Not sure of everyone's background (I'm new here), but I had "low" Vit D (that being 25D, my values were typically 20 ng/mL) and was told to supplement. I did that during May 2011, but also discovered looking back through all my labs that I had elevated calcium for at least 13 years. My endocrinologist tested my 1,25D (this is the active form of Vit D), it was off the charts high at 140.7 pg/mL. My PTH was low-normal, typically around 20 pg/mL. In fact, I'd been pursuing a possible diagnosis of hyperparathyroidism until my 1,25D came back so high.
I was advised to get the 1,25D down, and see how my calcium and PTH looked then. Started Benicar last year, and 1,25D and calcium went down, PTH came up into more normal ranges.
Some folks (e.g. Marshall) believe that it's the high 1,25D driving down the 25D - the 25D is the precursor, so if you have too much 1,25D it makes sense to me that your body would try to down-regulate the amount of precursor material. ETA: Therefore, he's saying low 25D is a *result* of the disease process rather than the *cause*. In most studies, the researchers only look at 25D...
Note that I am on a modified Marshall Protocol (they would say I'm not on it since I don't stricly follow their rules) - I think there is some value to parts of their approach but also think there is lots of room for other approaches, e.g. I'm also on higher levels of antibiotics, Malarone, LDN, just started Homeo KMAF.