Catalase Activity and Methylglyoxal Concentration in Plasma and Erythrocytes Across Different Clinical Phenotypes of Thyroid Disease: A Cross-Sectional Study

Background/Objectives: Thyroid disorders may involve systemic oxidative and carbonyl disturbances, but compartment-specific changes in catalase activity and methylglyoxal concentration across different clinical phenotypes remain unclear. This study aimed to evaluate catalase activity and methylglyoxal concentrations in erythrocytes and plasma across predefined clinical phenotypes representing different combinations of thyroid functional, autoimmune, and structural characteristics and to examine correlations among these variables within and between the erythrocyte and plasma compartments. Methods: This single-center cross-sectional study included 164 participants: 24 controls, 31 patients with hypothyroidism without autoimmune thyroiditis, 30 with autoimmune thyroiditis and hypothyroidism, 42 with euthyroid nontoxic nodular goiter, and 37 with euthyroid autoimmune thyroiditis. Catalase activity and methylglyoxal concentration were measured spectrophotometrically. Results: Erythrocyte catalase activity was higher in all patient groups than in controls. Plasma catalase activity was significantly higher in hypothyroidism without autoimmune thyroiditis and nontoxic nodular goiter, but not in autoimmune thyroiditis with hypothyroidism or euthyroid autoimmune thyroiditis. Erythrocyte MGO concentration was significantly lower in hypothyroidism without autoimmune thyroiditis and nontoxic nodular goiter, whereas autoimmune thyroiditis with hypothyroidism and euthyroid autoimmune thyroiditis did not differ significantly from controls. Plasma MGO concentration did not differ significantly among groups. In patients, catalase activity was not correlated with MGO within either compartment, whereas erythrocyte and plasma MGO concentrations were positively correlated. Conclusions: Thyroid disorders showed nonuniform group-level patterns of catalase activity and MGO concentration across the erythrocyte and plasma compartments. The findings support separate interpretation of erythrocyte and plasma measurements but do not establish distinct phenotype-specific biomarker profiles.