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Differential Diagnosis
Decreased exposure to sunlight, milk allergies, and adherence to a strict vegetarian diet can lead to vitamin D deficiency.
Vitamin D deficiency state or abnormal metabolism of vitamin D:
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A deficiency can also occur from malabsorption of vitamin D.
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Abnormal metabolism of vitamin D, ultimately to 1,25-dihydroxy vitamin D, is found most commonly in renal failure and liver failure.
Hypoparathyroidism may be congenital, idiopathic, or resulting from thyroid surgery and removal of parathyroid glands, iron overload, irradiation, or neoplasm
Calcium loss occurs from the kidney (may be drug-induced) and from the GI tract.
Pseudohypoparathyroidism is caused by unresponsiveness of the kidney to parathyroid hormone (PTH).
Magnesium depletion, as found in patients receiving cisplatin chemotherapy, can impair PTH secretion.
Phosphate administration can occur from enemas, laxatives, IV phosphate administration, chemotherapy, and rhabdomyolysis.
Pancreatitis is formation of calcium/fatty acid complexes lost in the feces.
An artifactually low calcium can be produced when a blood sample is mistakenly collected in a tube with a calcium-chelating anticoagulant, such as EDTA or citrate.
Pseudohypocalcemia can be produced by a low serum albumin concentration, since approximately 50% of the blood calcium is protein-bound.
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There is not a decrease of ionized (non-protein-bound) calcium in this condition.
Suggested Additional Lab Testing
The most commonly used parameter to assess for vitamin D is the total 25-hydroxy vitamin D, the most abundant metabolite of vitamin D. It has a much longer half-life than 1,25-dihydroxy vitamin D.
Creatinine and BUN should be used to assess renal function.
Serum or plasma phosphorus, since the clinical circumstances associated with decreased calcium are typically associated with an increase in phosphorus.
Serum PTH also differentiates hypoparathyroidism, where the value is low, from pseudohypoparathyroidism, where the value is normal or increased.
In patients with vitamin D-dependent rickets, alkaline phosphatase is typically elevated.
To assess magnesium depletion, measure serum or plasma magnesium. Serum or plasma amylase or lipase elevation is used as an indicator of acute pancreatitis.
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