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Vitamin D (1,25 OH2)
Related Subjects: |Ferritin |CEA |ESR |CRP |ALP |LDH |HbA1c |Alpha Fetoprotein |Anti-Hu ab |Biochemical Lab values
⚠️ Do NOT request 1,25-dihydroxyvitamin D (calcitriol) to assess vitamin D status. The correct test for deficiency, rickets, or osteomalacia is 25-hydroxyvitamin D. 💡 1,25-(OH)₂ D can be normal or even elevated in deficiency due to secondary hyperparathyroidism.
📖 About
- 25-hydroxyvitamin D (25-OH D): Main circulating form, long half-life (~2–3 weeks), best indicator of overall vitamin D stores.
- 1,25-dihydroxyvitamin D (1,25-(OH)₂ D, calcitriol): Active form, short half-life (hours), tightly regulated by PTH, calcium, phosphate, and fibroblast growth factor 23 (FGF23).
- Levels of 1,25-(OH)₂ D do not reflect vitamin D reserves. They may be normal or raised in deficiency due to secondary hyperparathyroidism.
- Extra-renal production of 1,25-(OH)₂ D can occur in sarcoidosis, TB, lymphoma, and granulomatous diseases → can cause hypercalcaemia.
🎯 Clinical Use of Each Test
- 25-OH Vitamin D: Test of choice for suspected deficiency, osteomalacia, rickets, falls, osteoporosis, malabsorption, chronic illness.
- 1,25-(OH)₂ Vitamin D:
- Investigation of unexplained hypercalcaemia (esp. granulomatous disease or lymphoma).
- Evaluation of rare inherited disorders (e.g. 1-α hydroxylase deficiency, vitamin D–resistant rickets).
- Assessment in advanced chronic kidney disease (where 1-α hydroxylation is impaired).
🧾 Sample Requirements
- 5 mL venous blood in gold-top serum separator tube (or rust-top in the Acute Unit).
🌟 Summary
25-OH Vitamin D = correct test for deficiency, osteomalacia, rickets. 1,25-(OH)₂ Vitamin D = only useful in specific scenarios (renal failure, granulomatous disease, rare inherited rickets). 💡 Ordering the wrong test is a common source of wasted investigations and diagnostic delay.
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