Background/Aims: A mild increase in thyrotropin (thyroid-stimulating hormone; TSH) is common among Down syndrome patients but is rarely detected by neonatal screening at birth. We hypothesized that Down syndrome was associated with fetal hypothyroidism and tried to determine whether Down syndrome fetuses had evidence of hypothyroidism. Methods: We performed a prospective observational study on 13 fetuses with Down syndrome diagnosed prenatally. TSH and free thyroxine (FT4) levels were measured in fetal blood. The results were analyzed and compared with the findings from fetal sonography and histopathology. Results: Of the 13 fetuses, 6 had TSH values greater than the 95th percentile, and FT4 values were low. None of the fetuses had sonographic evidence of goiter. The thyroids were normal by gross examination but exhibited small follicles and histological features suggesting delayed maturation. Conclusion: The absence of goiter even in fetuses with hyperthyrotropinemia suggests a mild decrease in TSH responsiveness.