Undifferentiated connective tissue disease (UCTD) is an autoimmune disease that can affect several systems in the body. Connective tissue disease (CTD) is classified as undifferentiated CTD when signs and symptoms are consistent with a CTD, but do not fulfill the diagnostic or classification criteria for one of the previously defined CTDs (for example, rheumatoid arthritis or lupus). Symptoms of UCTD vary, but more common symptoms include Raynaud phenomenon, arthritis, joint pain, fever, and symptoms involving the mucous membranes and skin such as dryness in the eyes and mouth, ulcers in the mouth, sensitivity to sunlight (photosensitivity), or hair loss. Various other symptoms may be present and may additionally involve the lungs, heart, muscles, or nervous system.

The underlying cause of UCTD is unknown, but it is a type of autoimmune disease, which means that the immune system malfunctions and mistakenly attacks a person's own healthy tissues. The diagnosis is made by ruling out other conditions that can cause the same symptoms, aided by blood and urine tests, and imaging studies such as X-ray, CT scan, or ultrasound. Treatment usually includes the use of non-steroidal anti-inflammatory drugs (NSAIDs), antimalarial drugs, and/or corticosteroids. People with photosensitivity may be helped by avoiding prolonged exposure to sunlight, and those with Raynaud phenomenon should avoid exposure to extremely cold temperatures.

More common signs and symptoms include:

• Raynaud phenomenon (a condition in which the blood vessels do not bring enough blood to the hands and feet)
• joint pain (arthralgia)
• arthritis
• symptoms involving the mucous membranes such as dryness in the eyes and mouth, or ulcers in the mouth
• symptoms involving the skin, such as sensitivity to sunlight (photosensitivity), a rash on the cheeks (malar rash), or hair loss (alopecia)
• fever

Many other signs and symptoms are possible and often overlap with those of well-defined connective tissue diseases such as lupus, mixed connective tissue disease, systemic sclerosis, rheumatoid arthritis, polymyositis, dermatomyositis, and Sjögren syndrome. Symptoms of any of these that may occur in people with UCTD include but are not limited to:

• other symptoms involving the mucous membranes or skin
• inflammation in the lungs and chest cavity, causing chest pain when breathing (pleuritic chest pain)
• inflammation of tissues that surround the heart (pericarditis), which also causes chest pain
• muscle weakness
• nervous system symptoms such as abnormal sensations, seizures, or mental disorders
• decreased number of white blood cells (leukopenia), red blood cells (anemia) or platelets (thrombocytopenia)

Undifferentiated connective tissue disease (UCTD) is type of autoimmune disease, which means that the immune system malfunctions and mistakenly "attacks" a person's own healthy tissues. However, like many autoimmune diseases, the reason a person develops UCTD is not known. Researchers suspect that both genes and environmental triggers interact to cause a person to develop UCTD.

A variety of other types of medications may be considered on an individual basis, such as calcium channel blockers for symptoms of Raynaud phenomenon. People with Raynaud phenomenon should also avoid exposure to very cold temperatures. Those with increased sensitivity to sunlight (photosensitivity) should try to minimize being exposed to sunlight for long periods of time. Wearing protective clothing and sunblock also helps protect against the effects of ultraviolet light.