Functional medicine is an approach. Metametrix is huge in functional medicine and it is the way treating the patient by viewing the patient as a person or as a whole and not as a 5 minute case. The testing is done differently and use of natural supplement are also important.
Naturopaths are usually functional medicine practitioners, and you will find MDs as well as chiropractors and even MS or PhDs. I have found very good MDs who practice functional medicine and environmental medicine are hard to find.