Untreated celiac disease can be life-threatening.
Celiacs who do not maintain a gluten-free diet also stand a much greater chance of getting certain types of cancer, especially intestinal lymphoma.
Untreated celiac disease can cause temporary lactose intolerance. Lactose is a sugar found in dairy products. To be digested it must be broken down by an enzyme called lactase. Lactase is produced on the tips of the villi in the small intestine. Since gluten damages the villi, it is common for untreated celiacs to have problems with milk and milk products. (Yogurt and cheese are less problematic since the cultures in them break down the lactose). A gluten-free diet will usually eliminate lactose intolerance. However, a number of adults (both celiacs and non-celiacs) are lactose intolerant even with a healthy small intestine; in that case a gluten-free diet will not eliminate lactose intolerance.
Celiacs often suffer from other food sensitivities. These may respond to a gluten-free diet--or they may not. Soy and MSG are examples of food products that many celiacs have trouble with. However, it should be noted that these other sensitivities, while troublesome, do not damage the villi. As far as we know, only gluten causes this damage.