I can respond to this post from personal experience only.   Disclaimer,
this is just personal experience.  I am not a dr, nor do I give medical
advice.  That said, I started taking PTU when Jonathan (my nursling) was
11 months old.  I was taking six 50mg pills a day.  Nothing was ever
said to me about having Jonathan's blood checked.  I was told it was
approved by the AAP for breastfeeding moms.  I am now down to just one
pill a day.  I asked my son's dr if he felt we should have Jonathan's
blood checked and he said it was such a small amount that he felt it
wasn't necessary, but he would do it if I wanted him to.   This is just
my experience, I know I wouldn't knowingly take anything that would harm
my son, nor would his dr allow it.

Maggie