Diagnostic Value of Serum 25-Hydroxyvitamin D Levels in Predicting Poor Glycemic Control Among Children with Type 1 Diabetes Mellitus: A ROC Curve and Decision Curve Analysis

Background/Objectives: Children and adolescents with type 1 diabetes mellitus (T1DM) may be particularly vulnerable to vitamin D deficiency; however, its association with glycemic control remains incompletely understood. This study aimed to determine the prevalence of vitamin D deficiency among children and adolescents with T1DM and to evaluate its relationship with glycemic control. Methods: This cross-sectional study enrolled individuals aged 1–18 years diagnosed with T1DM. Serum 25-hydroxyvitamin D concentrations were assessed, with deficiency defined as <20 ng/mL, insufficiency as 20–29 ng/mL, and sufficiency as ≥30 ng/mL. Glycemic control was defined as HbA1c < 9.0% (controlled) versus ≥9.0% (poor control). Multivariable logistic regression was performed to assess the independent association of 25-hydroxyvitamin D with glycemic control. Receiver operating characteristic (ROC) curve analysis and decision curve analysis (DCA) were conducted to evaluate the diagnostic and potential clinical utility of 25-hydroxyvitamin D levels. Results: A total of 266 participants were included; their median age was 13.0 years (IQR: 10.0–15.0), with a slight male predominance (56.8%). Overall, 64.3% of patients had suboptimal 25-hydroxyvitamin D status, including 30.1% with deficiency and 34.2% with insufficiency. Patients with poor glycemic control had significantly lower 25-hydroxyvitamin D levels compared to those with controlled diabetes (21.0 ng/mL [IQR: 17.0–26.0] vs. 35.0 ng/mL [IQR: 28.0–45.0], p < 0.001). A strong negative correlation was observed between HbA1c and 25-hydroxyvitamin D levels (Spearman’s ρ = −0.651, p < 0.001). Vitamin D insufficiency was significantly associated with poor glycemic control (aOR = 4.74, 95% CI: 2.48–9.32), while vitamin D deficiency was associated with substantially greater odds (aOR = 40.59, 95% CI: 15.55–129.23). The optimal cut-off for predicting poor control was 26.5 ng/mL, achieving a sensitivity of 75.5% and specificity of 82.2%. Decision curve analysis confirmed that the 25-hydroxyvitamin D model provided superior net benefit compared to treat-all and treat-none strategies across a threshold probability range of 32–85%. Conclusions: Vitamin D deficiency and insufficiency are highly prevalent among children and adolescents with T1DM. Lower 25-hydroxyvitamin D levels are independently associated with poorer glycemic control. Serum 25-hydroxyvitamin D demonstrates good diagnostic accuracy and potential clinical utility for risk stratification. Screening for 25-hydroxyvitamin D status and consideration of supplementation may serve as an adjunctive strategy to support metabolic management in this population.