Is Levothyroxine Safe in Pregnancy?

Levothyroxine is an FDA-approved synthetic thyroid hormone used to treat hypothyroidism. Although obstetricians try to minimize the number of medications women take during pregnancy, levothyroxine is safe for pregnant women with hypothyroidism. In fact, treating hypothyroidism is essential during pregnancy. If left untreated, hypothyroidism can cause anemia, muscle pain, congestive heart failure, and preeclampsia in a pregnant woman. It can also seriously affect the brain development of the fetus and lead to low birth weight. During the first trimester, the fetus completely depends on the mother for thyroid hormone. It is not until the second trimester that the fetal thyroid gland develops and the fetus can make its own hormone.

A pregnant woman sits on a couch with a glass of water and Levothroxine in her hand. (Image: Antonio_Diaz/iStock/Getty Images)

Hypothyroidism and Levothyroxine

The thyroid is a butterfly-shaped gland in the front of the neck that produces a hormone called thyroxine, or T4. T4 is needed for many basic functions, such as metabolism, digestion, normal hair and skin growth, and brain functioning. Hypothyroidism occurs when the thyroid does not produce enough T4 hormone. People with hypothyroidism may have fatigue, weight gain, dry skin, hair loss, constipation, memory loss, poor concentration and depression. In some countries, iodine deficiency causes hypothyroidism. In the United States, where iodine is added to table salt, people most commonly develop hypothyroidism because antibodies attack their thyroid glands. This condition is called Hashimoto's thyroiditis. Hypothyroidism can also be caused by radiation, certain medications and thyroid surgery. Levothyroxine may cause side effects when the dose is too high. These side effects often mimic hyperthyroidism, with palpitations, sweating, anxiety, diarrhea, feeling hot, weight loss, abdominal pain and headaches.

Hypothyroidism and Pregnancy

Women with hypothyroidism should have their thyroid tested as soon as they become pregnant, and then every six to eight weeks throughout pregnancy. Thyroid-stimulating hormone, or TSH, and Free T4, or FT4, are the two recommended tests pregnant women should get to check their thyroid. TSH is high in a person with untreated hypothyroidism because the pituitary gland releases TSH when the thyroid is not making enough hormone. FT4 is the amount of thyroxine “free” in the blood. In the bloodstream, T4 is converted to T3, which is the active form of the hormone. T3 and T4 bound to proteins in the blood are not active. During pregnancy, the number of thyroid hormone binding proteins is increased, so less T3 and T4 are free in the bloodstream. To compensate for this, the dose of levothyroxine usually has to be increased during pregnancy by 25 to 50 percent. Thyroid tests should be rechecked four weeks after changing the dose until the TSH is in the correct range.

There is a debate as to whether all women, not just those with pre---existing hypothyroidism or an increased risk for hypothyroidism, should have a TSH check during pregnancy. Risk factors for hypothyroidism include a family history, prior thyroid surgery or a goiter. Pregnant women who do not have these risks should still talk to their doctor about this test.