Levothyroxine in Pregnancy — Is It Safe?
Last reviewed: · Source: US FDA drug label database
Levothyroxine is safe and essential to continue in pregnancy — hypothyroidism must be well-controlled for healthy fetal development.
Is it safe to take Levothyroxine with Pregnancy?
Levothyroxine is not only safe in pregnancy — it is essential for women with hypothyroidism (underactive thyroid). Untreated or poorly controlled hypothyroidism in pregnancy is associated with miscarriage, preterm birth, and impaired fetal brain development. Thyroid hormone requirements increase by approximately 25-50% in pregnancy, so most women need a dose increase as soon as pregnancy is confirmed. Thyroid function must be monitored more frequently during pregnancy.
How do Levothyroxine and Pregnancy interact?
Levothyroxine is synthetic T4, identical to the thyroid's own output. Adequate maternal thyroid hormone is critical for fetal brain development, particularly in the first trimester when the fetal thyroid is not yet functional. Maternal T4 crosses the placenta and is the primary source of fetal thyroid hormone until fetal thyroid function begins around 10-12 weeks.
Data sourced from the US Food and Drug Administration (FDA) drug label database: NICE guideline — thyroid disease; OpenFDA levothyroxine drug label.
Who should be careful?
Women with hypothyroidism must have thyroid function checked as soon as pregnancy is confirmed and regularly throughout pregnancy. The levothyroxine dose will almost certainly need to be increased.
What if I've already taken Levothyroxine and Pregnancy?
Continue taking levothyroxine as prescribed. Contact your GP immediately to arrange an urgent thyroid function test and dose review.
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Frequently Asked Questions
Should I take levothyroxine during pregnancy?
Yes, absolutely. Levothyroxine is essential in pregnancy for women with hypothyroidism. Stopping is dangerous. Your dose will likely need to increase.
Does levothyroxine dose need to change in pregnancy?
Yes. Thyroid hormone requirements increase by 25-50% in pregnancy. Contact your GP for an urgent thyroid function test as soon as you know you are pregnant.
Is underactive thyroid dangerous in pregnancy?
Yes. Untreated hypothyroidism in pregnancy is associated with miscarriage, preeclampsia, preterm birth, and impaired fetal brain development. Well-controlled hypothyroidism on levothyroxine is not associated with these risks.
Can I take iron with levothyroxine in pregnancy?
Iron supplements (commonly prescribed in pregnancy) can reduce levothyroxine absorption if taken at the same time. Take levothyroxine at least 2-4 hours apart from iron supplements.