I think you missed my point. vitamin D levels in and of themselves aren’t an affliction. The “correct” level is established by looking at what everyone else has and also by trying to look for comorbidities. But the comorbidities are often hard to tease out; about the only disease we know is rickets. Everything else is weakly correlational for vitamin D.

How do you think they define healthy levels of other hormones like testosterone and estrogen? They look at the range of levels for them, they look to see when they think diseases start, and they say those are correlated and you should adjust.