Exophthalmos or proptosis is a state or condition in which the eyeballs protrude from the eye sockets such that they appear to bulge out from the patient's skull, it can be a manifestation of a few different other health conditions such as its most common cause in Graves' disease (autoimmune hyperthyroidism) as well as hypothyroidism, tumors (benign or malignant), trauma or injury to the eye, severe glaucoma, orbital cellulitis of infection of the eye, Erdheim-Chester disease (abnormal histiocyte multiplication), and Cushing's syndrome.
Most commonly though, it is a manifestation of Graves' disease or other thyroid-related conditions.