So much good information was given to you in all of these posts. the things that people said to you also helped me. I didn't know that we needed to ignore muscle weakness. After I say such things to my doctor and he does not speak to the symptom, I let it drop, thinking that this isn't something they need to know.
Thank you Bill for sending this website, and the info there.
It looks like you have already received very good information. MCTD is, in fact, a specific diagnostic category. It is NOT the same thing as UCTD (undifferentiated CTD), and it is correlated with the RNP antibodies. My mother had MCTD, but she didn't start with that diagnosis. She started with RA, and then gradually added the rest. As Bill pointed out, the typically MCTD is a mish mash of sx from PM, SSc, and SLE, although I've seen other combinations as well. My rheumy, who claims to be an expert in MCTD, says that is his experience overlap CTDs are much more common than MCTD. For example RA plus SLE (rhupus) is far more common that the PM, SSc and SLE combination. Nevertheless, the antibodies sort of clinch the diagnosis from my understanding. You cannot receive a diagnosis without those antibodies. Remember, though, that it's the symptoms that need to be treated, and it sometimes doesn't matter what the symptoms are collectively called, as long as you are receiving proper treatment for the symptoms. AI diseases are not easy to diagnose or treat. We all do the best we can.