Low serum vitamin D levels are associated with neonatal indirect hyperbilirubinemia and prolonged phototherapy duration: a retrospective case–control study

OBJECTIVE: To investigate the association between serum 25-hydroxyvitamin D [25(OH)D] levels, indirect hyperbilirubinemia severity, and phototherapy duration in term neonates. METHODS: This retrospective case-control study included 79 term neonates admitted to a tertiary neonatal intensive care unit between March 2023 and March 2024. Thirty-nine neonates requiring phototherapy for indirect hyperbilirubinemia (case group) were compared with 40 healthy term neonates (control group). Clinical and biochemical parameters, including bilirubin, 25(OH)D, calcium, phosphorus, magnesium, and alkaline phosphatase levels, were evaluated. Correlation, receiver operating characteristic [ROC], and multivariable regression analyses were performed to assess the association between vitamin D status and phototherapy duration in indirect hyperbilirubinemia. RESULTS: Serum 25(OH)D levels were significantly lower in neonates requiring phototherapy, with 75% demonstrating vitamin D deficiency (< 20 ng/mL). Lower vitamin D levels were associated with prolonged phototherapy duration (r = - 0.337, p = 0.036). Although no significant linear correlation was observed between serum 25(OH)D concentration and baseline bilirubin levels, neonates with vitamin D concentrations < 20 ng/mL exhibited significantly higher bilirubin levels than those with concentrations ≥ 20 ng/mL. ROC analysis demonstrated moderate predictive performance for prolonged phototherapy duration (AUC = 0.78, 95% CI: 0.67-0.88). The optimal cutoff value was 17.2 ng/mL, yielding a sensitivity of 84% and specificity of 69%. Multivariable regression analysis identified serum vitamin D level and baseline bilirubin concentration as significant predictors of phototherapy duration. CONCLUSION: Lower serum 25(OH)D concentrations were associated with indirect hyperbilirubinemia and prolonged phototherapy duration in term neonates. Larger prospective studies are needed to confirm these findings and clarify their clinical significance.