(1) Background: Although most cases of new-onset type 1 diabetes mellitus (T1DM) are managed without serious events, life-threatening complications do arise in a subset of patients. Our objective was to assess the correlation between elevated SIRI values and adverse events related to the onset of T1DM. (2) Methods: This retrospective study, spanning ten years, included 187 patients with new-onset T1DM divided into three groups based on SIRI tertiles. The primary outcome was the occurrence of acute complications during hospital admission, while the secondary outcome was prolonged Intensive Care Unit (ICU) admission. (3) Results: Patients with high SIRI values were more likely to experience higher disease activity, leading to longer ICU admission times and more frequent complications. Multivariate logistic regression analysis revealed that the SIRI was independently associated with acute complications (p = 0.003) and prolonged ICU length of stay (p = 0.003). Furthermore, receiver operating characteristic analysis demonstrated the SIRI’s superior predictive accuracy compared to venous pH (AUC = 0.837 and AUC = 0.811, respectively) and to the individual component cell lineages of the SIRI. (4) Conclusions: These findings emphasize the potential utility of the SIRI as a prognostic marker in identifying patients at increased risk during T1DM hospital admissions.

Background/Objectives: Hashimoto’s thyroiditis (HT) is an autoimmune disorder characterized by chronic inflammation of the thyroid gland. Recent evidence indicates that the inflammation may extend beyond the thyroid. The study aims to explore the potential of complete blood count (CBC)-derived indices as markers of systemic inflammation in HT. Materials and Methods: This cross-sectional retrospective study from 1 January 2015, to 31 December 2023 included 147 pediatric HT patients and 144 apparently healthy controls. Thyroid profiles, antibodies, CBC, and protein electrophoresis data were collected from patient records. CBC-derived indices were calculated and compared between the HT and control groups, as well as among HT subgroups. Results: The median age of HT patients was 13.6 years (range: 11.2–15.5 years), with 66% being girls. The control group had a similar age and gender distribution, with a median age of 13.7 years (range: 11–15.8 years) and 70.8% girls. Of the HT patients, 50% had subclinical HT, 15% were euthyroid, and 34% had overt thyroid dysfunction. HT patients showed significantly higher neutrophil and lymphocyte counts, as well as all evaluated CBC-derived indices than controls (p < 0.001)). These differences were not significant among HT subgroups. Logistic regression indicated a strong association between an elevated neutrophil-to-lymphocyte ratio (NLR) and HT diagnosis (p < 0.001), while ROC analysis confirmed NLR as the most accurate CBC-derived marker for distinguishing HT from controls. Conclusions: Elevated NLR levels in pediatric HT patients provide additional evidence that inflammation may extend beyond the thyroid gland. These results support the potential of NLR as a reliable and accessible biomarker for evaluating inflammation in Hashimoto’s thyroiditis.