MCTD was originally defined as symptoms of lupus, polymyositis, and scleroderma in 1972. The anti rnp antibody must be present to diagnose mctd. In recent years, mctd can include dermatomyositis, RA and even other autoimmune symptoms. Every case is different in presentation, response to treatment, and severity. Few have symptoms of all the possible diseases.
I presented with lupus and then pm but nothing else. Lupus can attack any part of our body so it is possible that it is what attacked your thyroid or it might be an entirely separate problem not related to mctd. The good news is that most cases are controllable and many are very mild. Mine was not but I have made a remarkable recovery and lead a normal, active life.
No point in worrying because no one, not even any doctor, can tell you how things are or are not going to progress.
MCTD (SLE Lupus, polymyositis, scleroderma). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, barretts esophagus, quadriplegic in 2005. Recovered and now active in downhill skiing, golf, hiking. Meds: amlodipine, benezapril, omeprazole, potassium, folic acid, vitamins, maxide.
Remain Optimistic and you can overcome.