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Studies investigating the effects of levothyroxine and pregnancy have found that levothyroxine does not appear to increase the risk of harmful effects to the fetus. For starters, thyroid hormones do not easily cross into the placenta. What’s more, having low thyroid hormones (i.e., untreated hypothyroidism) can negatively affect a pregnant parent and the fetus.

Therefore, it is generally recommended to continue taking levothyroxine once someone with hypothyroidism becomes pregnant, because not treating her underlying thyroid condition is likely to cause more harm to both parent and fetus. Due to its documented safety during pregnancy, the FDA considers levothyroxine to be a Pregnancy Category A drug.

Levothyroxine and Breastfeeding

According to the FDA, thyroid hormones, including levothyroxine, may be minimally excreted into human breast milk. So, medical providers should use caution when giving this drug to breastfeeding parents. Therefore, nursing and lactating individuals should check with their medical providers about their levothyroxine dosage.

FAQ

How long after taking levothyroxine can I get pregnant?

Experts have associated hypothyroidism with infertility and an inability to get pregnant, so treating an underactive thyroid with levothyroxine may increase the odds of becoming pregnant. Some research suggests that those with hypothyroidism can become pregnant within about six weeks after starting levothyroxine.

You can continue taking levothyroxine throughout your pregnancy, provided that you have close medical supervision from your licensed medical provider. 

Can levothyroxine cause miscarriage?

No studies have shown that levothyroxine can cause miscarriage or harm to a developing fetus. Levothyroxine is considered safe during pregnancy for both parent and child.

Do I need to increase my thyroid medication during pregnancy?

According to research cited by Harvard Medical School, people taking thyroid medications like levothyroxine may need to increase their dose within the first eight weeks of pregnancy. This behavior may help protect against hypothyroidism and fetal harm. 

Can thyroid medication affect breast milk?

Thyroid medications are unlikely to affect breast milk, and if so are thought to do so minimally. The FDA recommends that levothyroxine is administered with caution in nursing parents, but note that nursing parents must also have sufficient thyroid hormone levels in order to lactate. So, continued use of their medication is generally recommended.

When should thyroid levels be checked after pregnancy?

Hypothyroidism occurs in about 2 to 3 out of 100 pregnancies; some people already have hypothyroidism before they become pregnant. If expecting parents have hypothyroidism, their medical providers will likely test their thyroid hormone levels every 4 to 6 weeks within the first half of pregnancy, then at least one more time after 30 weeks. If patients continue having signs and symptoms of thyroid problems after pregnancy, their medical providers will likely continue checking patients’ thyroid levels until the symptoms resolve.

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