Here is a website explaining PTH and how it relates to calcium, vit. d, etc.
http://www.labtestsonline.org/understanding/analytes/pth/sample.html
Here is a blurb from one section regarding Vit. D.:
2. What does vitamin D have to do with PTH?
If you do not have enough vitamin D, your body will not be able to absorb calcium properly. Vitamin D regulates the intestinal absorption of calcium, while PTH regulates the activation of vitamin D. Too much and too little vitamin D can imbalance calcium metabolism.
You may try to use the Ask an Expert part of this forum.
I'm not so sure this is the right forum.