Propylthiouracil Pregnancy and Breastfeeding Warnings

Propylthiouracil Pregnancy Warnings

Propylthiouracil has been assigned to pregnancy category D by the FDA. Because propylthiouracil readily crosses placental membranes and can induce goiter and even cretinism in the developing fetus, it is important that a sufficient, but not excessive, dose be given during pregnancy. In many pregnant women, the thyroid dysfunction diminishes as the pregnancy proceeds; consequently a reduction of dosage may be possible. In some instances, propylthiouracil can be withdrawn several weeks or months before delivery. If propylthiouracil is used during pregnancy, or if the patient becomes pregnant while taking propylthiouracil, the patient should be warned of the rare potential hazard to the mother and fetus of liver damage. Since methimazole may be associated with the rare development of fetal abnormalities such as aplasia cutis and choanal atresia, propylthiouracil may be the preferred agent during organogenesis, in the first trimester of pregnancy. Given the potential maternal adverse effects of propylthiouracil (e.g., hepatotoxicity), it may be preferable to switch from propylthiouracil to methimazole for the second and third trimesters.

Propylthiouracil is known to cross the placenta. In one study, six hyperthyroid women were treated with propylthiouracil throughout pregnancy. Doses were withheld during labor and delivery. One to nine hours after the last dose of 100 to 150 mg, maternal propylthiouracil serum concentrations averaged 0.19 mcg/mL while cord blood concentrations averaged 0.36 mcg/mL. Pharmacokinetic evaluation suggested fetal clearance was slower than maternal clearance of propylthiouracil. To date, there is no evidence to support a teratogenic risk of propylthiouracil during pregnancy. However, mild hypothyroidism and neonatal goiter may occur. One review estimates a 12% incidence of propylthiouracil-induced neonatal goiter based on reported cases, although iodides were also used in some cases. As maternal hyperthyroidism may be associated with an increased risk of congenital anomalies, spontaneous abortion, prematurity, and perinatal death, medical and/or surgical intervention is usually indicated.

Propylthiouracil Breastfeeding Warnings

In one study, nine lactating women were administered 400 mg of propylthiouracil by mouth. The mean amount of propylthiouracil excreted during 4 hours after drug administration was 0.025% of the administered dose.

Propylthiouracil is excreted into human milk in small amounts and therefore likely results in clinically insignificant doses to the suckling infant. While adverse effects in the nursing infant are not reported, the manufacturer considers propylthiouracil to be contraindicated during lactation. However, propylthiouracil is considered compatible with breast-feeding by the American Academy of Pediatrics.

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