The thymus plays a crucial role in immune system homeostasis. Thymic abnormalities have been reported in many autoimmune diseases, but data for systemic sclerosis (SSc) and rheumatoid arthritis (RA) are sparse. The aim of this study was to evaluate the prevalence and correlates of radiological incomplete involution of the thymus in SSc and RA patients, and in a non-autoimmune group of controls.

All patients were at least 40 years old: 96 SSc patients (median age 59 years, 80% women) and 65 RA patients (median age 57 years, 88% women) were compared with 32 control individuals (median age 63 years, 62% women). Pulmonary CT-scans performed for lung assessment were available for all individuals. For the purpose of our study, complete involution of the thymus was defined as the absence of a residual thymus or a gland thickness, corresponding to the short axis on the axial slice, of less than 7mm. We defined incomplete involution of the thymus as a residual thymic tissue more than 7mm thick.

The frequency of incomplete thymus involution was significantly higher in SSc and RA patients (respectively 15 and 14%) than in the control group (0%; P<0.05). Incomplete thymus involution was associated with pulmonary restrictive syndrome in SSc patients, and with biotherapy and an absence of antinuclear antibodies in RA patients.

Our findings show that two autoimmune diseases, SSc and RA, are associated with incomplete thymus involution.