There are no clear international guidelines for dosing vitamin D based on deficiency severity. Therefore, a new clinical trial is needed to evaluate the benefits of early vitamin D supplementation in maintaining sufficient levels for critically ill patients. The investigators conducted a multicenter clinical trial in Taiwan focusing on vitamin D and critically ill patients. 240 patients with low calcidiol levels will be enrolled and be provided varying supplementation doses to maintain their serum calcidiol levels ≥ 30 ng/mL within 30 days of ICU admission. The serum levels of calcidiol and PTH will be measured on Day 0, Day 7, Day 14 and Day 30 before and after vitamin D supplementation. The results will serve as a valuable reference for intensivists when formulating appropriate vitamin D treatment strategy to maximize clinical benefits for critically ill patients.