We investigated the thyroid function of 151 patients with Down syndrome. Compared with a control group of 89 siblings nearest in age to their brother or sister with Down syndrome, the mean thyroid-stimulating hormone (TSH) value was significantly higher in patients with Down syndrome than in subjects without Down syndrome. However, the mean thyroxine (T4) levels in both groups were nearly the same. In the Down syndrome group there was a trend for TSH values to increase and for T4 values to decrease with advancing age. Of the 151 patients with Down syndrome, ten had both significantly elevated TSH levels (greater than or equal to 9.5 microU/mL) and significantly decreased T4 levels (less than or equal to 5.5 micrograms/dL), 21 had only abnormally high TSH values, seven had only markedly increased T4 levels (greater than or equal to 12.0 micrograms/dL), and three had only significantly decreased T4 levels. The intellectual function of patients with both abnormal TSH and T4 levels was significantly lower (mean IQ, 41.7) than that of Down syndrome patients with only increased TSH values (mean IQ, 53.8) and that of Down syndrome patients with normal thyroid function (mean IQ, 55.3). This study provides further evidence that there is an increased prevalence of thyroid dysfunction in patients with Down syndrome.