OsteoEd

Common Questions

What laboratory abnormalities are found in vitamin D deficiency?

Mildly reduced calcium and phosphate levels are commonly seen in vitamin D deficiency, but these levels may also be normal. Often, alkaline phosphatase is elevated due to the increase in bone turnover. A study of biochemical parameters in patients with biopsy-proven osteomalacia found that 47 percent had either decreased calcium or phosphate levels (only 12 percent had both) and 94 percent had increased alkaline phosphatase levels (1). Low 24-hour urine calcium (< 50 mg) is also seen and may be the first laboratory abnormality to occur.

In vitamin D deficiency, the increased PTH (parathyroid hormone) maintains a normal serum calcium concentration at the expense of the skeleton. Below is a schematic diagram of vitamin D and calcium regulation.

Figure 1.

Figure 1. Vitamin D deficiency causes a decrease in calcium absorption. The serum calcium is transiently lowered, which causes an increased production of PTH by the parathyroid gland. PTH stimulates the conversion of 25-OH vitamin D3 to 1,25-(OH)2 vitamin D3 by the kidney, which then acts on the gut to absorb more calcium and phosphate. PTH also causes resorption of calcium and phosphate from the bone. Both of these actions attempt to restore serum calcium and phosphate levels to normal. (Reproduced from Harrison's Principle of Internal Medicine, 14th edition, with permission by McGraw-Hill.)
  1. Bingham CT, Fitzpatrick LA. Noninvasive testing in the diagnosis of osteomalacia. Am J Med 1993; 95(5): 519-23.
Last updated 2009-06-04