What Changes to Expect to Your Thyroid During Pregnancy

Guidelines About TSH Levels and Other Factors During Pregnancy

Pregnant woman sleeping on couch
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During pregnancy, your thyroid gland needs to expand its production of thyroid hormone in order to meet not only your needs but the needs of your 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 in your thyroid during pregnancy include the following:

  • If you are not iodine-deficient, your thyroid will typically increase around 10 percent in size during pregnancy.
  • If you are iodine-deficient, your thyroid gland will typically increase from 20 percent to 40 percent in size during your pregnancy.
  • Typically, your production of thyroxine (T4) and triiodothyronine (T3) will increase by 50 percent during pregnancy.
  • You have a 50 percent increase in your daily iodine requirement.
  • The normal reference range for thyroid-stimulating hormone (TSH) has a lower high-end cutoff point during your first trimester, typically around 2.5 mIU/L.
  • During your first trimester, approximately 10 percent to 20 percent of you will test positive for thyroid peroxidase (TPO) or thyroglobulin (Tg) antibodies, even while your  TSH level remains in the normal reference range.
  • An estimated 16 percent of you 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 your third trimester.
  • Some 33 percent to 50 percent of you who are positive for TPO or Tg antibodies in the first trimester of your pregnancy 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 (within the normal TSH reference range) prior to conception.

Changes to Your 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, your doctor should use the following reference ranges to monitor your thyroid and adjust your thyroid medication during pregnancy:

  • 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 regularly monitored during your pregnancy. If thyroid disease runs in your family or you have symptoms of thyroid disease, it's also important to see your doctor for testing during pregnancy. 

Thyroid Disease During Your Pregnancy

If you are hypothyroid, it is crucial to be treated both before and during your pregnancy. Left untreated, or insufficiently treated, your hypothyroidism can cause developmental and motor delays in your child.

If you are hypothyroid and being treated prior to pregnancy, it is especially important to know that you should not only continue taking your thyroid hormone replacement medication but may need to increase the dose as much as 50 percent as soon as your pregnancy is confirmed.


In some cases, you may develop Graves disease' or hyperthyroidism, an overactive thyroid, during your pregnancy. Left untreated, hyperthyroidism can cause pre-eclampsia or premature birth for your baby, and which pose significant risks to your health and your baby's health and survival. If you have Graves' disease during pregnancy you are also at risk of developing a severe form of hyperthyroidism known as thyroid storm. If you have hyperthyroidism but are not treated, your baby is also at risk of being born with a low birth weight, a rapid heart rate, congenital defects, and other health problems.

In severe cases, the pregnancy may end in stillbirth. 

After giving birth, less than 10 percent of women develop postpartum thyroiditis––an inflammation of the thyroid gland. If you develop postpartum thyroiditis with significant symptoms, your doctor can treat you with thyroid medications. In many cases, you should be able to taper off these drugs within a year. 

A Word from Verywell

There are a few takeaways from the guidelines that can help ensure you have a healthy pregnancy and baby: 

  • Experts recommend that if you are planning a pregnancy, or you are pregnant, you should take a prenatal vitamin that includes at least 250 mg of iodine daily.  (It's also important to note that many prescription and over-the-counter prenatal vitamins fail to include the recommended amount of iodine.) 
  • If you are hypothyroid and planning for pregnancy, you should work with a knowledgeable practitioner to ensure that the TSH level is an optimal level to get pregnant and maintain that pregnancy. This includes having a plan in place with your practitioner to confirm your pregnancy as soon as possible, ideally before you miss a menstrual period, and to increase your dose by an agreed-upon amount as soon as the pregnancy is confirmed. 
  • If you are being treated for thyroid disease during pregnancy, you should request frequent thyroid blood testing, especially during the first trimester. You should also be checked at least once in your second and third trimesters, and after childbirth. 


Alexander E, et. al. “2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease during Pregnancy and the Postpartum.” Thyroid, Vol 27, Number 3, 2017. ​

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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