If a tumour in the colon caused thin stools, would the stools always be thin or could they alternate between normal size and thin?

It really depends on many factors like where in the intestine is the tumor, how big is it, etc. If your stools have changed, you should see a GI doctor for evaluation. A colonoscopy can be done where they look into your colon with a lighted scope while you are sedated. They would then visualize any abnormalities, including tumors, polyps, etc. Best of luck to you!

There would be many factors that would dictate the diameter of the stool, including where the obstruction is, how much fiber is in your diet, and what is the diameter of the narrowing. If you’re having any concerns, see a gastroenterologist. This can be easily checked out.