What Are the Concerns about Levothyroxine in Pregnancy?

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  • Written By: Lee Johnson
  • Edited By: John Allen
  • Last Modified Date: 11 January 2020
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There are no medical concerns about the use of levothyroxine in pregnancy. The only possible risk associated with the drug in pregnant women is if the dosage is too low in patients suffering from hypothyroidism. Levothyroxine is chemically identical to the natural version of thyroxine produced by the thyroid gland, and therefore a baby would be exposed to the chemical during pregnancy anyway. The US Food and Drug Administration (FDA) has given the drug an A classification, which is only given to medicines which have been extensively studied and found to be exceptionally safe. Pregnant women with existing hypothyroidism should contact a doctor to discuss a possible increase in dosage during pregnancy.

Taking levothyroxine in pregnancy is very safe. It is important that pregnant women only take the drug in the dosage suggested by a doctor, but in these cases the drug is absolutely safe. The small number of instances of fetus malformations during use of levothyroxine in pregnancy cannot even be attributed to the drug, because the thyroid disease it is used to treat could be the cause of the issues. Blood tests should be given to pregnant mothers to ensure safe levels of thyroid hormone, regardless of the safety of the drug.


All humans produce levothyroxine naturally in the thyroid gland. The synthetic version of the compound is identical to the natural version, so any fetus would be exposed to it even if the mother wasn’t taking any medication. The hormone increases the rate of cell metabolism, and is required in both infants and adults. These facts are responsible for the safety of levothyroxine in pregnancy.

The main risk associated with the use of levothyroxine in pregnancy is if mothers with hypothyroidism do not take enough of the drug. Hypothyroidism is the deficiency of thyroxine due to an underactive thyroid gland. Deficiency of the thyroxine hormone in a fetus can lead to many complications, including miscarriage. This is because the compound is required for the growth and development of the fetus. Without the hormone, infants would not develop brain, bone, and other tissues correctly.

Mothers suffering from hypothyroidism should discuss the dosage of levothyroxine in pregnancy with their doctor. The dependency of the infant on the mother’s natural hormones means that an increase in dosage is often necessary. An increased dosage can prevent the risks associated with deficiency of thyroxine in infants. It also illustrates the safety of the drug: too little of it is more of a cause for concern than is using at all.


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