Vitamin D

Overview

Vitamin D, measured as 25-hydroxyvitamin D [25(OH)D] in serum, assesses nutritional status and reflects production from skin UVB exposure or dietary/supplemental intake. It regulates calcium/phosphate homeostasis, bone mineralization, immune function, and cellular growth through nuclear receptor activation. Low levels indicate deficiency causing rickets/osteomalacia or osteoporosis risk, while high levels suggest toxicity; clinically useful for diagnosing deficiency in at-risk groups, guiding supplementation, and evaluating bone health disorders per Endocrine Society guidelines.

Clinical Use Cases

  • Diagnosing vitamin D deficiency in osteoporosis, rickets, osteomalacia.
  • Monitoring chronic kidney disease and malabsorption syndromes.
  • Assessing high-risk groups (limited sun exposure, dark skin, obesity).
  • Guiding supplementation in pregnancy or elderly patients.
  • Evaluating recurrent fractures or muscle weakness.

Specimen Types

  • Serum.
  • Plasma (EDTA).

Measurement Methods

  • Liquid chromatography-tandem mass spectrometry (LC-MS/MS, reference).
  • Automated immunoassays (chemiluminescent).
  • High-performance liquid chromatography (HPLC).

Test Preparation and Influencing Factors

  • No fasting required.
  • Recent supplementation (days-weeks) elevates levels.
  • Obesity lowers circulating 25(OH)D (sequestered in fat).
  • Liver/kidney disease impairs 25(OH)D metabolism.
  • Seasonal variation (lower in winter).

Synonyms

  • 25-hydroxyvitamin D.
  • 25(OH)D.
  • Calcidiol.

Further Reading

  • “Vitamin D Test” – MedlinePlus – 
  • Wikipedia