Diabetes insipidus, diabetes mellitus (DM), and SIADH (Syndrome of Inappropriate Antidiuretic Hormone) share the symptom of:
a. Excessive thirst (polydipsia)
Excessive thirst, along with excessive urination, is a common symptom in both diabetes insipidus and diabetes mellitus due to the disruption of fluid balance. In SIADH, excessive thirst can also occur, although it's primarily characterized by the opposite problem, which is retaining too much fluid and having dilute urine.