NORMAL RANGE - 11-70 ng/ml
Pathophysiology;
- 25-hydroxyvitamin D3, (25-OH Vit D3 ) , is the trivial name of 9,10-
- 25 OHD3 is a precursor for other vitamin D metabolites and has only limited biological activity in itself.
- Normally, epidermal cells that are exposed to sun light will produce cholecalciferol, which is then bound to a serum vitamin D binding protein and transported to liver.
- Hydroxycholecalciferol is the major form of vitamin D in the circulation.
- The vitamin D metabolite is transported to kidney where25- hydroxycholecalciferol, which is the most potent metabolite.
- This metabolite is transported to multiple organs and binds to nuclear receptors and causes biological response ( calcium absorption or resorption, increased bone resorption , etc )
- The vitamin D binding protein increases the serum half life of 25- OH cholecalciferol and serves to provide a stable serum concentration of vitamin D.
- The functional 25(OH) vitamin D3 1 alpha-hydroxylase is expressed in insulin producing pancreatic beta cells and expression is glucose- dependent. Local activation of vitamin D hormone by 25(OH) vitamin D3 1 alpha-hydroxylase may affect pancreatic beta cell function and differentiation via autocrine or paracrine mechanisms.
25-OH vitamin D3 is the best indicator for vitamin D status in ones body
- 25-OH vitamin D3 has a longer half life than vitamin D (2-3 days versus 1-2 weeks ).
- Levels of vitamin D fluctuate more widelywith exposure to sunlight and dietry intake as compared to 25-OH vitamin D3 .
- Also 25-OH vitamin D3 is present in much greater concentrations as compared to vitamin D.
Clinical significance'
- Allows precise diagnosis and better management of clinical or suspected cases of rickets or osteomalacia.
- An early indicator ( even before bone densitometry ) for osteoporosis.
- Ensure adequate control of poulations at risk ( old age pensioners, neonates, pregnant women and populations with mismatch between skin pigmentation and sun exposure )
- Confirm suspicion of sub-clinical deficiency state.
- Complement the diagnostic work up of some hepatic,renal or gastrointestinal diseases.
- Diagnose vitamin D intoxication, as a part of a differential diagnosis of hypocalcaemia.
What abnormal values mean
Decreased levels may indicate;
- Nutritional rickets /osteomalacia.
- Senile/postmenopausal osteoporosis
- Inflammatory bowel disease (eg chron's disease)
- Insufficiency of exocrine pancreas
- Short bowel syndrome
- Biliary cirrhosis,liver dysfunction
- Renal osteodystrophy
- Nephrotic syndrome
- Neonata hypocalcaemia
- Hypocalcaemia
- Treatment with anticonvulsant drugs (enhanced metabolism )
Elevated levels may indicate;
- Hypocalcaemia
- Vitamin D intoxication
REMARKS
The serum values fall with age and in pregnancy and depend on the extent of sunlight exposure with maximum values in the late summer and lowest values in the spring. Latitude , season , aging ,sunscreen use, clothing and skin pigmentation influence production of Vitamin D3 by the skin