I am really curious-how many people are we talking here, and how do they know they are making that much...do you mean that is what they pump while separated from baby? pumping once, twice?Ive read that most women's s supplies regulate to where they only make what their child needs, yet all of my friends and fam currently nursing make 8 or so ounces every 3-4 hours. And these aren't mama's to new borns... Their babies are anywhere from 2-7.5 months old.
Since I am a sahm, I know lots of other sahm's, who never or very rarely pump. So most of the moms I know personally, have absolutely no idea how much milk they make...and they 'successfully' breastfeed. Certainly I have never had a good idea how much I 'make.'
I think you have worked waaay too hard to let the possibly way out of the norm experiences of a few other mothers get you down.
On the other hand, I think it is a good idea to get your thyroid checked, given your history of slower than average baby weight gain.