Purpose <p>To investigate the relationship between platelet-to-lymphocyte ratio (PLR) or neutrophil-to-lymphocyte ratio (NLR) and Immune checkpoint inhibitor (ICI)-induced thyroid dysfunction.</p> Methods <p>This was a single-center retrospective observational study of patients with solid tumors receiving ICI therapy. Clinical characteristics of patients were assessed at baseline and during ICI therapy. Logistic regression was implemented to assess the association of PLR and NLR with thyroid dysfunction. Kaplan-Meier method was used to analyze the difference in time between the onset of hypothyroidism and thyrotoxicosis.</p> Results <p>A total of 355 patients with solid tumors were included in our study. Sixty-nine (19.44%) patients developed ICI-induced thyroid dysfunction after receiving ICI therapy, with a median (IQR) time to onset of 91(34–203.5) days. Patients with high PLR (H-PLR) had an increased risk of ICI-induced thyroid dysfunction (OR = 1.87, 95% CI 1.07–3.28, <i>P</i> = 0.028) compared to those with low PLR (L-PLR). Specifically, H-PLR was associated with ICI-induced thyrotoxicosis but not hypothyroidism (OR = 2.40, 95% CI 1.09–5.29, <i>P</i> = 0.030). Meanwhile, NLR was not correlated with ICI-induced thyroid dysfunction as a continuous (<i>P</i> = 0.699) or categorical variable (<i>P</i> = 0.914). The sensitivity analysis showed that H-PLR remains positively correlated with ICI-induced thyroid dysfunction.</p> Conclusion <p>PLR rather than NLR was associated with the occurrence of ICI-induced thyroid dysfunction. Furthermore, PLR may serve as a predictive biomarker for ICI-induced thyroid dysfunction.</p>