This test measures the levels of 1,25-dihydroxycholecalciferol, the active form of vitamin D3, in the blood. Vitamin D3 plays a crucial role in calcium and phosphorus metabolism, bone health, and overall immune function. Unlike the more commonly measured 25-hydroxy vitamin D, this test reflects the active hormone form of vitamin D, which is tightly regulated by the kidneys and parathyroid hormone.