A good example is when I had anaemia. I was correct, but if I self medicated (which some idiot told me to, I rejected their advice) I would have taken iron. Turns out it was actually a folate deficiency, but the GP knew to give me a blood test because I suspected I had it. At least in the UK, if they didn't like you telling them about your self diagnoses, the NHS website wouldn't exist. The correct thing to do with your self diagnoses is to take it to a doctor, not treat it yourself.