Vitamin D, 1, 25-Dihydroxy, Serum

1,25-Dihydroxyvitamin D, also called calcitriol, is the active form of vitamin D that your body uses to control calcium and phosphorus levels and support bone health. Unlike the more commonly tested '25-hydroxy vitamin D,' this test measures the activated hormone form and is used for specific medical conditions rather than routine vitamin D screening. Your kidneys are primarily responsible for making calcitriol, so kidney disease can lower these levels. Certain conditions like sarcoidosis can cause the body to make too much calcitriol, leading to high calcium levels. Your doctor will interpret this result alongside other blood tests to get a complete picture of your calcium and bone metabolism.

When elevated: Elevated calcitriol is associated with hypercalcemia, hypercalciuria, nephrolithiasis, and soft tissue calcification. In granulomatous diseases and certain lymphomas, autonomous extrarenal production of calcitriol by activated macrophages can cause unregulated hypercalcemia. High levels may also reflect primary hyperparathyroidism or vitamin D toxicity. When low: Low calcitriol impairs intestinal calcium and phosphate absorption, contributing to hypocalcemia, secondary hyperparathyroidism, and metabolic bone disease including osteomalacia and rickets. In chronic kidney disease, reduced renal 1-alpha-hydroxylase activity is a primary driver of renal osteodystrophy. Deficiency may also impair immune function and cell differentiation.

Calcitriol regulates calcium absorption and bone remodeling, supporting skeletal integrity under training stress and reducing fracture risk in high-impact sports. It also modulates immune function and inflammation, potentially affecting recovery and infection resistance during heavy training phases. However, this test is not routinely used for athletic monitoring—25-hydroxyvitamin D (the storage form) is the standard marker for assessing overall vitamin D status in athletes.