How to Treat Thyroid Disease in Pregnant Women

Posted November 24, 2015 by Stacy Bolzenius

Thyroid Disease in Pregnant Women

Hyperthyroidism affects one in every 500 pregnancies, while hypothyroidism affects three to five of every 1,000 pregnancies, according to data from the NIH, making it a significant women’s health concern. Because these diseases are relatively common and can be severe, all OB/GYNs need to remain up-to-date on current thyroid disease diagnosis and treatment practices for pregnant mothers.

Diagnosing Thyroid Diseases in Pregnant Mothers

Thyroid diseases are diagnosed through a combination of a patient’s medical history, symptoms, physical exams and blood tests (TSH, T3, T4 and TSI).

When there is a pre-existing condition or other symptoms that suggest hyper or hypothyroidism, TSH is the first blood test considered (although the various tests might be ordered together). It’s standard practice to order a TSH test if a patient has been previously treated for hyperthyroidism or hypothyroidism.

Hyperthyroidism can be definitively diagnosed by below-normal levels of TSH and elevated levels of T4. Less commonly, T4 levels may be normal but T3 levels might be elevated; tests with these results are less conclusive but may suggest hyperthyroidism. Additionally, a TSI test can show if there are TSI antibodies present in the body.

Hypothyroidism is diagnosed by elevated TSH levels and low T4 levels. In 2 to 3 percent of pregnancies, TSH levels will be high, but T4 levels will test normal. according to the NIH. These results are considered to show subclinical hypothyroidism, which is mild and without symptoms.

Treating Hyperthyroidism in Pregnant Mothers

If a pregnant woman’s hyperthyroidism is caused by Graves disease, thyroid function should be monitored monthly. Mild hyperthyroidism doesn’t require clinical treatment, but moderate and severe hyperthyroidism should be treated with medication.

During the first trimester, propylthiouracil (PTU) is preferred, while methimazole can be used in the latter two trimesters. Methimazole has been shown to cause fetal damage during the first trimester in rare cases, and therefore should be avoided during the first three months of pregnancy. Because small concentrations of these medications can cross the placenta, the lowest dose possible should be used to minimize impact on the fetus’ thyroid function.

When left untreated, pregnant women with hyperthyroidism are at an increased risk for severe preeclampsia, preterm delivery, heart failure, and possibly miscarriage.

Untreated Hyperthyroidism Increases a Pregnant Woman's Risk of Preeclampsia

Treating Hypothyroidism in Pregnant Mothers

Hypothyroidism is treated by a synthetic hormone, thyroxine. This is identical to T4 and safe to use during pregnancy. Prenatal vitamins, however, shouldn’t be taken within three to four hours of taking thyroxine as they can block the absorption of thyroid hormone. For patients already on thyroxine, the dosage is often increased during pregnancy.

Untreated hypothyroidism increases a woman’s risk of preeclampsia, though it is uncertain if subclinical hypothyroidism increases that risk. Inadequate treatment is also associated with low birth weight in neonates.

When Medication Is Not an Option

When medications aren’t effective, cause severe side effects or are otherwise not an option, surgery is the only safe alternative during pregnancy. Radioactive iodine treatments are not an option during pregnancy as they can damage the fetal thyroid gland. Likewise, any radiation treatments for thyroid cancer ought to be delayed until after pregnancy.

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