We recently assisted a 6-month-old boy with IPEX presenting with persistent diarrhea with failure to thrive, who had had the diagnosis of hypothyroidism in the neonatal screening and developed a Hashimoto's disease with high titers of anti-thyroid peroxidase (TPO), anti-thyroglobulin (TGB) and anti-TSH receptor autoantibodies. Here, TPO is linked to Failure to thrive.