Qualitative and quantitative assessment of thyroid scintigraphy in 536 hyperthyroid cats from Argentina

Feline hyperthyroidism is a prevalent endocrine disorder in geriatric cats, but studies regarding thyroid scintigraphic characteristics in South American populations are lacking. The aim of this study was to evaluate thyroid scintigraphic findings in cats with hyperthyroidism from Argentina. In this retrospective study, 536 hyperthyroid cats underwent 99mTc-pertechnetate thyroid scintigraphy, and the thyroid-to-salivary gland (T/S) ratio was determined. The median age of affected cats was 12 years (IQR 10-18 years). Bilateral asymmetric thyroid involvement was the most frequent scintigraphic presentation (38%), followed by bilateral symmetric (14.2%) and unilateral disease (31%) using size-based criteria. Multifocal uptake patterns were identified in 5.6% of cats, ectopic thyroid tissue in 11.2%, and scintigraphic features suggestive of thyroid carcinoma in 5.6%. The median T/S ratio was 2.8 (IQR 2.2-3.4) which exceeds the reference interval for clinically normal cats (<1.5). Cats exhibiting T/S ratios ≥2.1 demonstrated significantly higher serum total thyroxine (TT4) concentrations (8.1 μg/dl, IQR 6.8-10.2 μg/dl) than those with T/S ratios between 1.6 and 2 (5.3 μg/dl, IQR 4.9-6.2 μg/dl). A strong positive correlation was observed between T/S ratio and TT4 concentration (Spearman's ρ = 0.76, p < 0.001). These findings provide a comprehensive characterization of scintigraphic patterns in hyperthyroid cats from Argentina, constituting the largest cohort reported in the region to date. The comparatively high prevalence of ectopic thyroid tissue and suspected carcinoma highlights the relevance of scintigraphic assessment for diagnosis, staging, and therapeutic planning in cats with hyperthyroidism.