Is there a hosted commenting service that can be integrated with an existing user system?

I'm looking for a hosted web service like Disqus that allows me to use an existing user system rather then having my users sign up again on an external site. I've looked around at commenting systems and the systems I've found all require an account for their platform.

I already have users on my website and I don't want them to sign up for another account to be able to comment. Disqus does have something they call "Single Sign-On" (https://help.disqus.com/en/articles/1717139-getting-started-with-single-sign-on) which sounds just like what I need. The problem is that Single Sign-On is only available for Business level subscription, which sounds like something for huge websites. They even state that it's for sites with over 150 000 daily page views.