Background. Down syndrome (Trisomy 21) is a genetic disorder caused by an extra chromosome 21, leading to physical, cognitive, and medical impairments. One of the most frequent endocrine comorbidities is hypothyroidism, which is significantly more prevalent in this population than in the general pediatric population. Objective. To determine the prevalence and clinical characteristics of hypothyroidism in children with Down syndrome attending the Pediatric Endocrinology Clinic at Dr. Sardjito General Hospital Yogyakarta. Methods. This descriptive retrospective study reviewed medical records of children with Down syndrome diagnosed with hypothyroidism. Prevalence data were collected from January 2020 to December 2024. Clinical characteristics, including age, sex, TSH and FT4 levels, type of hypothyroidism, clinical features, and comorbidities, were analyzed from February 2024 to January 2025. Result. Among 915 children, 278 (30.38%; 95% CI: 27.60–33.28%) were diagnosed with hypothyroidism between 2020 and 2024. In the one-year analysis, 83 of 172 children (48.26%; 95% CI: 40.55–55.98%) were identified. Most diagnoses were made clinically (74.7%), while the rest were confirmed by karyotyping, with nondisjunction being the most common chromosomal abnormality. The most frequent clinical features were dysmorphic facies (57.83%), followed by umbilical hernia, low-set ears, sandal gap, simian line, and hypotonia. The most common comorbidities included congenital heart disease (22.89%), undernutrition, and developmental delay. The most prevalent type of hypothyroidism was subclinical (77.11%), followed by primary (10.84%) and central hypothyroidism (6.02%). Conclusion. Hypothyroidism is highly prevalent among children with Down syndrome, especially the subclinical type. Routine screening and monitoring are essential to prevent growth and developmental disorders. Keywords: hypothyroidism, Down syndrome, prevalence, clinical characteristics, retrospective study
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