Aim: The aim was to examine associations between thymus size and anthropometric measurements, sex, age, breastfeeding status, presence of siblings, household pets, and infections and allergies since birth in 8- to 13-month-old healthy Danish infants.

Methods: Data collected from 256 healthy infants enrolled in the ProbiComp study were used. Thymus size was assessed using sonographic measures, and thymic index (TI) and thymus weight index (TWI) was used as an absolute and a relative volume estimate, respectively.

Results: In terms of TI and TWI boys had approximately 15% and 5% larger thymus than girls (p<0.001 and p<0.02, respectively). TWI was larger in girls who were still breastfed than girls who were no longer breastfed (β 0.16 cm³ /kg; 95% CI: 0.004, 0.29; p=0.01), but no difference was observed for boys. Having household pets was associated with a larger TI (p=0.02), which seemed to be driven by associations for boys (β 1.38 cm³ ; 95% CI: 0.02, 2.74). No other factors associated with thymus size was identified.

Conclusion: Thymus size was associated with current breastfeeding in girls and with having household pets in boys. Sex-specific associations should be further explored in future studies on factors associated with thymus size.