One of my family members has been diagnosed with an underactive thyroid and was prescribed Synthroid. I also know a female who takes Synthroid and she continued her medication during pregnancy (don't know about breastfeeding since we lost touch before her baby was born). Hale has information on this medication in his latest edition and several studies are cited. He states "most studies indicate that minimal levels of maternal thyroid are transferred into human milk and that the amount secreted is low...the amount secreted after supplementing a breastfeeding mother is highly controversial and numerous reports conflict." Let us know how this turns out. Mary Kay Smith, CLE, IBCLC Romeoville, IL (near chicago)