The causes of frequent diarrhea in adults, also known as chronic diarrhea in clinical medicine, can be divided into many kinds, as follows: 1. 3, other diseases, including diabetes, hyperthyroidism and other autoimmune diseases, may also cause frequent diarrhea in adults. Therefore, it is recommended that if patients have frequent diarrhea, they must go to the hospital for examination as soon as possible to rule out the possibility of malignant diseases. Also, if you have frequent diarrhea, you should pay attention to timely hydration to avoid dehydration.