RESUMO
BACKGROUND: Hashimoto encephalopathy (HE) is a rare condition associated with autoimmune thyroid disease. We aimed to report the youngest patient with Down syndrome and HE with an unusual presentation. CASE REPORT: Six years and six months old boy with Down syndrome admitted due to loss of speech. His physical development was appropriate for his age and had no goiter. Neurological examination revealed the absence of eye contact and stereotypic movements. Autism spectrum disorder was considered based on his result on Gilliam autism evaluation scale. He had subclinical hypothyroidism with markedly elevated anti-thyroid peroxidase antibody level, rare spikes in the frontocentral area were found in electroencephalography, and cranial magnetic resonance imaging was normal. Neurologic improvement was observed to a treatment with glucocorticoid and thyroid hormone. CONCLUSION: HE might be considered in patients with Down syndrome along with progressive cognitive decline and autistic regression.