Vitamins
25-OH vitamin D25(OH)D
Storage form of vitamin D; the standard measurement of vitamin D status.
Sample
Serum
Clinical reference range
30 to 100 ng/mL
Sufficient (Endocrine Soc)
Conv: 30–100 ng/mLFunc: 40–60 ng/mL
Biology
What it measures
Cutaneous synthesis from 7-dehydrocholesterol → cholecalciferol → 25-hydroxylation in liver. Activated to 1,25(OH)₂D in kidney.
Clinical use
Why we order it
Diagnose deficiency, monitor supplementation. Measurement in healthy adults without risk factors is not routinely recommended (USPSTF I).
Lens · Clinical
Interpretation by lens
Clinical interpretation
Treat <20 ng/mL with 50 000 IU weekly × 8 weeks then maintenance. <30 with risk factors → 1000–2000 IU/day.
Functional interpretation
Functional optimal 40–60 ng/mL; values >100 increase hypercalcemia risk.
Differential
Causes of abnormal values
Causes of HIGH
- ↑Supplementation
- ↑Granulomatous disease (sarcoidosis, TB) — measure 1,25(OH)₂D
Causes of LOW
- ↓Limited sun exposure
- ↓Dark skin at high latitude
- ↓Obesity (sequestration)
- ↓Malabsorption
- ↓CKD
- ↓Anticonvulsants
Pitfalls
Pre-analytic & interpretation traps
- !Vitamin D2 supplementation may be underestimated by some immunoassays; LC-MS/MS preferred.
Evidence-graded claims
What the data says
B
Vitamin D supplementation prevents fractures and falls in older adults
Modest effect, especially with calcium.
C
Vitamin D supplementation reduces COVID-19 severity
Inconsistent RCT data.
E
Vitamin D prevents type 2 diabetes
VITAL and D2d trials largely negative.
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