MCTD vs. UCTD (Mixed Connective Tissue Disease vs. Undifferentiated Connective Tissue Disease)

Medical Author: William C. Shiel Jr., MD, FACP, FACR
Medical Editor: Melissa Conrad Stöppler, MD

I note that some viewers have been requesting some clarification of the meaning of the terms "mixed connective tissue disease" (MCTD) and "undifferentiated connective tissue disease" (UCTD).

Connective tissue diseases are a special group of rheumatic diseases (diseases that feature abnormalities of the muscles and/or joints) that can be associated with arthritis. The cause(s) for the connective tissue diseases is (are) unknown. They are characterized as a group by the presence of spontaneous overactivity of the body's immune (defense) system. This overactivity results in the production of unusual antibodiesthat are found in the blood. The antibodies themselves may or may not cause any problems in patients with connective tissues diseases, but they are commonly found in the blood as a characteristic feature.

The connective tissues are the structural portions of our body that essentially hold the cells of the body together. These tissues form a framework, or matrix, for the body. The connective tissues are composed of two major structural protein molecules, collagen and elastin. There are many different types of collagen protein that vary in amount in each of the body's tissues. Elastin has the capability of stretching and returning to its original length, like a spring or rubber band. Elastin is the major component of ligaments (tissues that attach bone to bone) and skin. In patients with connective tissue diseases, it is common for collagen and elastin to become injured by inflammation. Diseases in which inflammation of collagen tends to occur are also referred to as collagen diseases.