Mixed Connective Tissue Disease also presents itself with a red rash on the face in a butterfly pattern, which is what makes it "mixed" because of the group of symptoms similar to that of Systematic Lupus, Scleroderma, and Rheumatoid Arthritis. Over time, there is the possibility of it manifesting itself as one of these diseases entirely or a combination of two or even three (when certain antibodies of these diseases specifically become present in blood tests).

For example:

Anti-smith and Anti-dsDNA antibodies are strong indicators of Systemic Lupus.

SCL-70 antibody indicates Scleroderma.

Anti-RNP antibodies strongly indicate Mixed Connective Tissue Disease, but can also appear in those with Systemic Lupus (especially if Anti-Smith and/or Anti-dsDNA antibodies are present as well).

That's a really good question. In short, nobody really knows. RA attacks the joints and tendons, discord lupus affects the skin, and MCTD is a rare disease that's kind of a catch-all for signs and symptoms experienced with lupus, polymyositis and scleroderma. Not much is known about inflammatory autoimmune diseases but, it's kind of all the same. The immune system is "confused" and attacking your own body. The diagnosis is just a way of categorizing the symptoms depending on which systems and organs are being attacked. Since everyone displays symptoms differently and no one knows which systems/ organs your immune system will attack next or even if it will progress to other areas it's hard to say for certain if you will progress to also display other signs/symptoms associated with lupus.

My initial diagnosis was Mixed Connective Tissue Disease, but eventually was changed to SLE. I do not believe it morphed, my doc at the time could not get a definitive lupus diagnosis, but eventually did. Thus, my lupus is attacking my connective tissues. My RA, which was diagnosed five years ago, is attacking the joints and tendons.

I don't know the answer to your questions but a biopsy is one way to find out I just had one and waiting for results. I have SLE, and discord lupus, like I need another one. Is what you have painful? Because mine is. I have the red butterfly but on my arms. So not sure what to think. Apologize if I didn't help you. Praying for you.

Same thing happened to me. I started with a diagnosis of MCTD & 20 years later, I got a SLE diagnosis. My rheumatologist told me that it's not uncommon to have more than one autoimmune disease & some folks have 5.
With time, lab tests improve, but since there are so many overlapping symptoms I guess I'm just happy that Plaquenil works for me now. It didn't help when I had a working diagnosis of MCTD. Neither did Methotrexate. Eventually the symptoms mostly went away until I could tell by the way my skin was reacting that I must have Lupus now. This time my labs confirmed.