Maternal methimazole therapy does not affect thyroid function or intellectual development in breastfed infants with doses up to 20 mg daily. Taking methimazole right after nursing and waiting for 3 to 4 hours before nursing again should minimize the infant dosage. No cases of thyroid function alteration have been reported among infants exposed to methimazole via breastmilk. Some experts now recommend that methimazole should be considered the antithyroid drug of choice in nursing mothers.[1-3]
The American Thyroid Association recommends only monitoring infants for appropriate growth and development during routine pediatric health and wellness evaluations and routine assessment of serum thyroid function in the child is not recommended.[4] Rare idiosyncratic reactions (e.g., agranulocytosis) might occur, and the infant should be watched for signs of infection. Monitoring of the infant's complete blood count and differential is advisable if there is a suspicion of a drug-induced blood dyscrasia.