PTH and rickets: The unique and complex interactions between hypovitaminosis D, parathyroid hormone, and calcium (both in the serum and in intracellular compartments) make it especially difficult to tease apart how much of these effects are truly unique and distinct to vitamin D. Moreover, statistically significant associations between two factors do not prove that one has been the causative factor for the other, as these two factors may be closely related to a third factor.