Guidelines for Thyroid Disease in Pregnancy: Key Points

Sandra A. Fryhofer, MD


November 09, 2011

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Hello. I'm Dr. Sandra Fryhofer. Welcome to Medicine Matters. The topic is the new guidelines on hypothyroidism during pregnancy, from the American Thyroid Association and published in the journal Thyroid.[1] Here is why it matters.

Being pregnant can be a stress test for the thyroid. The size of the gland increases by 10%. Production of thyroid hormones T3 and T4 increases by about 50%. As a result, the normal thyroid-stimulating hormone (TSH) level during pregnancy is lower than the normal nonpregnancy level.

Recommendations for Testing for Thyroid Disease During Pregnancy

The new recommendations for TSH levels during pregnancy are the following:

  • First trimester: less than 2.5 with a range of 0.1-2.5

  • Second trimester: 0.2-3.0

  • Third trimester: 0.3-3.0.

If the TSH is greater than 2.5 at any time during pregnancy, T4 levels should be checked to determine whether the hypothyroidism is overt or subclinical.

If T4 is low, the diagnosis is overt hypothyroidism, which can impair the infant's neurocognitive development. There are also increased risks for premature birth, low birthweight, and miscarriage. Overt hypothyroidism must be treated.

If TSH is high and the T4 is normal, the diagnosis is subclinical hypothyroidism. In this case, the next step is to check for antithyroid peroxidase antibodies. Women who are antibody positive should be treated. The effect of subclinical hypothyroidism on fetal neurocognitive development is not clear. But one large study showed lower IQ tests in the children of untreated women.

Treatment is necessary when TSH is 10 or more, regardless of the T4 level. In addition, TSH should be monitored every 4 weeks during the first 20 weeks of gestation, then once again between 26 and 32 weeks.

Conception in Women With Hypothyroidism

What about women with hypothyroidism who want to get pregnant? It should be stressed that the need for extra thyroid hormone occurs as early as the first to 6 weeks of pregnancy. The new guidelines recommend that before trying to conceive, clinicians should adjust the patient's medication dose with a TSH goal of less than 2.5. Also, patients should increase their thyroid replacement dose by 25%-30% as soon as they miss a period or have a positive home pregnancy test.

Dosage often requires fine-tuning, so continue to check TSH every 4 weeks during the first half of pregnancy.

Medications Recommended During Pregnancy

The type of thyroid replacement medication that should be used during pregnancy is clearly stated: oral levothyroxine only. Other thyroid medications, such as T3 or desiccated thyroid, are not recommended.

Iodine needs increase during pregnancy by about 50%. So how much iodine do pregnant women need? The Thyroid Association backs the World Health Organization's recommendation of 250 µg daily for all pregnant or lactating women.

To ensure adequate intake, the guidelines recommend a daily oral supplement with 150 µg iodine for women who are pregnant, breastfeeding, or who are planning to get pregnant. Potassium iodine is best. The iodine dose in kelp and seaweed is not consistent enough.

A cautionary note: Daily iodine intake of more than 500-1100 µg g can trigger fetal hypothyroidism.


The guidelines are comprehensive and include 76 different recommendations on managing hypothyroidism, postpartum thyroiditis, and thyroid nodules during pregnancy. There's also a helpful tip for evaluating postpartum depression: Consider the thyroid and check TSH, T4, and thyroid antibodies.

One major precaution: Pregnancy should be avoided for 6 months after radioactive iodine therapy. Also, thyroxin dose should be stabilized before getting pregnant again.

The following factors put women at increased risk for thyroid problems:

  • Age over 30;

  • Living in an area with known iodine insufficiency;

  • Personal or family history of thyroid problems, including goiter, type 1 diabetes, history of miscarriage, infertility, or preterm delivery;

  • BMI of 40 or higher;

  • Certain medications, such as amiodarone or lithium; and

  • Having a recent test using iodinated contrast dye.

The guidelines are quite comprehensive and they're available online. I encourage you to take a closer look at them.

For Medicine Matters, I'm Dr. Sandra Fryhofer.


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