Babies of mothers taking antithyroid drugs have a higher risk of goiter, hypothyroidism, or even cretinism (a severe result of hypothyroidism that can cause mental retardation). That means that both PTU and methimazole are in the FDA's Category D during pregnancy, meaning that they pose a risk to a fetus, but the benefit may outweigh the risk. The risk of untreated hyperthyroidism for both mother and baby, for example, is considered greater than the risk of taking a low dose of antithyroid medication, and so antithyroid drugs are used -- very carefully -- in pregnancy. Typically, doctors will recommend the smallest possible dose that will control the condition.
Methimazole more easily crosses the placental membranes, and methimazole is associated with a higher risk of fetal side effects -- including birth defects in the scalp -- if taken during the first trimester of pregnancy. These scalp defects have not been seen in babies of mothers who took PTU, so PTU is the recommended drug during the first trimester of pregnancy. Methimazole is typically recommended for second and third trimester treatment.Can You Take Antithyroid Drugs While Breastfeeding?
Breastfeeding while on antithyroid drugs is controversial. Some physicians say that it is safe, but have preferred PTU over methimazole, since methimazole crosses into breast milk more easily. Other practitioners have concerns about use of any of these drugs during breastfeeding. Patients should discuss this with their physician, as the medication and dosage level may have an effect on the doctor's recommendations regarding breastfeeding.
Before you start antithyroid drugs, talk to your doctor about any history of liver problems, such as hepatitis or jaundice, as they can affect your ability to process antithyroid drugs safely, and your risk of side effects can be greater.
Also be sure to tell your doctor if you have or have ever had any blood disease, such as decreased white blood cell (known as leukopenia), decreased platelets (thrombocytopenia), or aplastic anemia.
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Ross, Douglas MD, "Patient information: Antithyroid drugs," UpToDate. Last updated: November 13, 2009
With thanks to Leslie Blumenberg for her research support for this article