Quantitative CT assessment of thymic involution in adults: Role of age, sex and smoking

Abstract

Objective: Thymus size and density progressively decrease through adulthood on CT. Many factors may influence thymic involution, resulting in varying CT appearances. This study aimed to evaluate thymic involution in adults using a detailed quantitative CT analysis—including volumetric soft-tissue segmentation and density measurements—and to investigate its associations with age, sex, and smoking.

Materials and Methods: Adults aged 18-50 years without known systemic disease, who underwent chest CT between June 2020 and October 2025 were retrospectively reviewed. Visual thymic scoring (0-3) and quantitative CT measurements—transverse and anterior-posterior diameters, right and left lobe lengths, overall thymus area, soft tissue area, and attenuation—were performed. These parameters were compared among thymic score groups, also based on sex and smoking status. Thymic scores were dichotomized into advanced (>50% fatty replacement, scores 0-1) and mild (≤50%, scores 2-3) groups. Logistic regression analysis was performed to identify independent predictors of advanced fatty replacement.

Results: A total of 136 patients (mean age 35.5±9, 48.5% male) were included. Age and sex significantly differed across thymic score groups (p<0.001 and p=0.018, respectively). Transverse diameter, CT attenuation, and soft tissue area increased with higher thymic scores (all p<0.05), although other diameters were not significantly different between score groups. Males exhibited significantly lower thymic scores and CT attenuation despite higher thymus diameters and area (p<0.05). Age and male sex were independently associated with advanced fatty replacement (p<0.05). Smoking was not an independent predictor despite negative correlations with thymic score, CT attenuation, and soft tissue area.

Conclusion: Thymic involution in adults is heterogeneous and primarily driven by age and sex. Fatty replacement predominates over size reduction, and males are more likely to exhibit advanced involution.