What Changes to Expect to Your Thyroid During Pregnancy

Understanding High and Low TSH Levels

Pregnant woman sleeping on couch
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During pregnancy, the thyroid gland needs to expand its function in order to meet the needs of both mother and her developing baby.

According to the Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum, some of the changes that take place include the following:

  • In women who are not iodine-deficient, the thyroid typically increases around 10 percent in size during pregnancy.
  • In women who are iodine-deficient, the gland typically increases from 20 to 40 percent in size during pregnancy.
  • Typically, production of thyroxine (T4) and triiodothyronine (T3) increases by 50 percent during pregnancy.
  • A pregnant woman has a 50 percent increase in her daily iodine requirement.
  • The normal reference range for thyroid-stimulating hormone (TSH) has a lower high-end cutoff point during the first trimester, typically around 2.5 mIU/L.
  • During the first trimester, approximately 10 to 20 percent of pregnant women are thyroid peroxidase (TPO) or thyroglobulin (Tg) antibody positive, with TSH in the normal reference range.
  • An estimated 16 percent of women who are within the normal reference range but positive for TPO or Tg antibodies during the first trimester will go on to have a TSH above 4.0 mIU/L during the third trimester.
  • Some 33 to 50 percent of pregnant women who are positive for TPO or Tg antibodies in the first trimester will develop postpartum thyroiditis.

    According to the authors of the thyroid guidelines: "Pregnancy is a stress test for the thyroid, resulting in hypothyroidism in women with limited thyroidal reserve or iodine deficiency, and postpartum thyroiditis in women with underlying Hashimoto's disease who were euthyroid prior to conception."

    Changes to Thyroid Function During Pregnancy

    Because normal thyroid function is different during pregnancy, the guidelines have established trimester-specific normal ranges for various thyroid tests, and specifically for the TSH test.

    According to the guidelines, if a laboratory has not established its own trimester-specific reference ranges for TSH, the following reference ranges should be used:

    • First trimester: 0.1-2.5 mIU/L
    • Second trimester: 0.2-3.0 mIU/L
    • Third trimester: 0.3-3.0 mIU/L

    If you have thyroid disease, you should have your thyroid monitored during your pregnancy. If thyroid disease runs in your family or you have symptoms of thyroid disease, see your doctor for testing. 

    Thyroid Disease During Pregnancy

    It's important to treat hypothyroidism, low thyroid levels, during pregnancy. Left untreated, hypothyroidism can cause developmental and motor delays in children born to mothers with the condition. If you have hypothyroidism, do not stop taking your medication while pregnant. 

    Some women may develop hyperthyroidism, high thyroid levels (or Graves disease), during pregnancy. Left untreated, hyperthyroidism can cause pre-eclampsia or premature birth for the baby. Women who have Graves disease during pregnancy are at risk of developing a severe form of hyperthyroidism known as thyroid storm.

    Babies born to mothers with untreated hyperthyroidism may have low birth weights, rapid heart rates, and congenital defects. In severe cases, the pregnancy may end in stillbirth. 

    After giving birth, some women (less than 10 percent) may develop postpartum thyroiditis –– an inflammation of the thyroid gland. Women with postpartum thyroiditis can be treated with thyroid medications. Most women taking these medications will be able to taper off within six to 12 months.  


    Stagnaro-Green A, et. al. Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum. Thyroid. Volume 21, Number 10, 2011. 

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