lethargic, depressed and cold, and your skin may be dry. If it persists you will need to take levothyroxine tablets. Most women are able to stop taking these tablets after six to 12 months, but around a third of women develop permanent hypothyroidism and need levothyroxine treatment in the long term.

If you have had postpartum thyroiditis, even though you have made a full recovery initially, it is recommended that you have your thyroid function checked before you try to conceive again and at the start of your next pregnancy to ensure that you have not developed hypothyroidism.

Some important points….

  • Tell your doctor if you are planning to become pregnant
  • An over- or under-active thyroid can prevent you from conceiving. Pregnancy can happen very quickly after your thyroid function returns to normal
  • Always tell your midwife or obstetrician if you have a thyroid disorder or have been treated for one in the past

If you are or have been treated for Graves’ disease, there is a small chance that your baby will develop temporary hyperthyroidism, but this can be monitored and treated during pregnancy and after the birth

  • If you are being treated for hypothyroidism it is recommended that the levothyroxine dosage be increased by 25-50mcg daily once you know you are pregnant
  • If you are taking antithyroid medication for hyperthyroidism, do not alter your dose without first speaking to your doctor
  • It is safe to breast-feed if you are taking levothyroxine tablets. If you are taking antithyroid tablets it is also generally safe to breast-feed, but speak to your doctor first
  • Postpartum thyroiditis is usually a temporary disorder that can clear up without treatment after a few months, but sometimes you will need a course of levothyroxine tablets
  • Postpartum thyroiditis can lead to hypothyroidism in future pregnancies and return after subsequent pregnancies so it is important to have a thyroid function test before you conceive and after each birth
  • Thyroid hormone reference ranges for pregnant women are different from those in the general non-pregnant population. This should be taken into account in interpreting thyroid function in pregnancy. (See: Your Guide to Thyroid Function Tests.)
  • It is well recognised that thyroid problems often run in families and if family members are unwell they should be encouraged to discuss with their own GP whether thyroid testing is warranted.

    If you have questions or concerns about your thyroid disorder, you should talk to your doctor or specialist as they will be best placed to advise you. You may also contact the British Thyroid Foundation for further information and support, or if you have any comments about the information contained in this leaflet.

    The British Thyroid Foundation

    The British Thyroid Foundation is a registered charity: England and Wales No 1006391, Scotland SC046037

    Endorsed by:

    The British Thyroid Association – medical professionals encouraging the highest standards in patient care and research

    The British Association of Endocrine and Thyroid Surgeons – the representative body of British surgeons who have a specialist interest in surgery of the endocrine glands (thyroid, parathyroid and adrenal)

    First issued: 2008

    Revised: 2011, 2015

    Our literature is reviewed every two years and revised if necessary.

    Thyroid Function Test

    Thyroid hormone level screen

  • Blood tests
  • Thyroid Function Test
  • The thyroid function test checks the levels of thyroid (thyroxine and triiodothyronine) and thyroid-stimulating hormones (TSH) in your blood. Depending on your levels, the results could indicate conditions such as hypothyroid (underactive) or hyperthyroid (overactive) thyroid.

    There are several, broad symptoms related to thyroid dysfunction. If you are noticing any of these, you should get tested for thyroid disorders.

    Signs of an overactive thyroid – hyperthyroidism:

    • Sudden weight loss
    • Fatigue
    • Mood swings, anxiety
    • Diarrhoea
    • Palpitations
    • Menstruation and fertility changes
    • Heat sensitivity
    • A swollen neck (enlarged thyroid gland)
    • Signs of an underactive thyroid – hypothyroidism
    • Weight gain
    • Low energy
    • Being sensitive to the cold
    • Feeling depressed
    • Dry and scaly skin
    • Aches and pains of muscles
    • Menstruation and fertility changes

    Please ensure that you are well-hydrated before you come in for your blood test.

    During the test

    The thyroid test is a simple blood test. A needle is inserted into a vein, usually on the inner arm near the elbow, and a small amount of blood is drawn. You may feel a pricking or scratching sensation.

    Your results will be sent to you by email. If you had a consultation with one of our doctors, the doctor will give you a call first to discuss the results with you.

    The thyroid, also referred to as a thyroid gland, is found in the neck, behind a cartilage known as the ‘Adam’s apple’. The thyroid is responsible for producing hormones which control metabolism and affect bodily growth rates and other functions. Your thyroid therefore plays an important role when it comes to your health. An imbalance of thyroid hormones can strongly affect your wellbeing.

