That's the Dx that my internist uses for me, although I've seen it on the Internet more frequently referred to as undifferentiated connective tissue disease. My understanding from my docs is that it's generally a placeholder diagnosis, when they know you've got something going on but your test results don't allow them to pinpoint which of the connective tissue autoimmune diseases you've got. My rheumy and my internist say they're waiting for my body to "declare itself" with a specific known disease. (OTOH, it could likely be that there's more connective tissue diseases than they've currently identified and that's why some people with mctd/uctd never differentiate.)
--SM
I've suspected it for some time. My followup after a battery of tests is in a few weeks. I'll post the results.