Infants with a larger thymus at 2 months of age had a significantly higher risk—over sixfold—for developing atopic dermatitis (AD) by age 2. The risk remained high—over fivefold—for early-onset AD before 6 months.
Study Overview: A prospective study of 300 full-term infants at Rigshospitalet, Copenhagen (2017–2019) tracked thymus size via ultrasound at birth, 2 months, and 12 months. AD severity was assessed using the Eczema Area and Severity Index (EASI). Out of 300, 290 infants completed the 2-year follow-up.
Results:
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AD prevalence by age 2: 34.1%
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High thymic index at 2 months:
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6.3x higher AD risk by age 2 (aHR: 6.32; P < .001)
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5.4x higher risk for early-onset AD (<6 months) (aHR: 5.35; P < .001)
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Moderate correlation between thymic index and EASI score at 2 months (r = 0.39); no correlation at birth or 12 months.
Clinical Insight: The association held even after adjusting for filaggrin (FLG) gene mutations and family history, indicating thymic size may independently predict AD risk.
Source: Jacob P. Thyssen, MD, PhD, University of Copenhagen; study published online July 29 in Allergy.
Limitations: Findings may be limited by potential bias from AD overrepresentation, variability in ultrasound quality, and reliance on thymus size alone.
Funding & Disclosures: Supported by multiple foundations including LEO, Lundbeck, Novo Nordisk, and others. Some authors reported financial ties or employment with sponsors.