Use is considered contraindicated and should be used during pregnancy only if the benefit outweighs the risk.

AU TGA pregnancy category: C
US FDA pregnancy category: D (IV); Not assigned (tablets)

Risk Summary: Use in pregnant women may increase the risk of adverse fetal effects (e.g., neonatal hypo- and hyperthyroidism, neonatal bradycardia, neurodevelopmental abnormalities, preterm birth, and fetal growth restriction). Untreated underlying arrhythmias in pregnant women pose a risk to the mother and fetus.

Comments:
-If this drug is used during pregnancy, or if the patient becomes pregnant while taking this drug, the patient should be apprised of the potential harm to the fetus.
-Patients planning pregnancy should be advised of the long half-life associated with this drug and its metabolite.
-Therapeutic levels may be difficult to maintain due to the increased volume of distribution and increased drug metabolism inherent in the pregnant state.
-Monitor the newborn for signs and symptoms of thyroid disorder and cardiac arrhythmias.

Animal studies have revealed evidence of embryofetal toxicity at doses less than the maximum recommended human maintenance dose. In humans, congenital goiter/hypothyroidism, hyperthyroidism, and neonatal bradycardia have been reported. There are no controlled data in human pregnancy.

Risk of arrhythmias may increase during labor and delivery; monitor patients continuously during labor and delivery.

AU TGA pregnancy category C: Drugs which, owing to their pharmacological effects, have caused or may be suspected of causing, harmful effects on the human fetus or neonate without causing malformations. These effects may be reversible. Accompanying texts should be consulted for further details.

US FDA pregnancy category D: There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience or studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.

US FDA pregnancy category Not Assigned: The US FDA has amended the pregnancy labeling rule for prescription drug products to require labeling that includes a summary of risk, a discussion of the data supporting that summary, and relevant information to help health care providers make prescribing decisions and counsel women about the use of drugs during pregnancy. Pregnancy categories A, B, C, D, and X are being phased out.

See references

The infant receives an estimated dose of this drug plus its active metabolite between 3.5% and 45% of the maternal weight-adjusted dosage, with a median dose of approximately 11%. Infant serum levels of this drug plus metabolite range from 14% to 74% of simultaneous maternal levels (the higher values reflect transplacental passage of the drug). Some authorities believe this drug can be used during breastfeeding with periodic monitoring of infant cardiac and thyroid function, especially if only one dose is administered.

Breastfeeding is not recommended during use of this drug; use is contraindicated per some authorities.

Excreted into human milk: Yes

See references