    Nodules, or small lumps, on the thyroid, or an enlargement of the gland, can be an indicator either of oestrogen dominance, when your natural hormone levels are out of balance, or of toxicity (either through diet, environmental factors, or chemicals in skincare products). If you have or think you have thyroid nodules, it is advised that you see a doctor to get checked out.

    Thyroid checks are recommended if you have a family history of thyroid disorder. Women can start experiencing thyroid-related symptoms as early as their late teens and 20s, particularly if another family member has a thyroid condition. In general, it is recommended you begin getting regular check-ups around age 30, or at any point when you have concerns.

    Thyroid Function Tests


    The major thyroid hormone secreted by the thyroid gland is thyroxine, also called T4 because it contains four iodine atoms. To exert its effects, T4 is converted to triiodothyronine (T3) by the removal of an iodine atom. This occurs mainly in the liver and in certain tissues where T3 acts, such as in the brain. The amount of T4 produced by the thyroid gland is controlled by another hormone, which is made in the pituitary gland located at the base of the brain, called thyroid stimulating hormone (abbreviated TSH). The amount of TSH that the pituitary sends into the blood stream depends on the amount of T4 that the pituitary sees. If the pituitary sees very little T4, then it produces more TSH to tell the thyroid gland to produce more T4. Once the T4 in the blood stream goes above a certain level, the pituitary’s production of TSH is shut off. In fact, the thyroid and pituitary act in many ways like a heater and a thermostat. When the heater is off and it becomes cold, the thermostat reads the temperature and turns on the heater. When the heat rises to an appropriate level, the thermostat senses this and turns off the heater. Thus, the thyroid and the pituitary, like a heater and thermostat, turn on and off. This is illustrated in the figure below:

    T4 and T3 circulate almost entirely bound to specific transport proteins, and there are some situations which these proteins could change their level in the blood, producing also changes in the T4 and T3 levels (it happens frequently during pregnancy, women who take control birth pills, etc).

    Another measurement done to assess the thyroid status of patients is the Free T4 measurement. The Free T4 avoids any change the proteins could have, giving us a more accurate value for the T4 level (see below).

    Blood tests to measure TSH, T4, T3 and Free T4 are readily available and widely used. Tests to evaluate thyroid function include the following:

    The best way to initially test thyroid function is to measure the TSH level in a blood sample. A high TSH level indicates that the thyroid gland is failing because of a problem that is directly affecting the thyroid (primary hypothyroidism). The opposite situation, in which the TSH level is low, usually indicates that the person has an overactive thyroid that is producing too much thyroid hormone (hyperthyroidism). Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid (secondary hypothyroidism). In most healthy individuals, a normal TSH value means that the thyroid is functioning normally.

    T4 circulates in the blood in two forms:

    1) T4 bound to proteins that prevent the T4 from entering the various tissues that need thyroid hormone.

    2) Free T4, which does enter the various target tissues to exert its effects. The free T4 fraction is the most important to determine how the thyroid is functioning, and tests to measure this are called the Free T4 (FT4) and the Free T4 Index (FT4I or FTI). Individuals who have hyperthyroidism will have an elevated FT4 or FTI, whereas patients with hypothyroidism will have a low level of FT4 or FTI.

    Combining the TSH test with the FT4 or FTI accurately determines how the thyroid gland is functioning.

    The finding of an elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to disease in the thyroid gland. A low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the pituitary gland. A low TSH with an elevated FT4 or FTI is found in individuals who have hyperthyroidism.

    T3 tests are often useful to diagnosis hyperthyroidism or to determine the severity of the hyperthyroidism. Patients who are hyperthyroid will have an elevated T3 level. In some individuals with a low TSH, only the T3 is elevated and the FT4 or FTI is normal. T3 testing rarely is helpful in the hypothyroid patient, since it is the last test to become abnormal. Patients can be severely hypothyroid with a high TSH and low FT4 or FTI, but have a normal T3. In some situations, such as during pregnancy or while taking birth control pills, high levels of total T4 and T3 can exist. This is because the estrogens increase the level of the binding proteins. In these situations, it is better to ask both for TSH and free T4 for thyroid evaluation.


    The immune system of the body normally protects us from foreign invaders such as bacteria and viruses by destroying these invaders with substances called antibodies produced by blood cells known as lymphocytes. In many patients with hypothyroidism or hyperthyroidism, lymphocytes make antibodies against their thyroid that either stimulate or damage the gland. Two common antibodies that cause thyroid problems are directed against thyroid cell proteins: thyroid peroxidase and thyroglobulin. Measuring levels of thyroid antibodies may help diagnose the cause of the thyroid problems. For example, positive anti-thyroid peroxidase and/or anti-thyroglobulin antibodies in a patient with hypothyroidism make a diagnosis of Hashimoto’s thyroiditis. If the antibodies are positive in a hyperthyroid patient, the most likely diagnosis is autoimmune thyroid disease.

