Observational studies among critically ill patients have shown strong associations between vitamin D deficiency and adverse outcomes, including increased length of stay, infection, and mortality. It is unknown whether vitamin D deficiency contributes directly to adverse outcomes or whether it is simply a biomarker of severity of illness or overall health status. However, vitamin D plays a key role in host defense, largely by stimulating production of the anti-microbial peptide cathelicidin (LL-37). We will test the hypothesis that administration of activated vitamin D (calcitriol) will increase serum levels of cathelicidin.

Inclusion Criteria:

• age ≥ 18
• Severe sepsis or septic shock
• Central venous catheter (for blood drawing)

Exclusion Criteria:

• Serum calcium ≥ 10.0 mg/dl or phosphate ≥ 6.0 mg/dl, assessed within previous 48 hours
• Current or recent therapy (within previous 7 days) with nutritional vitamin D at doses >1,000 I.U. per day or activated vitamin D at any dose
• History of solid organ or bone marrow transplant, primary parathyroid disease, metabolic bone disease, or sarcoidosis
• Expected to die or leave the ICU within 48 hours
• History of hypersensitivity or any allergic reaction to calcitriol
• End stage renal disease
• Acute Kidney Injury receiving intermittent renal replacement therapy
• Enrolled in a competing study
• Pregnancy