Vitamin D (25-Hydroxy)

Function

Vitamin D is a fat-soluble vitamin that functions more like a pro-hormone in the body. The 25-hydroxyvitamin D [25(OH)D] form is the major circulating form and is the best indicator of overall Vitamin D status. Its primary role is to facilitate the absorption of calcium and phosphorus in the intestines, which is critical for bone mineralization and density.

Why it is Ordered

This test is ordered to diagnose Vitamin D deficiency or toxicity. It is frequently requested for patients with bone pain, muscle weakness, or those at high risk for osteoporosis. Additionally, research has linked Vitamin D to immune function, cardiovascular health, and mood regulation, leading many clinicians to screen for it during routine physicals, especially in northern latitudes.

Associated Conditions

• Osteoporosis and Osteopenia: Thinning of the bones leading to fracture risk.
• Rickets/Osteomalacia: Softening of the bones due to poor mineralization.
• Hyperparathyroidism: Low Vitamin D can trigger the parathyroid glands to overproduce hormone to compensate for low calcium.
• Autoimmune conditions: Low levels are often observed in patients with Multiple Sclerosis or Rheumatoid Arthritis.

Source and Metabolism

Vitamin D is obtained through UVB sunlight exposure, diet (fatty fish, fortified milk), and supplementation. It undergoes hydroxylation in the liver to become 25(OH)D, and then in the kidneys to become the active 1,25-dihydroxyvitamin D.