Levothyroxine Pregnancy and Breastfeeding Warnings
Levothyroxine Pregnancy Warnings
Levothyroxine has been assigned to pregnancy category A by the FDA. Levothyroxine is a naturally occurring hormone, normally present in both maternal and fetal circulation. Levothyroxine, in replacement doses, is not expected to adversely affect the fetus. On the contrary, hypothyroidism may lead to poor pregnancy outcome. Thyroid replacement therapy should be maintained during pregnancy.
The Collaborative Perinatal Project monitored 50,282 mother-child pairs, of whom 537 were exposed to levothyroxine or thyroid during the first trimester. The standardized relative risk for malformations was 1.19 compared to a relative risk of 1.05 in the normal population. The standardized relative risks for cardiovascular malformations, polydactyly in Blacks, and Down syndrome were 1.61, 2.42, and 2.36, respectively. However, as the total number of malformations is small and because the contribution of thyroid disease itself cannot be excluded, the true risk of thyroid hormone use in the first trimester cannot be established based on these data. The extent to which levothyroxine is transferred from mother to fetus is controversial. Many early studies concluded only negligible amounts of levothyroxine are transferred to the fetus. However, a recent study evaluated maternal-fetal transfer of levothyroxine based on data obtained from 25 infants with a complete inability to form levothyroxine. Based on neonatal serum levothyroxine levels and an associated elimination half-life of 3.5 days, the authors estimated levothyroxine levels in cord blood to range from 35 to 70 nmol/L (normal T4, 80-170 nmol/L). As the infants were unable to synthesize levothyroxine, it was concluded that all levothyroxine present must have been of maternal origin. In the same study, 15 infants with thyroid agenesis were also evaluated. Serum concentrations of levothyroxine as well as elimination half-life were similar to those obtained from the infants with an inability to form levothyroxine. Again, these data support maternal transfer of levothyroxine late in pregnancy. TSH levels should be closely monitored during pregnancy. There is some evidence that hypothyroid women require more thyroxine during pregnancy.
Levothyroxine Breastfeeding Warnings
In one study, levothyroxine levels in 70 milk samples from 20 euthyroid women 17 to 39 days after delivery were determined by gas chromatography-mass spectrometry (GCMS). Levothyroxine was present in milk in concentrations less than 4 ng/mL. The authors suggested that, at most, a breast-fed infant would ingest approximately 10% of the recommended dose for hypothyroid infants. The majority of other studies have used radio immunoassay (RIA), a less specific assay, to measure thyroid hormone concentrations in human milk. Such studies have yielded significantly variable results. Earlier studies concluded that levothyroxine and triiodothyronine were present in sufficient quantities to treat a hypothyroid infant. However, most subsequent studies have concluded otherwise. In addition, the presence of thyroid hormone in breast milk does not appear to interfere with neonatal thyroid screening.
Levothyroxine is excreted into human milk in small amounts. Levothyroxine, in replacement doses, is not expected to cause adverse effects in the nursing infant. The manufacturer recommends that caution be used when administering levothyroxine to nursing women. However, adequate replacement doses of levothyroxine are needed to maintain normal lactation.
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