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)