Synthyroid is quite safe for breastfeeding. The best reference I've got on drugs in breastfeeding is Tom Hale, PhD, "Medications and Mother's Milk". It's published in paperback every year and is inexpensive ($20 or less). You can get it from Pharmasoft Medical Publishing 800-378-1317. Re: Synthyroid, Hale says "extremely low levels get into breastmilk. However, the thyroid function of the infant may need to be evaluated periodically." Linda L. Shaw MD, FAAP Altoona PA