If a doctor is an internal medicine doctor what abbreviations would be after his name?

The letters after the physician's name would depend on what medical degree they earned (e.g., a Doctor of Osteopathic Medicine would have D.O. after his/her name and a Doctor of Medicine would have M.D. after his/her name) and also would depend on their membership as a fellow in organizations such as the American College of Internists or the American College of Osteopathic Internists. If they had such membership, you would see the letters FACI or FACOI after their name as well.