High-dose intramuscular Vitamin D provides long-lasting moderate increases in serum 25-hydroxVitamin D levels and shorter-Term changes in plasma calcium

The best management of Vitamin D deficiency, defined as a 25-hydroxyVitamin D [(25(OH)D] level <50 nM, is unclear. Intramuscular (IM) injection of a large bolus of Vitamin D (≥100 000 IU) is used, but its safety is uncertain. In 10 adults given an IM injection of 600 000IU Vitamin D₃, we measured at baseline and at 1, 2, 3, and 4 weeks postinjection the serum levels of Vitamin D₃, 25(OH)D₃, 25(OH)D₂, total 25(OH)D, 3-epi-25(OH)D₃, and 24,25-dihydroxyVitamin D₃ [24,25(OH)₂D₃] using a standardized LC with tandem MS (MS/MS) assay; serum levels of 25(OH)D using the Abbott ARCHITECT i2000 immunoassay; and markers of bone metabolism. Bone markers and 25(OH)D (immunoassay) were remeasured at 24 weeks. All participants had baseline total 25(OH)D levels >50 nM. Serum 25(OH)D levels increased at 3, 4, and 24 weeks postinjection, peaking at 4 weeks [mean ± SEM of 126 ± 7.9 nM (immunoassay) and 100 ± 5.5 nM (LC-MS/MS)] but generally remained <125 nM, the upper limit recommended by the U.S. Institute of Medicine. Serum 24,25(OH)2D3 levels increased at 3 and 4 weeks postinjection. Serum ionized calcium levels were higher than baseline at 1, 3, and 4 weeks postinjection but remained within the clinically normal range. Other biochemical parameters, including other Vitamin D metabolites, plasma alkaline phosphatase, and parathyroid hormone levels, were unchanged. IM injection of a large bolus of Vitamin D effectively increases serum 25(OH)D levels without evidence of metabolic abnormality.