Feline hyperthyroidism progresses with the growth of autonomous thyroid nodules, making the assessment of disease severity necessary for determining appropriate treatment strategies. The neutrophil-to-lymphocyte ratio (NLR), a simple and cost-effective inflammatory biomarker known to be evaluated in inflammatory conditions, systemic illnesses, and malignancies, shows potential as a screening tool for severe hyperthyroidism. In this study, 70 client-owned cats with hyperthyroidism referred for radioiodine therapy were evaluated. Cats were grouped according to thyroid volume as: Group-1; small (< 2cm³), Group-2; medium (2.0–3.9cm³), Group-3; large (4.0–7.9cm³), and Group-4; very large (≥8cm³). Inflammatory biomarkers and total thyroxine levels were measured and compared among groups. Significant differences in NLR were observed among cats of Group-1 and Group-4, with cats in group 4 had a significantly higher NLR than those in group 1 (P<0.05). The optimal NLR cut-off for detecting very large (≥8cm³) thyroid tumor was 3.15, with 90% sensitivity, 60.0% specificity and an area under the receiver operating characteristic curve of 0.724, along with a negative predictive value of 97.30%. Significant positive correlation was found between thyroid volume and the NLR (r=0.283, P=0.018). These findings underscore that an elevated NLR may serve as a reliable predictor of increased tumor volume in cats with hyperthyroidism. In conclusion, the NLR is a valuable inflammatory biomarker for screening severe hyperthyroidism in cats with a high sensitivity and negative predictive value, making it reliable for differentiating severe cases of hyperthyroidism and guiding clinical decisions.

To Cite This Article: Chae Y, Yun T, Kim H and Kang BT 2025. Application of the neutrophil-to-lymphocyte ratio for predicting severity of hyperthyroidism in cats. Pak Vet J. http://dx.doi.org/10.29261/pakvetj/2025.220