<prompt> The most important regulator of serum 1,25 OH2 vitamin D concentration is:Serum calciumSerum magnesiumSerum 25 OH vitamin DParathyroid hormone  <response>: A major function of parathyroid hormone is to act as a trophic hormone to regulate the rate of formation of 1,25OH2 vitamin D. The mechanism by which parathyroid hormone exerts this effect may be secondary to its effects on phosphorus metabolism. Other hormones, including prolactin and estrogen, also may playa role in stimulating the production of 1,25OH2 vitamin D. 