My training is that you rule out a medical cause first for everything.
How can you rule out a disease which has not been described yet?
For instance, how could you rule out MuSK MG in 1998, when it was only discovered in 2000?"
Obviously you can't...though I've had MANY patients where I have said, "clearly this is a medical problem, even though they can't figure it out. Just because the doctors can't find the reason for it doesn't mean it isn't there."
I've had kids sent to me with unexplained rashes for "anxiety," where I have completely ruled out anxiety in a couple of visits and assured the parents it's a medical problem. I have many, many examples like that. I would never attach a diagnosis where it didn't belong just because the medical community couldn't name the disease. I think everyone knows neither medicine nor the mental health field are exact sciences.