Overview
Calcium Balance Depends on a Tightly Controlled System
Calcium is not merely a bone mineral. Ionized calcium helps regulate cardiac conduction, skeletal and smooth muscle contraction, nerve transmission, and coagulation.
Calcium is not merely a bone mineral. Ionized calcium helps regulate cardiac conduction, skeletal and smooth muscle contraction, nerve transmission, and coagulation. The body defends the serum calcium concentration aggressively because even a modest change can disrupt these functions. Vitamin D acts as a prohormone. Cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) undergo 25-hydroxylation in the liver to become 25-hydroxyvitamin D, written as 25(OH)D. This is the circulating storage form measured when vitamin D status is assessed. The kidney then converts 25(OH)D to calcitriol, the active hormone. Calcitriol increases intestinal absorption of calcium and phosphate. Parathyroid hormone (PTH) works alongside it to preserve serum calcium. When calcium intake or absorption is inadequate, PTH can maintain a normal serum calcium by increasing renal calcium conservation and mobilizing calcium from bone. That compensation creates an important clinical distinction: a normal serum calcium does not prove that calcium intake, vitamin D status, or skeletal mineral balance is adequate.