Unfortunately adequate data is not available on the adverse effects, if any, caused by thyroxine during pregnancy. Therefore the gynaecologist has to weigh the benefits v/s possible side effects. To be extra cautious, it is recommended that pregnancy be monitored more frequently than is usually done particularly ultrasound and blood tests.
In very small amounts, thyroxine is also excreted in mother's milk but its effect on neonates is not known. It is true that no major adverse effects have been reported from women who became pregnant when on thyroxine.