    Thyroglobulin (Tg) is a protein produced by normal thyroid cells and also thyroid cancer cells. It is not a measure of thyroid function and it does not diagnose thyroid cancer when the thyroid gland is still present. It is used most often in patients who have had surgery for thyroid cancer in order to monitor them after treatment. Tg is included in this brochure of thyroid function tests to communicate that, although measured frequently in certain scenarios and individuals, Tg is not a primary measure of thyroid hormone function.


    Because T4 contains much iodine, the thyroid gland must pull a large amount of iodine out from the blood stream in order for the gland to make an appropriate amount of T4. The thyroid has developed a very active mechanism for doing this. Therefore, this activity can be measured by having an individual swallow a small amount of iodine, which is radioactive. The radioactivity allows the doctor to track where the iodine molecules go. By measuring the amount of radioactivity that is taken up by the thyroid gland (radioactive iodine uptake, RAIU), doctors may determine whether the gland is functioning normally. A very high RAIU is seen in individuals whose thyroid gland is overactive (hyperthyroidism), while a low RAIU is seen when the thyroid gland is underactive (hypothyroidism). In addition to the radioactive iodine uptake, a thyroid scan may be obtained, which shows a picture of the thyroid gland (see Thyroid Nodules brochure).

    TSH (Thyroid-stimulating hormone) Test

    What is a TSH Test?

    TSH stands for thyroid stimulating hormone. A TSH test is a blood test that measures this hormone. The thyroid is a small, butterfly-shaped gland located near your throat. Your thyroid makes hormones that regulate the way your body uses energy. It also plays an important role in regulating your weight, body temperature, muscle strength, and even your mood. TSH is made in a gland in the brain called the pituitary. When thyroid levels in your body are low, the pituitary gland makes more TSH. When thyroid levels are high, the pituitary gland makes less TSH. TSH levels that are too high or too low can indicate your thyroid isn’t working correctly.

    Other names: thyrotropin test

    What is it used for?

    A TSH test is used to find out how well the thyroid is working.

    Why do I need a TSH test?

    You may need a TSH test if you have symptoms of too much thyroid hormone in your blood (hyperthyroidism), or too little thyroid hormone (hypothyroidism).

    Symptoms of hyperthyroidism, also known as overactive thyroid, include:

    Symptoms of hypothyroidism, also known as underactive thyroid, include:

    What happens during a TSH test?

    A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

    Will I need to do anything to prepare for the test?

    You don’t need any special preparations for a TSH blood test. If your health care provider has ordered other blood tests, you may need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.

    Are there any risks to the test?

    There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

    What do the results mean?

    High TSH levels can mean your thyroid is not making enough thyroid hormones, a condition called hypothyroidism. Low TSH levels can mean your thyroid is making too much of the hormones, a condition called hyperthyroidism. A TSH test does not explain why TSH levels are too high or too low. If your test results are abnormal, your health care provider will probably order additional tests to determine the cause of your thyroid problem. These tests may include:

    • T4 thyroid hormone tests
    • T3 thyroid hormone tests
    • Tests to diagnose Graves’ disease, an autoimmune disease that causes hyperthyroidism
    • Tests to diagnose Hashimoto’s thyroiditis, an autoimmune disease that causes hypothyroidism

    Is there anything else I need to know about a TSH test?

    Thyroid changes can happen during pregnancy. These changes are usually not significant, but some women can develop thyroid disease during pregnancy. Hyperthyroidism occurs in about one in every 500 pregnancies, while hypothyroidism occurs in approximately one in every 250 pregnancies. Hyperthyroidism, and less often, hypothyroidism, may remain after pregnancy. If you develop a thyroid condition during pregnancy, your health care provider will monitor your condition after your baby is born. If you have a history of thyroid disease, be sure to talk with your health care provider if you are pregnant or are thinking of becoming pregnant.


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  • Related MedlinePlus Health Topics

    The medical information provided is for informational purposes only, and is not to be used as a substitute for professional medical advice, diagnosis or treatment. Please contact your health care provider with questions you may have regarding medical conditions or the interpretation of test results.

    In the event of a medical emergency, call 911 immediately.

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