Should you breast feed, when your taking thyroid, like.75mg?

My daughter has been nursing her little boy and she says her doctor says it's fine too and he knows she's on thryroid. Her little one is now two years old and still nursing which I think is far too long and he's skinny and it makes me wonder if that thyroid she's taking is affecting HIM. I read up on Synthroid and it says do not nurse and breast feed. HELP give me some suggetions, pleaseShould you breast feed, when your taking thyroid, like.75mg?
More than likely the meds aren't causing the problem. The problem is she's not feeding him properly. At two he should be on solid foods and baby food, not the breast. More than likely her milk isn't produced enough and she is unknowingly starving that baby. Talk her into new feeding habits. That child will suffer from worse things than malnourishment if she keeps him on the breast for too long.
Always ask your ob-gyn before you take any meds. when you breastfeed. Remember anything you eat or drink, Your baby does too.