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Common Test-Ordering Errors
Part 1: Misordered Tests

1,25-Dihydroxyvitamin D
PTH-related Peptide
Uroporphyrinogen III Synthase

Vitamin D Pathway

Slide 9

April 2010

I’d like to briefly review the vitamin D pathway because I think that helps explain the confusion about what test to order. Sun exposure, converts 7-dihydrocholesterol to vitamin D in the skin. Vitamin D is then converted to 25-Hydroxyvitamin D in the liver, and is stored in the tissues. 25-Hydroxyvitamin D reflects vitamin D derived from both dietary sources and vitamin D synthesized in the skin. A small fraction of 25-Hydroxyvitamin D is converted to the active metabolite 1,25-dihydroxy vitamin D, mainly by the kidneys, in a process regulated by parathyroid hormone.

Vitamin D Pathway


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