Elevated Systemic Immune-Inflammation Index and Its Correlation with TSH Levels in Patients with Subclinical Hypothyroidism

INTRODUCTION: To investigate whether hemogram-derived inflammatory indices differ between patients with subclinical hypothyroidism and euthyroid controls and whether these indices are associated with thyroid-stimulating hormone (TSH) levels.
METHODS: In this retrospective cross-sectional study, 300 adults were included: 100 euthyroid individuals and 200 patients with subclinical hypothyroidism. Neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, monocyte-to-lymphocyte ratio, and systemic immune-inflammation index (SII) were calculated from complete blood count parameters. Correlations between TSH and laboratory variables were assessed in each group, and multivariable logistic regression identified variables independently associated with subclinical hypothyroidism.
RESULTS: SII values were significantly higher in the subclinical hypothyroidism group than in the euthyroid group (p=0.047). However, the between-group difference in SII showed a small effect size (Cohen’s d=0.29). In the subclinical hypothyroidism group, TSH was positively correlated with neutrophil count (r=0.201, p=0.045), creatinine (r=0.188, p=0.046), NLR (r=0.310, p=0.002), MLR (r=0.239, p=0.017), and SII (r=0.329, p<0.001). Older age (OR 1.058, 95% CI 1.035–1.082; p<0.001), female sex (OR 1.940, 95% CI 1.125–3.345; p=0.017), and higher body mass index (OR 1.055, 95% CI 1.006–1.107; p=0.048) were independently associated with subclinical hypothyroidism, whereas SII was not (OR 1.001, 95% CI 0.999–1.002; p=0.084).
DISCUSSION AND CONCLUSION: Subclinical hypothyroidism was associated with modestly higher SII values and positive correlations between TSH and selected hemogram-derived inflammatory indices. However, the small effect size of SII and its lack of independent significance after adjustment suggest that SII should be interpreted as a supportive and nonspecific marker of low-grade inflammation rather than an independent indicator of subclinical hypothyroidism.

Keywords: Subclinical hypothyroidism, thyrotropin, inflammation, blood cell count.