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49. How to diagnose vitamin D deficiency?

Vitamin D deficiency/insufficiency is defined on the basis of serum 25(OH)D

levels. This is because 25(OH)D levels are stable (half-life of 2–3 weeks) and

precisely reflect the vitamin D status of an individual. Serum 1,25(OH) 2 D is

not used to define the vitamin D status of an individual because vitamin D

deficiency is associated with normal/high levels of 1,25 (OH) 2 D as a conse-

quence of secondary hyperparathyroidism. There is also high variability in the

levels of 1,25(OH) 2 D, as it has a short half-life of 4hours. 1,25(OH) 2 D is highly

lipophilic, is labile, and circulates at a very low concentration. Further, assays

for 1,25(OH) 2 D are technically challenging because of difficulties in extraction

of 1,25 (OH) 2 D from its binding proteins.

50. How was the cutoff for defining vitamin D deficiency derived?

Various cutoffs have been proposed to define vitamin D deficiency by different

organizations based on clinical and biochemical parameters, including fracture,

serum alkaline phosphatase, PTH, intestinal calcium absorption, and bone min-

eral density. Currently, the level of 25(OH)D at which iPTH starts rising is used

to define vitamin D deficiency. Various studies have shown that 25(OH)D level

<30 ng/ml is associated with rise in serum iPTH, and a level <20 ng/ml is asso-

ciated with impaired intestinal calcium absorption. Hence, 25(OH)D level

<30 ng/ml constitutes vitamin D insufficiency, and a level <20 ng/ml consti-

tutes vitamin D deficiency.

Rickets–Osteomalacia

Clinical-Radiological evaluation

Fasting sample for calcium, albumin and phosphorous, alkaline phosphate, PTH and 25 (OH) D

Phosphopenic rickets

Low Phosphate

Normal/Mildly elevated PTH

Calcipenic rickets

Low/Low-normal calcium

High PTH

Low 25 (OH) D

Normal 25 (OH) D

• VDDR type 1 and 2


• Renal tubular defects


• Chronic kidney disease


• Vitamin D deficiency


• Celiac disease


• Malabsorption


• Familial hypophosphatemic rickets


• Oncogenic osteomalacia


• Renal tubular defects

Low alkaline

phosphatase

Hypophosphotasia

Elevated alkaline

phosphatase

• Early rickets


• Partially treated rickets


• Fluorosis


• Fibrogenesis imperfecta

Normal Calcium

Normal Phosphate

Fig. 5.11 Approach to a patient with rickets–osteomalacia

5 Rickets–Osteomalacia