Overactive Thyroid in Pregnancy

Thyrotoxicosis and hyperthyroidism can complicate pregnancy, and need to be properly diagnosed and treated, according to 2011 guidelines from the American Thyroid Association. istockphoto

Thyrotoxicosis is a medical term that can be used to refer to hyperthyroidism -- an excess of thyroid hormone. During pregnancy, thyrotoxicosis is defined as test results that show high levels of Free Thyroxine (Free T4) and/or high Free Triiodothyronine (Free T3). The 2011 "Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum” offer specific recommendations regarding thyrotoxicosis/hyperthyroidism in pregnancy.

Note: According to the 2011 Guidelines, due to the effect of the pregnancy hormone hCG, normal TSH values during pregnancy can be as low as 0.03 mIU/L in a pregnant woman. The Guidelines recommend that in a woman who has a TSH level of less than .1 mIU/L during her first trimester, doctors should take a comprehensive medical history and perform a physical exam. In addition, the Free T4 levels should be measured.

Other recommendations from the Guidelines are summarized here. 

What is Causing the Hyperthyroidism?

The challenge in pregnancy is to determine the cause of the hyperthyroidism. The most common cause of hyperthyroidism in pregnancy is gestational hyperthyroidism, also called “transient hyperthyroidism of hyperemesis gravidarum.” Another common cause of hyperthyroidism during pregnancy is Graves’ disease, the autoimmune condition that causes overactivity of the thyroid gland. Other non-autoimmune causes of hyperthyroidism include toxic multinodular goiter, toxic adenoma, and a situation known as factitious thyrotoxicosis -- hyperthyroidism due to overmedication with thyroid hormone medication.

Gestational Hyperthyroidism/Transient Hyperthyroidism

This type of hyperthyroidism is characterized by an elevated Free T4 level, and low or undetectable thyroid stimulating hormone (TSH) level, without any antibodies that would suggest Graves’ disease. It’s estimated that this type of hyperthyroidism occurs in 1% to 3 % of pregnancies, and is due to elevated hCG, a pregnancy hormone.

In some women, gestational hyperthyroidism can be a cause for a condition known as hyperemesis gravidarum, which is characterized by severe nausea, excessive vomiting, electrolyte disturbances, and usually loss of more than five percent of a woman's body weight -- unusual during pregnancy. Hyperemesis gravidarum usually occurs during the first trimester of pregnancy, and after various hormone levels peak during that trimester, thyroid levels typically normalize. Hyperemesis is sometimes treated with intravenous nutrition and hydration, anti-nausea drugs, or hospitalization. In rare cases, extreme hyperthyroidism due to hyperemesis gravidarum may involve short-term treatment with an antithyroid drug or short course of a beta blocker.

Graves' Disease


In the absence of clinical signs of Graves’ disease -- like an enlarged thyroid (goiter), bulging eyes or other thyroid eye-symptoms/ophthalmopathy, or other common symptoms – doctors will typically diagnose gestational hyperthyroidism in a woman with elevated Free T4 and low TSH. But if there is a suspicion of underlying autoimmune Graves' disease, the Guidelines also recommend measuring TSH receptor antibody (TRAb) levels. In some cases, Thyroid Stimulating Immunoglobulins (TSI) may be measured.

According to the Guidelines, radioactive iodine (RAI) scanning or radioiodine uptake (RAIU) tests -- tests that are frequently used to help diagnose Graves' disease and hyperthyroidism -- should not be done during pregnancy, as they pose a significant risk to fetal thyroid development. 


Typically, pregnant women receive antithyroid drug treatment (if newly diagnosed), or, if a woman is an existing patient, her dosage will be adjusted so that free T4 levels remain in the normal reference range for someone who is not pregnant.

The antithyroid drug of choice during the first trimester is propylthiouracil, because methimazole has a slightly higher (though very small) risk of birth defects.

The Guidelines recommend switching to methimazole for the second and third trimesters.

If a pregnant woman has a negative reaction to or debilitating side effects from antithyroid drugs, requires very high doses to control her hyperthyroidism, or has uncontrolled hyperthyroidism despite treatment, surgery may be recommended. Experts recommend that if surgery is needed, it be performed during the second trimester, when it is least likely to endanger the pregnancy.

Radioactive iodine (RAI) treatment is never be given to any woman who is or who might be pregnant, as it poses a risk to fetal thyroid function.


Stagnaro-Green, Alex, 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