Unknown The most important regulator of serum 1,25 (OH)2 vitamin D concentration is: A Serum calcium B Serum magnesium C Serum 25 (OH) vitamin D D Parathyroid hormone 💡 Explanation The most important regulator of serum 1,25 (OH)2 vitamin D concentration is: ✓ Correct Answer: D. Parathyroid hormone