The most important regulator of serum 1,25 (OH)2 vitamin D concentration is:

A major function of parathyroid hormone is to act as a trophic hormone to regulate the rate of formation of 1,25(OH)2 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 1,25(OH)2 vitamin D.