Clinical meaning
Vitamin D is a fat-soluble prohormone obtained from dietary sources (D2/ergocalciferol from plants, D3/cholecalciferol from animal sources) and cutaneous synthesis (UVB radiation converts 7-dehydrocholesterol in the skin to cholecalciferol). Vitamin D undergoes two hydroxylation steps: first in the liver by 25-hydroxylase to form 25-hydroxyvitamin D (calcidiol, the storage form measured clinically), then in the kidney by 1-alpha-hydroxylase to form 1,25-dihydroxyvitamin D (calcitriol, the active hormone). Calcitriol binds to the vitamin D receptor (VDR) in enterocytes, upregulating calbindin and TRPV6 calcium channels to increase intestinal calcium absorption from 10-15% (without vitamin D) to 30-40% (with adequate vitamin D). PTH stimulates renal 1-alpha-hydroxylase, linking the PTH and vitamin D axes. The NP must understand this pathway to logically evaluate why calcium is abnormal: is it a vitamin D production problem (limited sun, liver disease, CKD), an absorption problem (malabsorption, bariatric surgery), or a regulatory problem (PTH disorder)?
Diagnosis & workup
Diagnostics & workup: - 25-hydroxyvitamin D (calcidiol): the standard test for vitamin D status; deficiency <20 ng/mL, insufficiency 20-29 ng/mL, sufficiency 30-100 ng/mL - 1,25-dihydroxyvitamin D (calcitriol): order in CKD (low due to impaired synthesis), granulomatous disease (elevated due to ectopic production), and suspected vitamin D-dependent rickets - Serum calcium (total and ionized) with albumin correction: evaluate in context of vitamin D and PTH levels - Intact PTH: elevated in vitamin D deficiency (secondary hyperparathyroidism) -- PTH rises when 25-OH-D falls below ~30 ng/mL - Serum phosphorus: low in vitamin D deficiency (impaired intestinal absorption) and primary hyperparathyroidism (renal wasting) - Alkaline phosphatase: elevated in osteomalacia from increased osteoblast activity attempting to mineralize bone - DEXA scan: assess bone mineral density in patients with chronic vitamin D deficiency, osteoporosis risk factors, or hyperparathyroidism