Avoiding antithyroid drugs early in pregnancy reduces the likelihood of congenital defects in newborns, according to a Danish study. StineLindingAndersen and Peter Laurberg from Aalborg University Hospital said, “Internists should be aware of the effects on subsequent pregnancies when applying antithyroid drugs for the treatment of related diseases in young women. Antithyroid medications should be discontinued as early as possible in pregnant women.” Hyperthyroidism can seriously affect the health of the pregnant woman and the development of the fetus, but the application of antithyroid drugs early in pregnancy can lead to birth defects in newborns. Current guidelines recommend treatment with propylthiouracil (PTU) early in pregnancy, with a change to methimazole/carbimazole (MMI/CMZ) later. Dr. Andersen and colleagues analyzed data from the Danish Birth Registry, the Danish National Prescribing Registry, and the Danish Hospital Registry to determine the correlation between these antithyroid medications and birth defects, granulocyte deficiencies, and liver failure in the general population and in pregnant women. Nearly 30,000 people were taking antithyroid medications, with the majority receiving MMI/CMZ (n=27281) and a small proportion (n=5895) receiving PTU. The incidence of MMI-related side effects in the general population was about twice as high as the incidence of PTU-related side effects, but there was a nearly five-fold difference in this incidence, wrote the study published online Jan. 27 in the Journal of Clinical Endocrinology & Metabolism. The incidence of granulocyte deficiency was significantly higher with PTU (0.27 percent) than with MMI/CMZ (0.11 percent, P=0.02), while the incidence of hepatic failure was not significantly different between those treated with PTU (0.05 percent) and MMI/CMZ (0.03 percent, P=0.4). Compared to the general population, antithyroid drug-associated granulocyte deficiency and liver failure were uncommon in pregnant women. Of the 2206 pregnant women on antithyroid medication, 35% of the newborns had associated birth defects (340 cases/10,000 exposed to antithyroid medication). Andersen and Laurberg write, “We recommend that women treated with antithyroid medications intensify relevant testing during pregnancy. Discontinue the use of antithyroid medication if the results are positive after communicating with the physician. If the internist observes remission of the woman’s hyperthyroidism, we recommend discontinuing thyroid medication for observation and performing weekly thyroid function tests until mid-pregnancy.” They also advise, “If antithyroid medication is essential in early pregnancy, we recommend PTU. if the patient is planning a pregnancy, PTU therapy can be considered even up to the time of pregnancy. If antithyroid medication is still needed after early pregnancy, the internist has the option of continuing PTU therapy or changing to MMI/CMZ therapy.” Andersen and Laurberg conclude, “Early pregnancy is as important to the thyroid physician as late pregnancy is to the obstetrician. Untreated hyperthyroidism can complicate pregnancy and should be carefully managed and controlled by the internist. However, increased screening and evaluation of clinical indications early in pregnancy in women treated with antithyroid medications may reduce the incidence of adverse events due to antithyroid medications.”