American Board of Orthodontics (ABO) Practice Exam 2026 - Free Orthodontics Practice Questions and Study Guide

The regulating factor is the active hormone form of vitamin D, the 1,25-dihydroxy metabolite. This circulating hormone directly enhances intestinal absorption of phosphate by upregulating the transport mechanisms in enterocytes, so phosphate from the diet enters the bloodstream more efficiently and plasma phosphate rises.

The other forms aren’t doing this: 25-hydroxyvitamin D is mainly a storage reservoir with little direct effect on phosphate homeostasis; vitamin D3 is the precursor that must be hydroxylated to become the active form; calcitriol is simply another name for the same active metabolite, sharing the same effect on phosphate absorption.