Levothyroxine sodium has been taken by a large number of pregnant women and women of childbearing age without any form of definite disturbances in the reproductive process having been observed so far. Thyroid, hypo- or hyperactivity in the mother may, however, unfavorably influence the fetal outcome or well-being. Thyroxine is excreted in breast milk in low concentrations and this may be sufficient to interfere with neonatal screening for hypothyroidism.