What happens if jaundice left untreated in adults?

Depends: First and foremost we have to know the cause of the jaundice and whether the disorder is treatable, reversible, benign, malignant, worsening or gradually improving on its own,etc. For example, a malignant growth in the pancreas or gall bladder would be fatal. Some cases of cirrhosis may be stabilized. Hepatitis A or infectious mononucleosis will eventually resolve.